Methods of Soil Analysis, Part 3: Chemical Methods (SSSA Book Series) [1 ed.] 0891188258, 9780891188254, 9780891188667

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Published 1996

METHODS OF SOIL ANALYSIS PART3

Chemical Methods

Soil Science Society of America Book Series Books in the series are available from the Soil Science Society of America, 677 South Segoe Road, Madison, WI 53711 USA. 1. MINERALS IN SOIL ENVIRONMENTS. Second Edition. 1989. R. C. Dinauer, managing editor J.B. Dixon and S. B. Weed, editors 2. PESTICIDES IN THE SOIL ENVIRONMENT: PROCESSES, IMPACTS, AND MODELING. 1990. S. H. Mickelson, managing editor H. H. Cheng, editor 3. SOIL TESTING AND PLANT ANALYSIS. Third Edition. 1990. S. H. Mickelson, managing editor R. L. Westerman, editor 4. MICRONUTRIENTS IN AGRICULTURE. Second Edition. 1991. S. H. Mickelson, managing editor J. J. Mortvedt et al., editors

5. METHODS OF SOIL ANALYSIS: PHYSICAL AND MINERALOGICAL METHODS. Part 1. Second Edition. 1986. R. C. Dinauer, managing editor Arnold Klute, editor METHODS OF SOIL ANALYSIS: CHEMICAL AND MICROBIOLOGICAL PROPERTIES. Part 2. Second Edition. 1982. R. C. Dinauer, managing editor A. L. Page et al., editor METHODS OF SOIL ANALYSIS: CHEMICAL METHODS. Part 3. 1996. J.M. Bartels, managing editor D. L. Sparks, editor

Methods of Soil Analysis Part3 Chemical Methods Editorial Committee: D. L. Sparks A. L. Page P.A. Helmke R. H. Loeppert P. N. Soltanpour M. A. Tabatabai C. T. Johnston M. E. Sumner

Managing Editor: J.M. Bartels Editor-in-Chief SSSA: J. M. Bigham

Number 5 in the Soil Science Society of America Book Series Published by: Soil Science Society of America, Inc. American Society of Agronomy, Inc. Madison, Wisconsin, USA

1996

Copyright © 1996 by the Soil Science Society of America, Inc. American Society of Agronomy, Inc. ALL RIGHTS RESERVED UNDER THE U.S. COPYRIGHT LAW OF 1978 (P. L. 94-553) Any and all uses beyond the "fair use" provision of the law require written permission from the publishers and/or author(s); not applicable to contributions prepared by officers or employees of the U.S. Government as part of their official duties.

Second printing 1999. Third printing 2001.

Soil Science Society of America, Inc. American Society of Agronomy, Inc. 677 South Segoe Road, Madison, Wisconsin 53711 USA

Library of Congress Cataloging-in-Publication Data Library of Congress Catalog Card Number: 96-70096

Printed in the United States of America

CONTENTS

Page Foreword ix Preface xi Contributors .. .. ... . ... ... .... .. .... ... .... .... .... .... ... .. ... .. .. ... ... ... ..... ... .. .... ... .. .. ... ... .. .... .. xiii Conversion Factors for SI and non-SI Units .......... ...................... ........ ........ xvii

1

2

3

4

5

6

7

8

9

10

Sampling Roger G. Petersen and Lyle D. Calvin ......................................

1

Quality Assurance and Quality Control E. J. Klesta, Jr. and J. K. Bartz ..................................................

19

Dissolution for Total Elemental Analysis L. R. Hossner .................................. ...... .................... .... ........ .....

49

Atomic Absorption and Flame Emission Spectrometry Robert J. Wright and Tomasz I. Stuczynski ..............................

65

Inductively Coupled Plasma Emission Spectrometry and Inductively Coupled Plasma-Mass Spectroscopy Parviz N. Soltanpour, Greg W. Johnson, Stephen M. Workman, J. Benton Jones, Jr., and Robert 0. Miller .................................................................

91

Neutron Activation Analysis Philip A. Helrnke .................. .................. .... ..... ......... .... ....... ... ...

141

Elemental Analysis by X-Ray Fluorescence Spectroscopy A. D. Karathanasis and Ben F. Hajek .......................................

161

Liquid Chromatography M. A. Tabatabai and W. T. Frankenberger, Jr. ..........................

225

Differential Pulse Voltammetry Larry M. Shuman ..................................................... .................

247

Fourier Transform Infrared and Raman Spectroscopy C. T. Johnston and Y. 0. Aochi .................................................

269

V

CONTENTS

vi

11

Electron Spin (or Paramagnetic) Resonance Spectroscopy Nicola Senesi ............................................................................. 323

12

X-Ray Photoelectron Spectroscopy R. K. Vempati, T. R. Hess, and D. L. Cocke ............................ 357

13

X-Ray Absorption Fine Structure Spectroscopy Scott Fendorf and Donald L. Sparks ......................................... 377

14

Salinity: Electrical Conductivity and Total Dissolved Solids J. D. Rhoades.............................................................................

417

15

Carbonate and Gypsum Richard H. Loeppert and Donald L. Suarez .............................. 437

16

Soil pH and Soil Acidity Grant W. Thomas....................................................................... 475

17

Lime Requirement J. Thomas Sims.......................................................................... 491

18

Aluminum Paul M. Bertsch and Paul R. Bloom ......................................... 517

19

Lithium, Sodium, Potassium, Rubidium, and Cesium Philip A. Helmke and Donald L. Sparks ................................... 551

20

Beryllium, Magnesium, Calcium, Strontium, and Barium Donald L. Suarez ....................................................................... 575

21

Boron R. Keren ..................................................................................... 603

22

Silicon R. Lewis Jones and Gary B. Dreher .......................................... 627

23

Iron Richard L. Loeppert and W. P. Inskeep ..................................... 639

24

Manganese R. P. Gambrell ........................................................................... 665

25

Chromium Richmond J. Bartlett and Bruce R. James................................. 683

26

Copper and Zinc Stewart T. Reed and D.C. Martens............................................ 703

CONTENTS

27

28

29

30

vii

Molybdenum and Cobalt John L. Sims ................ ..............................................................

723

Nickel, Cadmium, and Lead Michael C. Amacher ........... .......................................................

739

Mercury James G. Crock ..........................................................................

769

Selenium and Arsenic P. M. Huang and Roger Fujii ................... ... ...... .................... .....

793

31

Bromine, Chlorine, and Fluorine W. T. Frankenberger, Jr., M.A. Tabatabai, D. C. Adriano, and H. E. Doner ................................................ 833

32

Phosphorus Shiou Kuo ........ ............................................................. ..... ........ 869

33

Sulfur M. A. Tabatabai ........... .......................................................... .... 921

34

Total Carbon, Organic Carbon, and Organic Matter Darrell W. Nelson and Lee E. Sommers ................................... 961

35

Organic Matter Characterization Roger S. Swift ........................................................................... 1011

36

Extraction of Organic Chemicals Brij L. Sawhney ......................................................................... 1071

37

Nitrogen-Total John M. Bremner ....................................................................... 1085

38

Nitrogen-Inorganic Forms R. L. Mulvaney .......................................................................... 1123

39

Nitrogen-Organic Forms F. J. Stevenson ........................................................................... 1185

40

Cation Exchange Capacity and Exchange Coefficients Malcolm E. Sumner and William P. Miller ........ ....................... 1201

41

Charge Analyses of Soils and Anion Exchange Lucian W. Zelazny, Liming He, and An M. Vanwormhoudt ........................................................ 1231

viii

CONTENTS

42

Redox Measurements of Soils W. H. Patrick, Jr., R. P. Gambrell, and S. P. Faulkner .............. 1255

43

Kinetic Methods and Measurements Donald L. Sparks, Theodore H. Carski, Scott E. Fendorf, and Charles V. Toner, IV .................................. .............. ........... 1275

44

Equilibrium Modeling in Soil Chemistry S. V. Mattigod and J.M. Zachara .............................................. 1309

FOREWORD Analytical methods are the foundation of a scientific discipline. This was recognized by the Soil Science Society of America when an effort was initiated in 1957 to give recognition to the body of analytical methods developed specifically to characterize soil composition and properties. Publication of the first edition of the "Methods of Soil Analysis" in 1965, under the editorship of Dr. C.A. Black, marked a milestone in the development of the field of soil science. Although there existed several books on soil analysis prior to 1965, this publication was the first authoritative treatise collectively authored by soil scientists under the joint sponsorship of the American Society of Agronomy and American Society of Testing and Materials, and published as volumes in the Agronomy Monograph series. The publication quickly became the primary reference book on methods for analyzing many soil physical, chemical, and microbiological properties. After the Soil Science Society of America created the Book Series, the Boards of Directors of the American Society of Agronomy and Soil Science Society of America reached an agreement in 1993 "to publish all future reprints, revised editions, and new versions of Methods of Soil Analysis and all subsequent parts as part of the SSSA Book Series." The third edition of Methods of Soil Analysis will now have three volumes. The volume covering the microbiological and chemical methods was published in 1995. The current volume will cover the chemical methods, and the volume on physical and mineralogical methods is under preparation. This volume includes coverage of newer methods for characterizing soil chemical properties as well as several methods for characterizing soil chemical processes. It should continue to serve as the primary reference on analytical methods and provide soil and environmental scientists additional tools to advance our knowledge of soil properties and soil processes. H. H. Cheng, president Soil Science Society of America

ix

PREFACE The second edition of Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties was published in 1982. It was edited by AL. Page, R.H. Miller, and D.R. Keeney. The 2nd edition is recognized as the benchmark reference on chemical and microbiological analyses of soils. It has been used widely by soil scientists and professionals in other fields. More than 11,000 copies have been sold. Due to major advances in analytical equipment and methodology, the desire to include new chapters on analyses of soil chemical processes, and the need to include additional material on microbiological analyses, Part 2 has been divided into two parts and revised. The first book, Methods of Soil Analysis, Microbiological and Biochemical Properties (Part 2), was published in 1995 as SSSA Book Series No. 5. This book, Methods of Soi/Analysis: Chemical Methods, is Part 3 of SSSA Book Series No. 5. This book contains 44 chapters, written by 70 authors from throughout the world. A new chapter on quality assurance and quality control is included. Updated chapters are included on the principles of various instrumental methods and their applications to soil analysis. Additionally, new chapters are included on Fourier transform infrared, Raman, electron spin resonance, x-ray photoelectron, and x-ray absorption fine structure spectroscopies. The application of these methods to analyzing soil chemical reactions is currently one of the major research areas in the soil and environmental sciences. Chapters are included on analyses of soil chemical properties including soil salinity, carbonate and gypsum, soil pH and acidity, lime requirement, cation and anion exchange capacities, and organic matter. Methods for the analyses of soluble, sorbed, and total concentrations of 34 elements are also included. Additionally, these chapters include useful background information on the chemistry of the elements. A new chapter on methods for organic chemical extraction is included. A new aspect of this book is the addition of procedures for analyzing important soil chemical processes. These include redox and surface charge (points of zero charge) analyses, and kinetic methods and measurements. Chapter 44, the last chapter, discusses equilibrium modeling in soil chemistry. The editorial committee, that was instrumental in the planning of the book and in the reviewing process, was composed of: D.L. Sparks, Chairman, University of Delaware, Newark, DE. AL. Page, University of California, Riverside, CA. P.A. Helmke, University of Wisconsin, Madison, WI. R.H. Loeppert, Texas A&M University, College Station, TX. P.N. Soltanpour, Colorado State University, Fort Collins, CO. M.A. Tabatabai, Iowa State University, Ames, IA. C.T. Johnston, Purdue University, West Lafayette, IN. M.E. Sumner, University of Georgia, Athens, GA. xi

xii

PREFACE

The editorial committee expresses its sincerest gratitude to the anonymous reviewers for their careful and thoughtful evaluation of the manuscripts. Many thanks are extended to Jon Bartels of the SSSA Headquarters Staff for his outstanding editorial assistance. Methods of Soil Analysis: Chemical Properties should be useful not only to students and professionals in soil science, but to those in allied fields such as engineering, chemistry, geosciences, and marine studies, who are increasingly interested in soils. D.L. Sparks, Editor-in-Chief University of Delaware Newark, Delaware 19717-1303 AL. Page, Associate Editor University of California, Riverside Riverside, California 92521 P.A. Helmke, Associate Editor University of Wisconsin Madison, Wisconsin 53706-1299 R.H. Loeppert, Associate Editor Texas A&M University College Station, Texas 77843

CONTRIBUTORS Domy C. Adriano

Professor and Head, Biogeochemical Ecology Division, Savannah River Ecology Laboratory, Drawer E, Aiken SC 29802

Michael C. Amacher

Research Soil Chemist, USDA-FS, Forestry Sciences Laboratory, 850 N. 1200 E., Logan UT 84321

Y.O.Aochi

Staff Research Associate, Department of Soil and Environmental Sciences, University of California, Riverside CA 92521

Richmond J. Bartlett

Professor of Soil and Environmental Chemistry, Department of Plant and Science, Hills Building, University of Vermont, Burlington VT 05405

J. K. Bartz

Chemist, Washington State Department of Ecology, 1315 W. 4th Avenue, Kennewick, WA 99336

Paul M. Bertsch

Professor, Advanced Analytical Center for Environmental Science, University of Georgia, Savannah River Ecology Laboratory, Drawer E, Aiken SC 29802

Paul R. Bloom

Professor of Soil Science, Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, St. Paul MN 55108

John M. Bremner

Distinguished Professor Emeritus, Department of Agronomy, Iowa State University, Ames IA 50011-1010

L.D.Calvin

Professor Emeritus of Statistics, Department of Statistics, Oregon State University, Corvallis OR 97331

Theodore H. Carksi

Research Associate/Supervisor, Agricultural Products, Stine-Haskell Research Center, E.I. duPont de Nemours and Company, Building 210N, Room 204, Elkton Road, P.O. Box 30, Newark DE 19714

D.LCocke

J.M. Gill Professor of Chemistry, Department of Chemistry, Lamar University, P.O. Box 10022, Beaumont TX 77710

James G. Crock

Analytical Geochemist, Branch of Geochemistry, Analytical Chemistry Services Group, U.S. Geological Survey, Denver Federal Center, Mail Stop 973, Building 20, Box 25046, Denver CO 80225-6046

Harvey E. Doner

Professor of Soil Chemistry, Division of Ecosystem Sciences, ESPM, University of California, 101 Giannini Hall, Berkeley CA 94720-3110

Gary B. Dreher

Head, Analytical Geochemistry Section, Illinois State Geological Survey, 615 E. Peabody Drive, Champaign IL 61820

S.P. Faulkner

Assistant Professor, Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge LA 70803-7511

Scott Fendorf

Assistant Professor of Soil Science, Soil Science Division, University of Idaho, Moscow ID 83844-2339

xiii

xiv

CONTRIBUTORS

W.T. Frankenberger, Jr.

Professor of Soil Microbiology and Biochemistry, Department of Soil and Environmental Sciences, University of California, Riverside CA 92521-0424

Roger Fujii

Research Chemist, Water Resources Division-California District, U.S. Geological Survey, 2800 Cottage Way, Sacramento CA 95825

R.P. Gambrell

Professor, Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge LA 70803-7511

Ben F. Hajek

Professor, Department of Agronomy and Soils, 201 Funchess Hall, Auburn University, Auburn AL 36849

Liming He

Research Assistant, Scripps Institute of Oceanography, University of California, La Jolla CA 92093-0202

Philip A. Helmke

Professor of Soil Science, Department of Soil Science, University of Wisconsin, 1525 Observatory Drive, Madison WI 53706-1299

T. R. Hess

Gill Chair, Department of Chemistry, Lamar University, P.O. Box 10022, Beaumont TX 77710

L. R. Hossner

Professor of Soil Chemistry, Department of Soil and Crop Sciences, Texas A&M University, College Station TX 77843-2474

P.M.Huang

Professor of Soil Science, Department of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8 Canada

William P. Inskeep

Professor, Department of Plant, Soil and Environmental Sciences, Montana State University, Bozeman MT 59717

Bruce R. James

Associate Professor of Soil Chemistry, Department of Agronomy, University of Maryland, College Park MD 20742

Greg W. Johnson

Spectroscopist, Matheson Gas Products, 1861 Lefthand Circle Drive, Longmont CO 80501

C.T. Johnston

Associate Professor of Soil Chemistry, Department of Agronomy, Purdue University, 1150 Lilly Hall, West Lafayette IN 47907-1150

J. Benton Jones, Jr.

Vice President, Micro-Macro International, Inc., 183 Paradise Boulevard, Athens GA 30607

R. Lewis Jones

Professor of Soil Mineralogy and Ecology, Department of Natural Resources and Environmental Sciences, University of Illinois, 1102 S. Goodwin Avenue, Urbana IL 61801

A.O. Karathanasis

Professor of Soil Mineralogy and Pedology, Department of Agronomy, University of Kentucky, N-122 Agricultural Sciences Building North, Lexington KY 40546-0091

Rami Keren

Director, Institute of Soils and Water, The Volcani Center, ARO, P.O. Box 6, Bet Dagan 50250, Israel

E. J. Klesta, Jr.

Director of Technology, Cozz Iron & Metal, Inc., 605 Alexandria Drive, Naperville IL 60565

ShiouKuo

Soil Scientist, Department of Crop and Soil Sciences, Washington State University, 7612 Pioneer Way E., Puyallup WA 98371-4998

Richard H. Loeppert

Professor, Department of Soil and Crop Sciences, Texas A&M University, College Station TX 77843-2474

CONTRIBUTORS

xv

D. C. Martens

W.G. Wysor Professor of Agriculture and Life Sciences, Department of Soil and Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg VA 24061-0404

S. V. Mattigod

Battelle Pacific Northwest Laboratory, P.O. Box 999, Richland WA 99352

Robert O. Miller

Soil Scientist/Extension Soil Specialist, Department of Land, Air and Water Resources, Hoagland Hall, University of California, Davis CA 95616-8627

William P. Miller

Professor of Soil Science, Department of Crop and Soil Sciences, 3111 Miller Plant Sciences Building, University of Georgia, Athens GA 30602-7503

Richard L. Mulvaney

Professor of Soil Science, Department of Natural Resources and Environmental Sciences, University of Illinois, 1102 S. Goodwin Avenue, Urbana IL 61801

Darrell W. Nelson

Dean and Director, Agricultural Research Division, 207 Agricultural Hall, University of Nebraska, Lincoln NE 68583-0704

W. H. Patrick, Jr.

Boyd Professor and Director, Wetland Biogeochemistry Institute Louisiana State University, Baton Rouge LA 70803-7511

Roger G. Petersen

Professor Emeritus of Statistics, Department of Statistics, Oregon State University, Corvallis OR 97331

Stewart T. Reed

Assistant Professor of Soil Science, CESTA, 306E South Perry Paige Building, Florida A&M University, Tallahassee FL 32307

J. D. Rhoades

Laboratory Director, USDA-ARS, U.S. Salinity Laboratory, 450 W. Big Springs Road, Riverside CA 92507

Brij L Sawhney

Soil Chemist, The Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven CT 06504

Nicola Senesi

Professor of Soil Chemistry and Director, Istituto di Chimica Agraria, Universita di Bari, Via Amendola 165/A, Bari-70126, Italy

Larry M. Shuman

Professor of Soil Chemistry, Department of Crop and Soil Sciences, Georgia Agricultural Experiment Station, University of Georgia, Griffin GA 30223-1797

John L. Sims

Professor Emeritus of Agronomy, Department of Agronomy, N-122 Agricultural Science Building North, University of Kentucky, Lexington KY 40546-0091

J. Thomas Sims

Professor of Soil Science, Department of Plant and Soil Science, 149 Townsend Hall, University of Delaware, Newark DE 19717-1303

Parviz N. Soltanpour

Professor of Soil Science, Department of Soil and Crop Sciences, Cll 7 Plant Sciences Building, Colorado State University, Fort Collins CO 80523

Lee E. Sommers

Professor of Soil Science and Department Head, Department of Soil and Crop Sciences, Colorado State University, Fort Collins CO 80523

Donald L. Sparks

Distinguished Professor of Soil Science, Department of Plant and Soil Sciences, University of Delaware, Newark DE 19717-1303

xvi

CONTRIBUTORS

Frank J. Stevenson

Professor Emeritus, Department of Agronomy, University of Illinois, 1102 S. Goodwin Avenue, Urbana IL 61801-4798

Tomasz I. Stuczynski

Soil Scientist, Institute of Soil Science and Plant Cultivation, Osada Palacowa, 24-100 Pulawy, Poland

Donald L Suarez

Research Leader, USDA-ARS, U.S. Salinity Laboratory, 450 W. Big Springs Road, Riverside CA 92507

Malcolm E. Sumner

Regents' Professor of Soil Science, Department of Crop and Soil Sciences, 3111 Miller Plant Sciences Building, University of Georgia, Athens GA 30602

Roger S. Swift

Chief, CSIRO, Division of Soils, PMB 2, Glen Osmond 5064, Australia

M. A. Tabatabai

Professor of Soil Chemistry, Department of Agronomy, Iowa State University, Ames IA 50011-1010

Grant W. Thomas

Professor of Agronomy, Department of Agronomy, University of Kentucky, Lexington KY 40546-0091

Charles V. Toner, IV

Graduate Research Fellow, Department of Plant and Soil Sciences, University of Delaware, Newark DE 19717-1303

An M. Vanwormhoudt

49 Lillibrooke Crescent, Maidenhead, Berkshire SL6 3XJ U.K.

R. K. Vempati

Department of Chemistry, Lamar University, P.O. Box 10022, Beaumont TX 77710

Stephen M. Workman

Technical Manager, Analytical Technologies, Inc., 225 Commerce Drive, Fort Collins CO 80524

Robert J. Wright

Research Soil Scientist, USDA-ARS, Environmental Chemistry Laboratory, Building 007, Room 224, BARC-West, Beltsville MD 20705

J.M. Zachara

Chief Scientist, Pacific Northwest Laboratory, P.O. Box 999, Mail Stop K3-61, Richland WA 99352

Lucian W. Zelazny

Thomas B. Hutchison, Jr., Professor of Soil Chemistry/Mineralogy, Department of Crop and Soil Environmental Science, Virginia Polytechnic Institute and State University, Blacksburg VA 24061-0404

Conversion Factors for SI and non-SI Units

xvii

10-2

104 10-2

10-3

cubic meter, m 3 cubic meter, m3 cubic meter, m3 liter, L (10-3 m3) liter, L (10-3 m3) liter, L (10-3 m3) liter, L (10-3 m3) liter, L (10-3 m3) liter, L (10- 3 m 3)

hectare, ha square kilometer, km2 (103 m)2 square kilometer, km2 (103 m)2 square meter, m2 square meter, m2 square millimeter, mm2 (l0-3 m)2

2.47 247 0.386 2.47 X 10-4 10.76 1.55 X 10-3

9.73 X 35.3 6.10 X 2.84 X 1.057 3.53 X 0.265 33.78 2.11

kilometer, km (103 m) meter, m meter, m micrometer, µm (10-6 m) millimeter, mm (10-3 m) nanometer, nm (10-9 m)

Column 1 SI Unit

0.621 1.094 3.28 1.0 3.94 X 10-2 10

To convert Column 1 into Column 2, multiply by

Volume

Area

Length

acre-inch cubic foot, ft3 cubic inch, in3 bushel, bu quart (liquid), qt cubic foot, ft 3 gallon ounce (fluid), oz pint (fluid), pt

acre acre square mile, mi2 acre square foot, ft 2 square inch, in 2

mile, mi yard, yd foot, ft micron,µ inch, in Angstrom,A

Column 2 non-SI Units

Conversion Factors for SI and non-SI Units

102.8 2.83 X 10-2 1.64 X 10-5 35.24 0.946 28.3 3.78 2.96 X 10-2 0.473

0.405 4.05 X 10-3 2.590 4.05 X loJ 9.29 X 10-2 645

1.609 0.914 0.304 1.0 25.4 0.1

To convert Column 2 into Column 1, multiply by

;_

("'.)

rr,

--3

z

-

00

-

0

z z

i::,

rr,

-

0 :,::,

""l

("'.)

z

::

0

-

z < t"1

0

==

megapascal, MPa (106 Pa) megapascal, MPa (106 Pa) megagram, per cubic meter, Mg m-3 pascal, Pa pascal, Pa

9.90 10 1.00 2.09 X 10-2 1.45 X 10-4

Pressure

atmosphere bar gram per cubic centimeter, g cm- 3 pound per square foot, lb ft- 2 pound per square inch, lb in-2

square centimeter per gram, cm2 g- 1 square millimeter per gram, mm 2 g- 1

Specific Surface

pound per acre, lb acre- 1 pound per bushel, lb bu- 1 bushel per acre, 60 lb bushel per acre, 56 lb bushel per acre, 48 lb gallon per acre pound per acre, lb acre- 1 pound per acre, lb acre- 1 ton (2000 lb) per acre, ton acre- 1 mile per hour

Yield and Rate

pound, lb ounce (avdp), oz pound, lb quintal (metric), q ton (2000 lb), ton ton (U.S.), ton ton (U.S.), ton

(continued on next page)

square meter per kilogram, square meter per kilogram, m2 kg- 1

10 1000

m2 kg- 1

kilogram per hectare, kg ha- 1 kilogram per cubic meter, kg m- 3 kilogram per hectare, kg ha- 1 kilogram per hectare, kg ha- 1 kilogram per hectare, kg ha- 1 liter per hectare, L ha- 1 tonnes per hectare, t ha- 1 megagram per hectare, Mg ha- 1 megagram per hectare, Mg ha- 1 meter per second, m s- 1

0.893 7.77 X 1.49 X 1.59 X 1.86 X 0.107 893 893 0.446 2.24

10-2 10-2 10-2 10-2

gram, g (10-3 kg) gram, g (10-3 kg) kilogram, kg kilogram, kg kilogram, kg megagram, Mg (tonne) tonne, t

2.20 X 10-3 3.52 X 10-2 2.205 0.01 1.10 X 10-3 1.102 1.102

Mass

0.101 0.1 1.00 47.9 6.90 X 103

0.1 0.001

1.12 12.87 67.19 62.71 53.75 9.35 1.12 X 10-3 1.12 X 10-3 2.24 0.447

454 28.4 0.454 100 907 0.907 0.907

~>
40%) andMn oxide (> 1%) with H20 2 and HCI to reduceMn in Mn02 (Jackson,1958), and then with aqua regia to dissolve as much of the oxides as possible, obviates this problem (Kanehiro & Sherman,1965; see section on "Method for Pretreatmentof Soils High in Ferric Oxide and Manganese Dioxide").

TOTAL ELEMENTAL ANALYSIS DISSOLUTION

53

Fusionwith SodiumPeroxide Sodium peroxide, Na202' (melting point 460°C) is a strongly alkaline fusion agentthat simultaneouslyactsas a strongoxidant. Resistantcompoundsof Zr, Ti and Cr that are difficult to decomposeby Na2C03aloneare readily attacked by an excessof Na202or a Na20z-Na2C03mixture. Fusionwith Na202 ensures efficient decomposition of resistant borates, silicates, phosphates,sulfates, borides, carbides, nitrides and halides (Donaldson, 1979; Donaldson, 1980; Bhargavaet aI., 1980). Sodium peroxidealone,or with modifying admixturesof alkali carbonatesor hydroxides,is often usedto decomposerefractory materials suchas chromiteand zirconium oxide ceramics,bauxites,chromites,zircon, tantalite-columbites,beryl, ilmenite sand, and fired clays (Sulcek & Povondra, 1991). Sodium peroxide melts without dangerof explosion, however, it can be dangerouswhen its melt is in contact with substancesthat are easily oxidized such as various forms of C, elementalS, and AI or other metal powders(Sulcek & Povondra,1991). Porcelain,Ag, Fe, Ni, Pt and corundumcruciblesare significantly corroded by Na202 at temperaturesabove 400°C. Zirconium and glassy C crucibles resist molten Na202' Decompositionwith mixed fusion agentsof peroxide-carbonatewill suppressthe corrosiveeffect of the peroxidealoneon the crucible and will decreasethe fusion temperature.Use of a reducingflame or fusion in an Or free atmosphereof an electricfurnacewill reducelossesof the zirconiumcrucible during fusion operations.Glassy carbon crucibles are suitable for fusion with Na202up to 700°Cbut are rapidly oxidized at highertemperatures.The massloss from a glassy carbon or zirconium crucible at a fusion temperatureof 500 to 600°C is about4 to 8 mg per fusion with Na202 (Sulcek& Povondra,1991). Microwave Digestion Decompositionof soils with acids heatedin a microwaveoven resultsin a rapid, accelerateddissolution procedure.The sourceof microwaveradiation is a magnetron,and its power can be regulatedfrom 1 to 750 W. Microwaves are a form of electromagneticenergy.This form of energyhas alternatingelectric and magneticfields (or waves)which travel at the velocity of light, about 300000 km S-1 (Gilman, 1988).A frequencyof 2450 MHZ (wavelengthof 12.2 cm) hasbeen usedfor heatingof aqueoussolutions.Domesticmicrowaveovensoperateon this frequency and can be utilized directly for dissolutionof solids in aqueoussolution. Microwave energyis below the visible light and infrared regionsof the electromagneticspectrumand above radio waves.If a material absorbsmicrowave energyit will increasein temperature. Aqueous acid solutions canbe heated to the required temperaturein a microwavefield within a few minutes. Microwave heatingis many times more efficient than classicalheatingtechniques.Microwave radiation acts as a source of intenseenergyto rapidly heatthe sample,however,a chemicalreactionis necessaryfor completedissolution in the acid mixture. Microwave heatingis inter-

HOSSNER

54

nal and externalasopposedto conventionalheatingwhich is only external.Better contactbetweenthe particlesand acids is the key to rapid solubility. Local internal heatingtaking place on individual particles can result in their rupture, thus exposingfresh surfaceto acid attack.Thermalconvectivecurrentstend to agitate the mixture and exposefresh surfacesto fresh solution(Nadkarni,1984a;1984b). Pressurizedcontainersenhancechemicaldissolutionby allowing higher samplesolution temperatures.They also decreasethe amountof acid neededand reduce or eliminatethe loss of volatile elements. Aqueoussolutionsof acidsabsorbmicrowaveradiationlessefficiently than water. The efficiency of absorptionof microwave energyby acids decreasesin the following order (Kingston & Jassie,1986)

The amount of energy absorbedincreaseswith incre40% Fe203,1 % Mn02' or both, in the following manner.Placean accurately weighedquantity (0.5 to 1 g) of ignited soil (Lim & Jackson,1982) in a 150-mL beaker.To dissolveMn02, add 10 mL of 2 M HCI and 30 mL of H20 2. Digest first at room temperatureand then on a low temperaturehot plate and evaporatethe contentsto dryness.Add 10 mL of 15 M HN03 and 20 mL of 12 M HCI. Digest the sampleon a hot plate at low heatfor a few hours,and evaporate the contentsto dryness.After cooling the beaker,dissolvethe solublematerials with about 20 mL of 6 M HCI using a stirring rod to aid solution. Transfer the contentsof the beaker quantitatively to a low-ash filter paper. Wash the beakerand paperseveraltimes with 2 M HCI and a few more times with water. Savethe filtrate and washingsfor silica and other elementalanalyses. Placethe filter paperplus residuein a platinumcrucible, oven dry the crucible and contentsat about100°C,and ignite them with a Meker burner.Startthe ignition with a low flame on the side of the crucible, and gradually increasethe temperatureat the bottom of the crucible until it is the color of cherry red. Continueignition until all the C hasbeenburnedaway. Adjust the cover so that the crucible is aboutone-fifth open at the start and aboutone-halfor more open at the end of ignition. After cooling thecrucible, usethe residuefor the Na2C03 or Na202fusion.

TOTAL ELEMENTAL ANALYSIS DISSOLUTION

S9

METHOD FOR FUSION WITH SODIUM CARBONATE

SpecialApparatus 1. Platinumcrucible, 30 mL with cover

Reagents 1. Sodiumcarbonate(Na2C03)'anhydrous 3. Hydrochloric acid (HCI), 12 M, (36%) 2. Hydrochloric acid (HCI), 6 M

Procedure Add 4 to 5 g of anhydrousNa2C03to a 30-mL platinum crucible containing 0.5 to 1.0 g of soil or to the residueproducedin the section on"Procedures" under "Method for Pretreatmentof Soils High in Ferric Oxide and Manganese Dioxide." Thoroughlymix the Na2C03with the soil usinga spatulaor a glassrod with a fire-polishedtip. Place1 g of Na2C03on top of the mixture. Placethe coveredcrucible at a slight angleon a triangle, and warm it over a Meker burner.Do not let the reducingportion of the flame comein contactwith the crucible, and do not let the flame envelopthe crucible.Heat the crucible with a low flame for the first 10 min. Cautiouslylift the cover to let air in through a slit after the initial dangerof spatteringhas passed.Gradually increasethe heat to almosta full flame of the burner. Increasethe Meker burnerflame intensity until the bottom of the crucible is the color of cherry red (about900°C). Maintain this temperaturefor 10 to 20 min. Lift the coverperiodically to provide anoxidizing environment.During the latter stagesof fusion, adjustthe coverso that the crucible top is aboutone-fourth open.Increasethe heatuntil the crucible bottom is the color of bright cherry red (aboutlOOO°C), and continueheatingfor 5 to 10 min. If thereare any patchesof fused materialsticking to the sidesof the crucible, heat theseplacesto causeall the material to run togetherin one mass.At the end of this period, there should be no effervescence,and the molten mass should be quiescent.Remove the cover, and heatthe crucible a few minutesmore to finish the fusion. Removethe flame, and after a secondor so, graspthe crucible with tongs, androtateit in sucha way that the fusion will solidify alongthe sides.This action facilitates removal of the solidified melt. Formation of bubbles or miniature cracksduring the first momentsof cooling indicatesincompletedecomposition. In such an event, remelt the cake, and heat the crucible until it is the color of bright cherry red for another15 min. If the sampleis high in silica, add water to the thoroughly cold crucible until the cake is coveredto a depth of at least 0.5 cm. Heat the crucible cautiouslywith a low flame to loosen the cake. Avoid boiling the water. Transferthe loosenedcake to a Pyrex beaker(or to a Teflon

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60

beaker). Wash the crucible with water to transfer any loose particles to the beaker.Add approximately5 mL of 6 M HCI to the crucible,heatcautiouslywith a low flame or an electrichot plate to disintegratethe remainingcake,and transfer the crucible contentsto the beaker.Use a policemanand a few dropsof 6 M HCI to transferall materialsadheringto the coverinto the beaker.Someanalysts preferthe techniqueof flexing a warm platinum crucible as a way to removethe cakefor analysis. In the caseof an obstinatesample,place the cold crucible on its side in a 250-mL beaker,and add enough water to cover the crucible. Gently heat the beakerto loosenthe cake. Cover the beakerwith a watch glass,and add20 mL of 12 M HCI throughthe lip of the beaker,addinga small portion at a time to prevent any loss through violent effervescence.Use a flattenedstirring rod to help loosenthe cakeanddisintegratethe lumps,taking carenot to scratchthe crucible. Heat the beakerat low heatto hastendisintegrationof the cake. Lift the crucible by meansof a stirring rod, and washoff adheringmaterialsinto the beaker. If the fusion has beenconductedproperly, no hard gritty particlesshould be present,and the liquid, except for gelatinousparticles of silica, should be clear. If the samplewas pretreatedfor removal of Fe and Mn oxides before the Na2C03 fusion, add the filtrate and washings saved from the section "Procedures"under"Method for Pretreatmentof Soils High in Ferric Oxide and ~anganese Dioxide" to this acid solution. Transferthe combineds~lution to a 200-mL volumetric flask and bring to volume with water. Comments The use of a Meker burner is necessaryfor obtaining a temperature between900 and 1200°C.This burneris constructedso that a very hot oxidizing flame is possible. A fusion carried out at the color of bright cherry red heat (about 1000°C) is adequatefor sampleshigh in silicate content,however,with soils low in silica and high in free oxides, the sample should be ignited to 1200°C, as indicatedby an orangecolor of the crucible bottom. A longer ignition period also is requiredfor thesesoils. Soils that are high in Mn02 are digested(see section on "Method for Pretreatmentof Soils High in Ferric Oxide and ManganeseDioxide") beforethe Na2C03fusion or the formation of manganatecan be expectedduring fusion. A bluish-green-coloredfusion cake is an indication of the presenceof manganate. This oxidized form of Mn will attackPt in acid chloride solution becauseof the releaseof free Cl. To prevent damageto the crucible, the manganatemay be reducedby the addition of 1 mL of 95% ethanolto the crucible beforethe addition of HCl. The liquid in the beakercontainssilica in solution as solublesilicic acid or as insoluble particlesand all the other soil constituentsin solution as chlorides. The liquid can be evaporatedto drynessto renderthe silica insoluble and thereby separatedfrom the rest of the elements.The silica becomessomewhatsoluble as basic salts if dehydration takes place above 130°C (Jackson, 1958). Dehydrationof silica after the addition of a few milliliters of HCI04 provides approximatelya lO-fold decreasein the amount of silica dissolvedduring the

TOTAL ELEMENTAL ANALYSIS DISSOLUTION

61

washingand also permitsdehydrationup to 200°C(Smith, 1940;Jackson,1958). The filtrate left after separationof dehydratedsilica contains the other constituents.The filter papercarrying the dehydratedSi02 may be oven dried, ignited in a platinum crucible and weighed.Since evaporationof the acid solution is carriedout at a fairly low temperature,the amountof glassfrom the Pyrexbeaker going into solution is almost negligible, and the Teflon is inert.

METHOD FOR SODIUM PEROXIDE FUSION

SpecialApparatus 1. Zirconium crucible, 30 mL with cover 2. Teflon beaker,400 mL with cover

Reagents 1. Sodiumperoxide(Na202) 2. Hydrochloric acid (HCl), 12 M

Procedure Thoroughly mix a 0.4- to 1.0-g sample,or the residuefrom the sectionon "Procedures"under"Method for Pretreatmentof Soils High in Ferric Oxide and ManganeseDioxide" with a 5- to IS-fold excessof Na202 in a 30- mL zirconium crucible. Cover the mixture with a I-g layer of Na202' Cautiously fuse the mixture using a Meker burner,or to the dark red heatof a Meker burner,until the contentsof the crucible are the color of cherry red and clear. Keep the contents in a molten state for approximately 30 s to insure complete decomposition. Allow the melt to cool, then transfer the crucible to a covered400-mL beaker containingapproximately50 mL of water and 10 mL of concentratedHCl. When the melt hasdissolved, removethe crucible and beakercover, rinsing them thoroughly with water. Cover the beakerand evaporatethe solution to approximately 50 mL. The solution shouldbe kept almostcompletelycoveredduring the initial evaporationto avoid loss by spray. Cool to room temperaturethen transfer the contentsof the beakerto a 200-mL volumetric flask and dilute to volume with water.

Comments Sodiumperoxidefusion and its efficiency can be modified by additionsof alkali carbonates,hydroxides, and borates. Sodium peroxide is often used to decomposerefractory and resistantmaterials such as chromites,zircon, beryl, and fired clays. Admixtures of carbonate,hydroxide, or boron oxidewill suppress the corrosive effect of the peroxide alone on the crucible material and decreasethe fusion temperature.Sodium peroxide alone or with modifying admixtureshasbeenrecommendedfor fusion of resistantberyl, chromiumores,

HOSSNER

62

fired clays,Ti concentrates,iron oresand zirconiumsands.Titanium and chromium oresthat were difficult to decomposeby Na2C03alonewere readily attacked by an eightfold excessof a 3 + 1 Na202-NaKC03mixture (Schinkel, 1984). Chromiteswere decomposedby fusion at the temperatureof liquefaction of the mixture with a 10- to 15-fold excessof the fusion agent(Donaldson,1980). Fusion also can be accomplishedby meansof a muffle furnace. Placethe crucible and its contents,coveredwith a lid, in a cold muffle furnace. The temperatureis slowly increasedto attain 650°C. The liquid is then swirled to permit decompositionof any aggregated particles and the heatingcontinuedfor another 5 min. The contentsare cooled, extractedwith a minimum amountof water and acidified with HCI or H2S04 to completedissolution. Vitreous carboncruciblesavoid contaminatingthe subsequentsolution but are expensiveand have a limited life. Zirconium cruciblesare much less expensive, can be usedfor Na202fusion and have a wide rangeof usefulness.

METHOD FOR DECOMPOSmONWITH MICROWAVE DIGESTION SpecialApparatus 1. Microwave oven with 650 W output and polypropyleneturntable 2. Polypropylenebottle, 250 mL with polypropyl screw-oncap

Reagents 1. Aqua regia. Just before use, mix one part of 15 M (70%) nitric acid (HN03) with threepartsof 12 M (36%) hydrochloricacid (HCI) 2. Hydrofluoric acid (HF), 29 M, (48%) 3. Boric acid (H3B03), 1.5% solution

Procedure Weigh 0.1 g of sample,groundto

Q)

.::i. "-"

>-

2423

0.07%

I

0.05%

t

C)

0::

w Z w

0.18%

I

I

t

1836

I I

1525

18% 82%

\

\

Fig. 6-1. Decayschemefor 42K.

state.The 1525 keY level of 42K also is populatedby small contributionsfrom higher energylevels. The most intensegammaray for the analysisof K is therefore the 1525 keY line. The sensitivity of analysiscalculatedfrom Eq. [8] must thereforebe adjustedupward by a factor of 5.6 (100%/18%)to accountfor the percentageof all decays that produce the gamma ray used analytically. The gammarays are called 42K gammarays though they result from the de-excitation of 42Ca. The energiesand intensitiesof gammarays emitted by radioisotopesproducedby neutronirradiation are compiledin forms convenientfor NAA (Filby et aI., 1970; Lis et aI., 1975a,b;Erdtmann & Soyke, 1975a,b).When selectinga gammaray to be usedanalytically, one must considerthe half-life of the isotope and its abundance,the intensitiesand energiesof the gammarays, and the potential spectral interferenceof gammarays from other radioisotopes.The relative concentrationsof elementsin the sample must be consideredin making these decisions.Radioisotopesand gammarays suitable for NAA of soils and plants are given in Table 6-1. Potentialspectraland nuclearinterferencesalso are presented.The review by Laul (1979) gives a thoroughevaluationand discussionof interferences.

Detectionof GammaRays The radiationsemittedby decayingnuclei can only be measuredwhen the radiationsinteractwith matter.Photons,including gammarays, interactwith matter by three mechanisms:the photoelectriceffect, Comptoneffect, and pair production. The natureof theseinteractionsmust be understoodto properly interpret gamma-rayspectra.The relative probability of eachtype of interactiondepends on the energy of the photonsand the compositionof the absorber,which in the caseof interestis the detector. The photoelectriceffect involves the total absorptionof the energy of the photon by an atom in the detector,which causesthe ejection of a K or L shell electron.The ejectedelectronusually interactswith the other atomsof the detec-

HELMKE

148

Table 6-1. Summaryof radioassayinformation pertainingto neutronactivationanalysisof soils. Interferences Nuclide

Half-life

24Na

15.0 h

38Cl

37.3 min

42K 47ea 49ea 46Sc

12.36h 4.54d 8.7 min 83.8d

51Cr

27.7d

56Mn

155 min

59Fe

44.56 d

6OCO

5.272yr

64CU

12.8 h

65Cu 65Ni

5.1 min 152 min

65Zn

243d

Energies of gamma rays used

Count set

Spectral Isotopeor reaction§§ Half-life

keY

d

1368.4 1731.9 2754.1 1642.7 2167.6 1524.7 1297.0 3084.1 889.3 1120.5 320.1

3 det

[24Mg (n,p) 24Na]:j: [27AI (n,lX) 24Na]:j:

0

[41K(n,lX) 38Cl]:j:

846.7 1811.2 192.2 1099.2 1291.6 1173.2 1332.5 511.0 1345.8 1039.0 366.5 1481.7 1115.5

0

3 10 0 10,40 10,40

152Eu§

12.7 yr

1299.2

182Ta1l 177Lu§ [54Fe (n,lX) 51Cr]:j:

115d 6.74d

1121.2 321.4

46Sc

12.7 yr 72d 83.9d

1112.2 1115.3 1120.5

181Hf§ l3lBa§ 181Hf§

42.5 d 12 d 42.5 d

133.1 133.7 136.3

76As§

26.4 h

562.8

40 10,40 0.3#

[~+

annihiiation]1I

0 0# 10,40

152Eu 1~

72Ga

14.12h

76As 75Se

26.32h 120d

82Br

35.34 h

86Rb 87mSr 99Mo

18.82d 169.8min 66.7h

llOmAgtt 115Cd

255 d 53.5 h

122Sbtt 124Sbtt 1281

64.34h 60.2d 25.0 min

629.9 834.0 559.1 136.0 264.7 554.3 698.3 776.5 1077.2 388.5 181.1 366.4 657.7 492.3 527.9 564.1 1691.1 443.3 526.4

Energy

0# 3 40# 40# 3,10 10,40 0# 3# 40 3# 3,10 40 0#

(continuedon next page)

149

NEUTRON ACTIVATION ANALYSIS Table 6-1. Continued.

Nuclide

Half-life

Energies of gamma rays used keY

134es

2.062yr

l3lBa

12.0 d

140La

40.27 h

Interferences Count set

Spectral Isotopeor reaction§§ Half-life

Energy

d 124Sb§

60.3 d

602.7

10 3,10

23SU fission 47Ca§

4.5 d

489.2

145.4 531.0

40 10

23SU fission IS3Sm§ 23SU fission

46.8 h

531.4

46.8 h

103.2

3,10

lS2mEu lS2Eu

23~p§ 23~p§

9.30h 12.7 yr

0 40

99.7 103.7

l60Jb 17SYb

72.1 d 4.19 d

344.2 121.8 778.9 1408.1 298.6 282.5 396.1

56 h 56h

7SSe§ lS4Eu:j::j: l3lBa:j: 233Pa'll

177.0 208.3 133.1 482.2 1189.0 1221.4 1231.0 77.6 191.4 411.8 311.9 228.1 277.6

40 3,10 40 10,40

120 d 16 yr 12.0 d 27d 56 h 72d 27 d

121.1 123.1 124.2 300.1 285.6 392.5 398.5

131Ba

56h 12.0 d

209.7 133.7

191Pt§ l6OJb§ 182ya

3.0d 72.1 d 115d

409.6 309.6 229.3

604.7 795.8 373.2 496.2 328.8 487.0

141Ce 147Nd

1596.5 32.55 d 10.98d

IS3Sm

l60yb 177Lu 181Hf

32.02d 6.71 d 42.5 d

182Ta

115d

197Hgtt

65.0h

198Autt 64.7 h 233Pa(fh) 6.95 d 13~p(U) 56.3 h

40 10

10,40 10

23~p§

16OJb§ 133Pa§ 23~p'll

40 3#

3,10 10,40 3,10

t de =doubleescape. :j: Not seriousfor most soil samples. § Potentially serious,peaksnot resolvedor marginally resolved,but interfering peak is usually insignificant in most soil samplesif sampleis radioassayed at optimum time. 'II Serious,similar size peaksnot resolvedfor nearly all samples,correctivemeasuresnecessary. # Radiochemicalseparationusually requiredbeforeradioassay. tt Elementpresentat concentrationsbelow detectionlimit for INM in most soil samples. :j::j: Not serious;152Eu/154Euratio is constantfor all samples,andthe half-lives are too long to cause any error. §§ The symbols,n, p, a, and 11+ representa neutron,proton,alphaparticle,and positron,respectively.

tor, with the net result that all of the gamma-rayenergy is depositedinto the detector.The hole left by the ejectedelectronis filled by a higherenergyelectron, accompaniedby the emissionof characteristicx-rays. The photoelectriceffect is the desiredphenomenafor gamma-rayspectroscopy.

HELMKE

150

During the Compton effect, the incident gammaray transferspart of its energyto a bound electron.The gammaray emergeswith a lower energy,while the electronis ejectedfrom the atom. The scatteredgammarays have a continuousenergyspectrum,with their energydependingon the angleof interaction.The Compton effect is important for gammarays having energiesgreaterthan 0.5 MeV. This phenomenonis the dominantcontributorto the generalbackgroundin the low-energyregionsof gamma-rayspectra,especiallyif the samplescontain radioisotopeswith intense,high-energygammarays. Pair productionresultswhen high-energygammarays interactwith nuclei to producean electron-positronpair. The minimum energy of the gamma ray neededfor this phenomenonis 1.02 MeV, which is the energyof the equivalent rest massof the electron-positronpair. Any extraenergyis carriedaway as kinetic energyof the pair. The positroncan interactwith an electronin the detectorand both articles are annihilatedto form two photonsof 511 keVeach.Gamma-ray spectrathereforealmostalwayscontaina peakat 511 keY. Also, single and double escapepeaksfor intensegamma rays with energiesabove 1.02 MeV will occur. The energiesof the escapepeakswill be 511 and 1022 keY lessthan that of the photopeak.The escapepeaksresult when one or both of the 511 keV annihilation gammarays escapefrom the massof the detector. EQUIPMENT

NeutronSources The limited availability of neutron sourcesis the major deterrentto the more widespreaduse of NAA. Researchnuclearreactorsat university and governmentfacilities are most commonly used for irradiations. A completelist of available research and training reactorscan be obtainedfrom the Departmentof Energy Research Reactor Database through the FEDIX system (Internet 192.111.228.33).Forty reactorsin the contiguousUSA are currently listed. The flux of thermal neutronsin reactorscan be as high as 1015 neutrons cm-2 S-1 but valuesnear1013 neutronscm-2 S-1 are more common.Neutrongeneratorsthat utilize the interactionof a radioisotopewith selectedtargetsare available. Their thermal neutronflux is generallytoo low to be useful for the analysis of most naturalmaterials,but they are usedfor someindustrial and in vivo analyses.The costsof neutronirradiation vary, but start at about $150/hr(1992 information). From 20 to 50 samplesand standardscan be irradiatedsimultaneously so the cost per sampleis moderate.

Gamma-RayDetectors A nuclearradiationdetectoris a device that convertsthe energyof nuclear radiationsinto an electricalsignal having an amplitudeproportionalto the energy of the radiation. Successfulanalysisby INAA requiresa detectorwith sufficient efficiency and energyresolutionto measurelow count ratesand to resolve gamma-raypeaksdiffering in energy by only a few kiloelectronvolts. Modem

NEUTRON ACTIVATION ANALYSIS

151

semiconductordetectorshave almost completely replacedscintillation, such as thallium dopedsodium iodide crystals [NaI(Tl)], or gas-filled detectors.This is especiallyso for INAA becausepractical gamma-rayspectroscopyrequiresthe much superiorenergyresolutionof Ge detectors.Scintillation detectorsare still sometimesusedfor radiochemicalNAA becauseof their superiorefficiency. Most modemsemiconductordetectorsfor gamma-rays,known as intrinsic detectors,are madefrom hyperpureGe. They were introducedin 1976 and now dominatethe market. The previousgenerationof semiconductordetectorsintroducedin 1965 requiredthe diffusion of Li into part of the crystal to compensate for the excessp-typeimpuritiesoriginally present.TheseGe(Li) detectorsrequire a continuoussupply of liquid N to cool the cryostat and crystal becauseof the mobility of Li at room temperature.Their propertiesare destroyedif they are without liquid N for evena few minutes.Intrinsic detectorsrequire cooling only when in useto reduceleakagecurrentsgeneratedby mobile carriersat room temperature.There are still many Ge(Li) detectorsin use. Interactionof gamma-rayswith the detectorcreateselectricalcarriers,electrons and positive holes, which are collectedby an applied electrical field. The collectedchargeis proportionalto the energylost by the incident radiation. The averageenergyneededto producean electron-holepair in Ge at 7rK is 2.98 eY, which comparesto about300 eV in Na(Tl). This differenceis responsiblefor the greatly superiorenergyresolutionof Ge detectors. The resolutionof gamma-rayspectrometers varieswith gamma-rayenergy. It is usually reportedfor the 122 keY peakof 57CO, the 662 keY peakof 137CS, and the 1333 keV peakof 6OCo. The resolutionis expressedas the full width-half maximum (FWHM) in kiloelectronvoltsat the specifiedpeaks.Detectorswith a resolutionof 1.60 to 1.90 keY at 1333 keY are preferredbecausethey can fully resolveclosely spacedpeaksof severalimportant elements (e.g., Sc and Zn; and Br, As, and Sb). The efficiency of the detectorincreasesas the size of the detectorincreases, especially for high-energygamma rays. Low-energy gamma rays are efficiently stoppedeven by small detectorsor those madeof a low atomic number material such as Si. Efficiencies rangefrom 10 to 100% relative to a 7.6 by 7.6 cm Na(I) detector.The choice of efficiency is largely dictatedby cost. Detectors with efficienciesfrom 15 to 25% are adequatefor NAA, especiallysince analytically useful gamma-raysoccur at energiesbelow 2800 keY, and most are below 2000 keY. The photopeak/Comptonratio is an indicator of the ability of a detectorto measurelow-intensity, low-energypeaksin the presenceof higher energyradiation. It is usually measuredas the ratio of the photopeakheight of the 1333 keV peak of 6OCO to the height of the highest point of the Compton spectrumjust below the Comptonedge.The peak/Comptonratio increasesas the efficiency and resolutionof the detectorincreases,with commonvaluesranging from 35 to 55 for detectors with15 to 25% efficiencies. The cryostat housesthe detectorcrystal and activatedC to provide cryostatic pumpingto maintain a vacuum.It also containsa preamplifiermatchedto the characteristicsof the detector.Cryostatsare made in either vertical or horizontal configurations.The cryostatis seatedin a dewarfor liquid N, usually with

HELMKE

152

a capacity of 31 L. Appropriate hardwareis neededto securethe detectorand sampleholder within the shielding to reproduciblyposition the samplesrelative to the detector. The detectorsrequireshieldingagainstbackgroundradiationfrom the natural U and Th decayseriesand 40K. Commercialshieldsare available,but very satisfactoryshieldscan be fabricated atlesscost. About 2.0 cm of Pb is adequate for NAA applications.Lead shielding thicker than 12 em increasesthe background becauseof the increasedvolume for cosmic ray interactions.Lining the inside of the lead shield with Cd and Cu sheetswill eliminate the Pb x-rays (75 keV) if they presenta spectralinterference.Plexiglasscan be usedto adsorbthe Cu x-rays if needed.

MultichannelAnalyzers Multichannelanalyzersare neededfor pulseheightanalysisandfor recording the gamma-rayspectrum.Other requiredmodulesinclude an amplifier and a high voltagesourcefor the detector.Electronicmodulesconform to the Nuclear Instrumentation Module(NIM) standardand are generallyinterchangeablewith respectto physicalsize and signal characteristics. Multichannel analyzersare available in single box (hardwired) units or computer-based configurations.A computer-based system requiresa multichannel buffer with an analogto digital converterand an interfaceto the computer. Suchsystemsoften can acceptinput simultaneouslyfrom severaldetectors.The software available with computer-basedsystemsgreatly simplifies data reduction and management.The multichannel analyzershould have a minimum of 2048 channelsper detectorbut 4096 channelsor larger is preferredfor INAA. The cost of a completesystemin 1992without shieldingfor the detector(detector with cryostat,preamplifier anddewar; multichannelanalyzerwith amplifier and high-voltagepower supply) startsat about $25 000. About another$10 000 is neededfor a computer-basedsystem withsoftware.The cost is considerably higher if a large detectoris purchased.

GENERAL METHODS

RadiationSafety The amountsof radioactivity producedduring neutron irradiation of samples are potentially hazardousand proper precautionsand shielding must be used.Gram amountsof sampleirradiatedfor severalhourswill requireseight or more cm of lead shieldingfor severaldaysafter irradiation to limit the doserate to less than 50 mr h-1 at 30 cm from the surfaceof the container.The intensity of radiation decreasesquickly as 56Mn (half-life of 155 min), 24Na (half-life of 15 h), and other short-lived radionuclidesdecay. The usual practice is to not removethe samplesfrom the lead containerusedto transportthe samplesuntil the samplesare radioassayed.Dose ratesmust be monitoredwith a portablesurvey meterand personnelmust wear either film badgesor thermoluminescence

NEUTRON ACTIVATION ANALYSIS

153

dosimeters.The U.S. Nuclear Regulatory Commission(USNRC, 1983) limits personnelexposureto 50 mr d-1 and 100 mr wk- 1• Prudenthandlingof the samples will keeppersonnelexposuresto almostundetectablelevels. The USNRC regulates the use of radioactive material in the USA. Individual researchersusually receiveauthorizationto use and possessradionuclides through their institutional USNRC license. Fume hoods, lead bricks for shielding,and a lead containerfor sampletransportmust be available.The institutional healthphysicistshouldbe consultedto obtain training in radiationsafety and contaminationcontrol and advice about the adequacyof the facilities for radionuclides.

Preparationof Samplesfor NeutronIrradiation Grinding Samples Samplesof soils and rocks for activationanalysisshouldbe groundto pass a 150-~m (l00-mesh)plastic sieve to insure a homogenoussample.This size requirementis sometimesmore severefor activation analysis,especiallywhen samples30 min can be easily and accuratelydeterminedin most soils by INAA (Koons & Helmke, 1978). Additional elementswith isotopeshaving half-lives 0.15 nm of the gas-filled detectors.This makesthe scintillation detectorsmore efficient at radiation wavelengths0.20 nm (cutoff elementsMn or Cr). Therefore,it is not unusualto interchangethe two typesof detectorsdependingon the wavelengthrangeof interestor combinethem, with the flow detectormountedin front of the scintillation detectorthroughan auxiliary collimator. This doesnot improve the resolutionof the tandemsystem,however, sinceit is essentiallythat of the scintillation counter.Scintillation detectors also may produceescapepeak phenomenabut only by very high-energyx-ray photons. Both scintillation and proportionalcountersproducesmall pulsesthat cannot be measureduntil they havebeenamplified. This is achievedby adjustingthe voltage on the detectorto give maximum gain, or amplification and by using a linear amplifier. Best resultsare obtainedwith the lowest possiblevoltageon the detector alongwith a high amplifier gain, i.e., attenuation.Settingsmust be consistentwith maximumamplificationand minimal noisefrom the amplifier. There are severalother practical problemsassociatedwith the use of scintillation and gas-flow proportionalcounters,which are discussedlater.

X·RAY FLUORESCENCESPECTROSCOPY

171

Solid-statedetectorsor semiconductorsdevelopedin the 1960s,like gasfilled detectors,dependon the photoelectricprocessinitiated when an x-ray photon enters the detection system which producespulses proportional to the adsorbedx-ray energy.However, this processtakesplace in a solid insteadof a gaseousmedium and the ionization produces"electron-hole"pairs rather than "electron-ion" pairs. The most widely used solid-statedetectorin x-ray spectroscopyis the lithium-drifted Si detectoror Si(Li). Pure Si is an intrinsic semiconductorwhich has fully occupiedvalence bandsseparatedby energygapsfrom their higherenergyconductionbands.The energyof incident x-ray photonsabsorbedby intrinsic semiconductorsis transferred by photoelectroninteractionsto valenceelectronswhich are excited and jump acrossthe bandgap to the conductionband.The producedconductionband electronsact as negativechargecarrierswhile the electronholesgeneratedin the valence band act as positive charge carriers. By applying a suitable voltage acrosssuch a semiconductor,the ion pair chargescan be sweptout and collected by an electrodeset (anodeand cathode).Since the numberof electron-hole pairs producedare proportionalto the energyof the incident x-ray photons,the magnitudeof the chargescollectedalso areproportionalto the x-ray photonenergy andthus,the semiconductoris inherentlya true proportionaldetector.To compensatefor B impurities in Si that reduceits efficiency as an intrinsic semiconductor, a certainamountof Li is diffused throughthe Si that producesa detector with high inherentsensitivity. The pulsescollectedat the electrodeare amplified and passedto the multichannelanalyzer,which sortsthe pulseselectronicallyaccordingto their amplitudesinto different channelsand countsthe numberof pulsescollectedin each channel.Becauseof their superiorenergy resolutionover other counterssolidstate detectorsoffer practicaladvantagesfor numerousanalytical applications. They do not require a dispersingcrystal systemor the goniometerto rotate at specified Bragg diffraction anglesand thereforerequire much less space;they can sample the complete spectrum of emitted x-ray energiessimultaneously avoiding the time consumingnecessityfor sequentialscansat different energy (wavelength)settings; they can be placed much closer to the emitting x-ray source(sample)without requiring intensity-reducingcollimator, and therefore, they providesubstantialefficiency increasein x-ray energyconversion.The most outstandingattribute of thesedetectorswhen used with energy dispersivesystems is the speedof multielementanalyses,which often can be completedwith adequateprecisionin secondsrather than minutesor even hours requiredby the wavelengthdispersivesystems. On the other hand, solid-statedetectorshave some disadvantageswhich should be consideredin order to ensuresatisfactoryoperation.They mustbe maintainedconstantlyat low temperature(usually immersedin a dewarof liquid N2) which requiresregularrefilling to preventexcessiveLi diffusion and serious degradationof their performance.The detectorcan occasionallybe brought to room temperaturefor a short period provided the voltage is switched off. The surfaceof the detectoris very sensitiveto contaminationso that the housingis kept permanentlyundervacuumwhich requiresthicker windows that reducethe

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KARATHANASIS & HAJEK

bestpossibletransmissionand therefore,detectionand resolutionof low energy x-rays.

Pulse-HeightSelection All the detectors(gas-filled, scintillation, solid-state)are proportional in the sensethat they producepulseswith amplitudeproportionalto the energyof the incidentx-rays. After amplification,thesepulsesare directedto electricalcircuits which are used to distinguishbetweenpulsesof different amplitude,and therefore,x-rays with different energies(wavelengths).Thesecircuits are called pulse-heightanalyzersor pulse-heightselectors. The rangeof pulse heightsthat passesthe selectoris called a window, or channel,and the lower thresholdis termedthe baseline,or lower level. Both window and baselinemay be adjusted,and the signal that passescan be fed to a counting device or to a rate meter. Pulse-heightselectioncan thus improve the peak/backgroundratio by rejecting the harmonics, much of the instrumental background"noise," and often but not always,the nearbyand overlappingpeaks due to otherelements,including an escapepeak.It is particularlynecessarywhen the scintillation counter is used for detecting relatively soft radiation (A. = 0.15-0.40nm) and when the gas-flow counteris usedfor detectionof radiation from the light elements(A. > 0.45 nm). Theremay be a problemof shift in pulse height with the latter. This may be due to: (i) drift in the voltage applied to the detector;(ii) somevariation in the density of the gascausedby variationsin the temperatureor pressureof the gas; or (iii) the incident intensity being so great that photonsare entering the detector beforethe precedingphotonshave been resolved,i.e., within the deadtime of the counter. With an energy-dispersivespectrograph,a multichannelanalyzeris used with at least 1024 channels.This receivesthe amplified output from the silicon (lithium) detectorand sortsthe pulsesby energyinto the channels.When a sufficiently large count (-UP) has beenaccumulatedin the channelcorresponding to the highestintensity,the countsin eachchannelare transmittedto the recorder, oscilloscope,or digital printer so that the intensity over the entire spectrummay be displayedsimultaneously.Correctionsmay be appliedelectronicallyfor dead time, background,"pulse pileup," and spectralline interferenceby the computer associatedwith the spectrograph.Modem computershave sufficient capacityto make appropriatecorrectionsfor absorptionand enhancementby the sample matrix. To monitor the rate at which pulsesare collectedby the detectorsystema ratemeteror a scaler-timersystemis used.The ratemeteris essentiallyan integrating (averaging)device with analoguevoltage output that varies proportionally with the count rate. The finite time interval over which the averagingof count rate occursis the time constantof the systemwhich typically rangesfrom 0.1 to 10 s. Ratemeteroutputsare usually directedto a chart-recorderwhich is convenientfor qualitative or quantitativeanalysis.On a scaler-timersystemthe amplified detectorpulsesare displayedon a digital device(scaler)and electronically countedby a timer over finite time intervals. The analystcan then select

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the counting mode by either fixing the time constant(presenttime mode) and measurethe counts accumulated,or by fixing the total number of countsto be accumulated(presetcount mode)and measurethe time interval it takesto collect them.

Counting Efficiency The overall efficiency of the counterelectronicssystemin detectingincident x-ray photonsis the productof absorptionefficiency and detectionefficiency. All countershave a thin window (mica, Be, Mylar) throughwhich the x-rays passbeforethey reachand are absorbedby the counterchamber.The absorption efficiency, therefore,can be expressedas

[9] whereIw = fraction of x-rays absorbed by the window, andIe = fraction of x-rays absorbedby the counter.The detectionefficiency (Eder) on the other hand is (1 11), where11 fractional countinglossesoccurreddue to unresolvedpulses(especially at high counting rates and long-time constants).Therefore, the overall countingefficiency (E) is given by the equation [10] Since Edet for most countersis nearly 100%, E mainly dependson Eabswhich is usually low for very short wavelengths(high energy)that passthrough the window and the counter without being absorbed,and high for long wavelengths becauseof the increasedabsorptionby the window. Thus, the bestcountingefficiency is attainedwhen fw is minimum and fe is maximum. For example,the counting efficiency of a scintillation counter for wavelengths0.02 to 0.20 nm usually exceeds90%. For longer wavelengths,however, and with a beryllium window ofO.127-nmthicknessabout 25% of the Ca (0.336 nm)and nearly 95% of the Si (0.713 nm) radiation is lost. On the other hand, the counting efficiency of a gas-flow proportionalcounterusing a P-lO (90% Ar-lO% methane)mixture and a Mylar window 0.00625mm thick, is nearly 100% for CaKa and KKa radiations, but drops to 60% for Fe (0.194 nm) and 30% for AI (0.834 nm).

INSTRUMENTATION There are two main types of XRFS's. The wavelength-dispersivespectrometers(Fig. 7-1a), which utilize the diffracting propertiesof a single crystal to dispersethe wavelengthsof the x-ray spectrumemitted from the sample,and the energy-dispersivespectrometers (Fig. 7-1b) that use electronicsorting of the pulsesof different amplitudeproducedby different energyx-ray photons.In the latter, a multichannelpulse height analyzermeasuresthe pulse height distribution and subsequentlydisplaysthe spectrum.

KARATHANASIS & HAJEK

174

x-ray tube spectrometer circle

\J _-

v \

secondary x-rays

AI and A2 ' \

::i(

sample

collimator

,

detector

, A2

......---~~\ \_ ) detector (a) Wavelength-dispersivespectrometer

EJ

A, counts- - - -

MeA ____

nf-- x-__

A2coWlts-- - - U

secondary ray-s--...y,'Y

Aland A2

Si (Li) detector

(b) Energy-dispersivespectrometer Fig. 7-1. X-ray fluorescencespectrometers-inthis example,elements1 and 2 in the sampleemit characteristicwavelengthsA.I and A.2' Thesewavelengthsare separatelymeasuredby crystal diffraction in (a) or by pulse-heightanalysisin (b) (whereMeA =multichannelanalyzer).

Wavelength-Dispersive Spectrometers Thesespectrometersmay be built either as single-channelsequentialor multichannel simultaneousspectrometers.Single-channelunits have only one counterwhich is mechanicallycoupledwith the analyzingcrystal. Thus, when the crystal is set at a particularBragg angle8, the counteris automaticallyset at the correspondingangle 28. The various spectrallines of different elementsare measuredsequentiallyby moving the counter (goniometer)from line to line either manuallyor by a mechanicaldrive. In automaticsingle-channelspectrometers,the angularpositions28" 282, ..., at which lines A." A.2, ... for the analyzed elementswill be reflectedare presetor programmed.The counter movesrapidly from position to position, and accumulatesenoughcounts for each elementto makean accurateintensity measurement. Thesemeasurements are recordedon a printer or sent to a computerfor conversionto concentrationsof the elements involved. Multichannel simultaneousspectrometersare most efficient where a predeterminedsuite of elementsis to be measuredrepetitively over a long period of time. They usually haveas many channels(crystalsand counters)as there

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are spectrallines (elements)to be measured(as many as 30). For eachchannela crystal and counterare fixed at the correct angularposition to measurea selected spectralline. All the analyzingchannelsreceive the samefluorescentradiation from the sample,while one nondispersivecontrol channelreceivesfluorescent radiation directly from a standardand servesto monitor the output of the xray tube. Since the data are acquiredsimultaneouslya large numberof samples can be analyzedin a short time. The wavelengthdispersivespectrometers(WDS) could utilize either a flat crystal or a curved (focusing) crystal for x-ray dispersion.The flat crystal is mountedon the central arm of a rotating goniometerso that the Bragg angle 8 can be varied by simple rotation of the crystal mount (Fig. 7-2a). Beam divergenceis limited by primary and secondarycollimators consistingof a seriesof parallel metal plateswhich are positionedin front of the x-ray sourcebetween the sampleand/or the crystal detector.X-rays parallel to the platespassthrough the collimator while those diverging significantly are absorbedby the plates. This reduceddivergenceimprovesthe resolutionof the spectrometerbut source x-ray intensity lossesin incident and dispersedradiation are unavoidable.The closer the spacingbetweenthe collimator platesthe better the resolutionbut at the expenseof greaterintensity losses.Therefore,a variety of interchangeable coarse-and fine-spacedcollimators are employedto achievethe best analytical conditiol).s. Fluorescentx-rays incident on the flat crystal are diffracted into the counterby lattice planesparallel tothe crystal surface.Sinceno focusingoccurs, the beamdispersedby the crystal is relatively broad, which necessitatesa relatively wide counterreceiving slit. In the curved crystal spectrometer(Fig. 7-2b) fluorescentx-rays incident on the crystal are focusedand diffracted on a slit (in place of the collimator) in front of the detector. The analyzing crystal has its reflecting planesbent to a radius 2R equal to the diameterof the focusing (Rowland) circle and its surface curved to a radius R equal to the radius of the Rowland circle. The essential requirementsof this geometricconfigurationare for the x-ray source,the surface of the dispersingcrystal, and the entranceslit to the detectorto all lie on the circumferenceof the circle so that D = 2R sin 8

[11]

where8 = angle of incident tothe crystal x-ray beamand D = distancefrom the emitting slit (ES) or the receiving slit (RS) to the crystal. This distanceis kept equalby rotating both crystal and counterin sucha way that the counterrotation angle is always twice as large as the crystal rotation angle. A variety of interchangeableslits is usedto accommodatethe proper resolution-intensityrequirements. In both flat and curve crystal configurationsthe angle 28 at which a particular wavelengthis reflected dependsonly on the d-spacingof the analyzing crystal. Since the longest wavelengthsthat can be diffracted are equal to 2d, small d-spacingcrystals are usedfor short wavelengths(elementswith high Z) and large d-spacingcrystals are used for long wavelengths(low Z elements). Someof the most commonlyusedanalyzingcrystalsare listed in Table 7-2.

KARATHANASIS & HAJEK

176

sample

(a) Flat-crystalgeometry

o sample R

I

I

I I

o

I

\

,

protective \ cover

I

\

.

\

\

,

focusmg I circle I I

"

... _--- ..... -"'" "

,,

I

I

~

detector

(b) Curvedcrystal geometry Fig. 7-2. WavelengthdispersiveXRFs with flat-crystal (a) and curved-crystal(b) geometry(ES = emitting slit and RS =receivingslit).

Energy DispersiveSpectrometers Thesespectrometershave becomewidely used since the late 1960s following advancesin electronicand digital processingtechnologythat have made them convenientin operationand maintenanceand practicalfor routine spectroscopicdeterminations.The essentialpartsof thesespectrometersare: (i) a solidstatedetector,usually a silicon(lithium) counterfitted with a field effect transis-

177

X-RAY FLUORESCENCESPECTROSCOPY

Table 7-2. Characteristicsof commonly useddispersioncrystals(Williams, 1987). LowestZ detectable (max e= 70°) Crystal Topaz Lithium fluoride Lithium fluoride Germanium Quartz Quartz Pentaerythritol Ethylenediaine d-tartrate Ammonium hydrogen phosphate Mica (muscovite) Potassiumacid phthalate Rubidium acid phthalate Thallium acid phthalate Lead steratedecanoate Lead melissate

Abbreviation LiF LiF Ge PET EDDT ADP KAP RAP, RbAP TAP, TIAP PbSD

(hkl)

2d(A)

(303) (220) (200) (111) (1011) (1010) (002) (020) (101)

2.712 2.848 4.028 6.532 6.686 8.50 8.742 8.804 10.648

(002) (1010) (1010) (1010)

19.8 26.6 26.1 25.75 100 160

K

L

Efficiency

23(V) 23(V) 19(K) 16(S) 15(P) 14(Si) 14(Si) 14(Si) 12(Mg)

59(Pr) 57(La) 49(In) 40(Zr) 40(Zr) 37(Rb) 37(Rb) 37(Rb) 33(As)

Average High Intense Average High Average High Average Low

9(F) 8(0) 8(0) 8(0) 5(B) 4(Be)

26(Fe) 23(V) 23(V) 23(V) 20(Ca) 20(Ca)

Low Average Good High Average Average

tor (FET) preamplifierboth cooled by liquid Nz, and (ii) a multichannelanalyzer (MeA). They are mechanicallysimple systemsbecausethey do not needthe analyzing crystal required by the wavelengthspectrometers,but electronically complexbecauseof the presenceof the MeA. The silicon(lithium) detectorcollects the chargesproducedfrom the interaction of the emittedx-ray photonswith the drifted Si and the preamplifierintegratesthem and converts them to low voltage pulseswith amplitudesproportional to the energyof the emittedx -ray photons.Thesepulsesare amplified, filtered for noise and convertedto numerical values (digitized) using a seriesof amplifiers, single channelanalyzersand analog-digitalconvertersbefore being directedto the MeA. The MeA measuresand sorts the digitized pulsesinto a seriesof computer memory registersor channels.Each channel representsa particular energy interval of the x-ray spectrumin which correspondingpulsesare collected and counted.Typical MeA's have1024channelsof lO-eV incrementsspanningx-ray spectrain the rangeof 0 to 10.24keV (0.12 nm), which encompassalmostall of the characteristicx-ray lines routinely used in XRF analysis.A timer, which is usually a "live time" clock, controlsthe time intervalsover which countsare collected and providesautomaticdead-timecorrectionby stoppingthe clock when the system is temporarily insensitive to processfurther pulses. The processed data can be output in a visual display or a printer as countsper channelor as a graphicimageof the completespectrum. The utility of thesespectrometersis basedon the excellentenergyresolution of the Si(Li) detector, which is much better than any other proportional detectorand the ability of the MeA to perform rapid pulse-heightanalysismuch fasterthan a single channelwavelength spectrometer. The MeA can measurethe intensitiesof all spectrallines emitted from the samplesimultaneouslyand in a very short time (usually 1-2 min), unlesselementsof very low concentrationsare to be determined.

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Advantages/Disadvantages of Wavelength-Dispersive Spectrometers and Energy-DispersiveSpectrometers One of the major advantagesof energy-dispersivespectrometers(EDS) spectrometersis their speedand conveniencewith which they can be used to completecomplexmultielementanalysisevenby the most inexperiencedoperators. The absenceof a dispersingcrystal eliminatesproblems associatedwith optical geometricrequirementsand crystal contaminationas well as errors arising from mechanicalwear and tear of the resettingcomponentsof the wavelength-dispersivespectrometers (WDS), which ultimately lead to poor analytical precision. In addition, since countsfor all elementsare accumulatedduring the samemeasurement cycle, they can be obtainedfrom the samespot or areaof the sample.The sequentialdeterminationof successiveelementsby WDS requires extensivebombardmentof the target sampleareawhich may damageor create diffusion gradientsin thermally sensitivesamples,thus causinga serioussource of errorsin the intensity measurement.Alternatively, changingthe targetsample areamay causesimilar errorsin nonhomogeneous samples, and using low-beam currentsmay lead to unacceptablylow x-ray intensities. The intensity of the fluorescentx-rays emittedfrom the sampleis generally reducedin WDS before it reachesthe counterdue to the presenceof the collimator(s)and the inefficient diffraction ofthe analy'zingcrystal.Theselossesare eliminated in EDS so that the radiation entering the counter is quite intense. Alternatively, low-power x-ray tubesor low-intensity radioactivesourcescan be usedto excite the sample.This low-power equipmenthas provided the impetus behindthe developmentof miniature tubesand various types of simple portable of limited capability but useful in ore researchand environmental spectrometers investigations. The resolution of EDS is better for short wavelengthsand high-energy radiationwhile that of WDS is more efficient for long wavelengths(Ka lines of elementslighter than Kr with Z = 36) and low-energyx-rays. This often yields much betterimagesand more detailedresolutionat low beamcurrentswith EDS and completelyinadequateresolutionwith WDS. On the other hand, there are somepractical disadvantages associatedwith EDS. Although the energy resolution of the silicon(lithium) detector is about three times betterthan the gas-filled proportionalcounteralone, the presenceof the dispersingcrystal and the focusing of the dispersedx-ray beamprovidesan overall better resolution in WDS. This means that overlapping interferences betweenadjacentspectralemissionsare more pronouncedand more difficult to correctwith EDS. Furthermore,WDS generallyhave betterpeakto backgroundratios, especially for low concentrationsand for light elementsbeyondthe effective working rangeof the EDS detector.Becauseincreasedelectronicnoisecontributesto higher backgroundsin EDS, their ability to detect and measurelow concentrations especially of light elements with low-energy spectra is significantly reduced. Problems also exist in the electronic processingof the silicon(lithium) detectorsignals.The proportionalityrequirementbetweenx-ray photonenergies

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179

and pulse amplitudesis disturbedfor count rates exceeding15 000 counts per second(cps), thus causingpoor performanceat evenmoderatelyhigt total count rates.This disadvantagebecomesmore seriousconsideringthat wb ile the total countof a WDS reflects x-raysof a single wavelength,the total count of an EDS correspondsto pulsesof all sizescollectedat the sametime. For example,if the radiation emitted from a sampleconsistingof two elementsA and B produces count rates of 150 000 and 1 000 cps, respectively,their intensity is measured incorrectly becausethe total count rate exceedsthe linear rangeof the counterelectronicsystem.In order to correct the problem, a reductionof power in the excitationsourceby a factor of 10 will reducethe Ka count rate of the B element to only 100 cps, which in retrospectmay not be adequateto resolve it from the background.Furthermore,becauseof the precisionrequiredin pulseshaping,filtering, and discrimination,the processis relatively slow, requiring deadtimes in the rangeof 10 to 20 f.1S which are about 10 times greaterthan thoseof WDS. At high-count rates even the "live clock" device is inadequateto. compensatefor pulse pile-up effects, which may result in the production of spuriouspulsesor enhancement of backgroundintensities. The needfor constant coolingof the detector-preamplifiersystemwith liquid N2 is somewhatinconvenientand the detectormay be seriously damagedif the dewar is accidentallyallowed to run out of liquid N2 while the EDS unit is on.

EquipmentSelectionand Safety In spite of the readily available descriptionsof XRF spectrometersand their componentsin the literature, often it is difficult for a prospectivebuyer to decideon the relative merits of commerciallyavailableequipmentand the types that are best suited to the analyst'sneeds.Obviously the determinantfactor in this caseis the type of applicationand the objectiveof the laboratory.For example, a process controltype, which specializesin the laboratoryanalysisof only a few elementson a relatively similar matrix, would require a relatively simple spectrometertunedto the elementsof interest.In contrast,a generalpurposeanalytical laboratory which analyzesmany elementsin dissimilar matrices may require a more sophisticatedspectrometer.Another important factor in equipment selectionis the sampleload, which may rangefrom a few samplesper week to a few hundredsamplesper day. Generally, the higher sample load makesa high-speedXRFS desirable.This requirement,however,dependson the sample type and the required preparation.For ready-to-runsamplesor those requiring minimum preparation,a high-speed,simple-to-operateXRFS may be ideal. For samplesrequiring elaboratepreparationsand computations,the advantageof the high speed has essentially no effect on the total analytical time involved. Variations in the type of XRF equipmentand sample preparationalso require operatorswith different training. While a processor quality control spectrometer can havean operatorwith generalknowledgein lab proceduresand limited x-ray training, a versatilespectrometerin a generalpurposelaboratoryrequiresa technician highly trained in practical x-ray spectroscopy.

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The choiceof an XRFS alsowill dependon spaceandotherfacilities available (samplepreparation,storage,electric power, cooling capacity,ventilation, computerinterfacing capabilities, etc.). Most XRFS's use either a radioactive sourceor an x-ray tube for excitation. A radioactivesourcedoes not require a power supply, and thereforeit is smallerin size and more versatile.However, it requiresadditional safety measuresbecauseit cannot be turned off. Also, efficient excitation of only a small range of spectrallines representinga few elementscan be accomplishedwith the properchoiceof radioisotopes.X-ray tubes also are availablein a wide rangeof voltage ratings,with the high voltage rating tubesbeing usedwith WDS and the low voltagetubeswith EDS. The high-voltage tubesrequire a longer and generally more complex power supply and cooling systemthat increasesthe size and the cost of the spectrometer. Sincethe x-ray sourceis most efficient in exciting elementswith Z slightly less than that of the target, severalmethodsare usedto modify the radiation emittedby the sourceincluding different voltage settings,using other targets,or utilizing variousfilters. Provisionsfor suchmodificationsshouldbe built into the instrument. Improvised modifications often are not effective and may create safetyhazards. As discussedearlier, WDS provide better resolutionfor low atomic number elements.To overcomethis limitation EDS are frequently equippedwith minicomputersthat are able to provide deconvolutionand peak-strippingcl!-pabilities for resolvingoverlappingpeaks.The degreeof effectivenessof the methods dependson the concentrationof the elementsmeasuredand the particular algorithm used. Since radiation of only a particular wavelength(only one element) is measuredby anyonesettingof the crystal in WDS, while the entire xray spectrumis countedat onceby EDS, processingthe pulses(deadtime) takes much longer in the latter system.Therefore,for accuratemeasurements of small concentrationsEDS will require very long times and since most of the counts will be coming from the matrix they will provide a very low countingefficiency. However, for large amountsof severalelementspresentin a sampleEDS can measureall of themsimultaneouslyin a relatively short time.MultichannelWDS also could measureseveralelementsat a time, but they are generally presetso that it is difficult to changethe elementsmeasured. The maintenancere.]uirementswill vary dependingon the type of spectrometerusedbecausethe WDS are mostly basedon mechanicalsystemswhile the EDS on electronicsystems.Someof the relatively simple maintenancecan be doneby the laboratorypersonnel,but other maintenancetasksare more complex and require trained specialistsprovidedby the vendor. Several systemsof recording data are available from the simplest ratemeter-stripchart recordercombinationto the microcomputercontrolled signal output servicesusing teleprintersor screendisplays. The microcomputersalso are usedfor datamanipulation(storage,corrections)as well as calculationof the final concentrations.Since there is no generalconsensuson the best procedure for correcting interelementinterferences,severalsoftware programsare available for this purpose,eachone with ratherspecificapplications.Therefore,compatibility of the programwith the spectrometerand computertype and the range of the elementsof interestis necessary.Becausemost of the spectrometerman-

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ufacturersdo not make the computer hardware and softwareusedwith the spectrometer, compatibility problemsfor certain applicationsmay arise even using computerperipheralsprovidedby the manufacturer. Besidesdata manipulations,anotheradvantageof the computer-controlled systemis that it can operateunattended,thus allowing that time to be usedfor other tasks,such as samplepreparationand record keeping.The availability of the automaticsamplechangerthat is able to handlemany samplesat a time contributes to big time savingsespeciallyin laboratorieswhere large numbersof similar samplesare processed. Although it is well known that radiationcould causebumsand physiological changesin the humanbody, it is humannatureto be at times carelessin the presenceof danger.Available statisticsshow that thereis at leastone accidentper 200 x-ray installations.Unfortunately,many of theseaccidentsoccur to people with considerableexperience.Since the effects of radiation exposureare not observeduntil considerabletime haspassed,it is imperativethat peopleusing xray equipmenthavethoroughinstructionson x-ray safety.The operatorof x-ray apparatusis generally exposedto two dangers,electric shock and radiation injury. Both of thesehazardscan be reducedto negligible proportionsby proper equipmentdesign and reasonablecare on the part of the operator.All modem instrumentshave factory installed safety interlocks, which prevent accidental exposureto x-rays. Therefore,it is importantthat theseinterlocksare maintained functional by the x-ray user. X-ray fluorescencespectrometersrequire severalhundredvolts for their detector power supplies and several thousandvolts for the x-ray generator. Therefore,specialprecautionsmust be usedto preventelectricalaccidents.Most of the currently availablex-ray spectrometersare shockproofedand haveseveral electrical interlocks to prevent radiation leakage.However, they should be inspectedregularly to be surethat they are in goodworking order. Portablecounters (radiation survey meters)are available for surveyingvarious areasaround the equipment forpossibleradiationleaks. Film badgesshouldbe worn regularly by people who spend significant amounts of time near x-ray equipment. Governmentregulationsregardingradiation safety are very strict. More information aboutx-ray safetyregulationscan be found in specialgovernmentpublicationswhile a useful summaryabout radiation units, survey meters,tolerance levels and reportedaccidentshas beenpublishedby Jenkinsand Haas(1973).

ANALYTICAL CAPABILITIES QualitativeAnalysis Wavelength-and energydispersivespectrometersare both well suitedfor qualitativeanalysisof elementswith Z ~ 9 within their detectableranges.A vacuum path is necessaryfor detectionof elementswith Z : 80), possiblespectralinterferencesand overlapsshould be considered,especially in trace-elementanalysis. For example, the strongest

186

KARATHANASIS & HAJEK

BaLa line (0.2776nm) is normally avoidedin traceelementanalysisbecauseof proximity to the TIKa line (0.2750nm), which is much strongerin soil and geological samples.Instead the Ba42 (0.2404 nm) is used, which is efficiently excitedby a chromium x-raytube. In WDS the selectionof the proper analyzingcrystal, primary collimator spacing,and Bragganglesettingdependson the elementsto be analyzed.A precise spectrometercalibration using reliable standardsis accomplishedby slow scanningacrossthe peakof a particularelementto locate its maximum intensity, or by scanningacrossthe peakat a uniform rate and taking.readingsat 80% of the maximumintensityat eithersideof the peak.The peakmaximumcentroid can then be takenas the position halfway betweenthe two readings.In computer-controlledWDS spectrometersthe position of the peakmaxima is computed automaticallyfrom a seriesof intensity measurementsacrossthe approximate position of a characteristicspectral line, which may last only a few seconds. Crystal selection becomessomewhatcomplicatedonly in overlapping ranges (Table 7-2), where the better resolutionof the high angle rangesmay be compromisedby lower dispersingefficiency or longeranalyticaltime. Coarsercollimatorsare normally usedfor major elementsbecauseof higher peakintensities but for traceelementanalysiswhere,betterresolutionis critical fme collimators ( narrow slits) are preferred. If there is a choice betweendetectors,a gas flow proportional counter shouldbe usedfor wavelengths>0.14 nm, while a scintillation counteris more suitablefor wavelengths

V

v=Q

IncidentVisible (laser)Source

----~C>

Detector

Sample

0

I

Sample

Inelastically Scattered Light

Detector

hvi

hvo

Laser Excitation Line

Fig. 10-3. Schematicdiagramof IR absorptionand Ramanscatteringprocesses .

lating electromagneticfield with a moleculewill push the positive chargesone way and the negativechargesin the oppositedirection. If theseatomic displacementscreateor induce a changein dipole momentin the samplethen the molecule will absorb infrared energy and the correspondingvibrational mode is termedIR-active. A schematicillustration of the IR absorptionprocessis shown in Fig. 10--3. Raman Scattering

Vibrational modes also can be studied using a fundamentally different processknown as Ramanscattering.When photonsfrom a visible, monochromatic light source(e.g., an argon-ionlaser) interact with a sample,most of the scatteredphotonsare not perturbed;i.e., they have the sameenergyas the incident photons.However, a small portion of the incident photons,roughly on the

JOHNSTON& AOCHI

274

order of 1 out of every 108, are inelastically scattered by the sample. ConventionalRamanspectroscopyis basedon the processshown on the right side of Fig. 10--3 which is termedStokesscattering.A small amountof energyis transferredfrom the incident photon to the sampleresulting from a vibrational transition from the ground stateto an excited level. The energyof this inelastically scatteredphotonwill be lessthan that of the incident photons,with the differencein energiescorrespondingto the energyof the vibrational transition. A small portion of the light scatteredby the samplewill be shifted to lower frequenciesby discrete amounts correspondingto the vibrational energy levels encountered.The position of a band in a Ramanspectrumis always givenin wavenumbersand is measuredas the differencein energybetweenthe frequency of the laser (in cm-I ) and that of the inelastically scatteredband. Several excellenttreatisescover both IR absorptionand Ramanspectroscopy,including Wilson et aI., 1955; Long, 1977; Turrell, 1972; and Heffelfinger, 1971. In order for a vibrational mode to be Raman-active(Le., observedin the Ramanspectrum),the vibrational mode must producea changein the induced molecular polarizability of the molecule.The conceptof molecularpolarizability can be visualizedby picturing an electroncloud aroundthe vibrating atomsof interest;changesin polarizability relate to how easily this electroncloud can be deformedby the pulsating electromagneticradiation. Recall that the selection rules for IR spectroscopyrequire a changein the induceddipole moment. One practicalaspectof this differencein selectionrules betweenRamanand IR spectroscopy involves applicationsin aqueousmedia. Raman-activebandsof water are comparativelyweakerthan their IR counterparts.Thus, the presenceof water can be toleratedwith fewer problemsin Ramanspectroscopythan in IR spectroscopy.Unfortunately,the sensitivity of Ramanmethodsis generallylessthan that of FIlR spectroscopy,so this advantagemay be of little benefit.

Vibrational Analysis The typeof information that canbe obtainedfrom vibrationalspectroscopy can be subdividedinto two broad categories.The first is directed towards the identification of soil constituentsand the secondat characterizingthe chemical and physicalpropertiesof soil materialsand their respectiveinterfacial processes. The simplestapproachis to examinethe vibrational spectrumof a soil material for the presenceof diagnosticbandsthat indicatethe occurrenceof a particular component.This applicationis analogousto the use of powderx-ray powder diffraction methodsto identify the presenceor absenceof a particularcrystalline solid phase.The observedband positionsfor a soil material can then be comparedto the spectraof known referencematerialsor to tabulatedfrequencies from literature references.Advantagesof this approachare that it does not requireany detailedunderstandingof spectroscopy,is amenableto routine analysis by a nonspectroscopist, and can be donequickly. This modeof spectralinterpretationof vibrational spectracan be aided by the useof groupfrequencies.The characteristicvibrational frequenciesassociated with certain submoleculargroups of atoms are termed group frequencies. Thesediagnosticbandscan be usedto suggestthe presenceof a particularcom-

FI1R & RAMAN SPECTROSCOPY

275

ponent or functional group. Examplesof the submoleculargroups commonly identified in soil-relatedapplicationsare listed in Table 10-1. Infrared analysis of humic substances, for example,relies heavily on group frequenciesto indicate the type and distribution of functional groups present.Invariably, a vibrational band can be assignedto several possible functional groups. Thus, additional experimentalcriteria must be used to confirm the assignment.In the case of humic substances,correlationof the vibrational data with 13e nuclearmagnetic resonance(NMR) data and elemental analysis can provide more definitive assignmentsfor the vibrational bands(Johnstonet aI., 1994). One further difficulty with relying upon group frequenciesaloneis that the bandsassociated with a particularfunctional group can shiftas a function of pH, metal binding, or ionic strength. Vibrational spectraalso can be used to provide insight about molecular structure or interfacial processes.This application requires a more detailed understandingof vibrational spectroscopy.The most commontypesof diagnostic propertiesused in these studies include shifts in position of a vibrational mode,changesin the bandwidth or line shape,or the appearanceof new bands. Sincethe spectralinformation of interestfor theseapplicationsis typically manifested by a change in the vibrational spectrum,spectralanalysistools are frequently required for theseapplications.Thesetechniquesinclude spectralsubtraction, bas~line correction, band fitting, and deconvolutionmethods,and are discussedin the section"SpectralAnalysis." Table 10--1. Group frequenciesof soil constituents. Components Organic O-H stretchingof carboxylic acids, phenols,alcohols N-H stretchingof amines,amides Aromatic C-H stretching Aliphatic C-H stretching C=O stretchingof carboxylic acids,amides,ketones salts of carboxylic acids asymmetricCOO- stretching symmetricCOO- stretching C-H bendingof -CHz- and -CH3 C-O stretching,O-H bendingof -COOH C-O stretchingof polysaccharides Inorganic Clay mineralsand oxides O-H stretchingof structuralOH O-H bendingof structuralOH Si-O-Si stretching Sorbedwater O-H stretching O-H bending Carbonates Phosphates Sulfates

3500-3200 3400-3200 3150--3000 2970--2820 1750--1630 1650--1540 1450--1360 1465-1440 1250--1200 1170--950

3750--3300 950---ll20 1200-970 3600-3300 1650--1620 1600-1300 900--670 1300-850 600-550 1200-1100 680-600

JOHNSTON& AOCHI

276

Fixed M irror

Fig. 10-4. Schematicdiagramof a FTIR showingthe source,beamsplitter,moving mirror, fixed mirror, samplecompartment,and detector.

FOURIERTRANSFORM INFRARED SPECTROMETERS

Principlesof Operation Fourier transform infrared spectrometersare superior to dispersive IR spectrometersfor virtually all applications(Griffiths & de Haseth,1986). This advantagecannotbe realized,however,if the key componentsare not appropriately matched.The most importantcomponentsare briefly discussedbelow.

Interferometer The interferometeris the central componentof a FTIR spectrometer.The schematicof a Michelson interferometer-based FfIR spectrometeris shown in Fig. 10-4. Light from the IR sourceis passedonto a beamsplitter,whereapproximately half of the incident light is reflected onto a moving mirror, and the remainderis transmittedthrough the beamsplitteronto a fixed mirror. The moving mirror introducesa phasedifference betweenthe reflected and transmitted beamsby varying the pathlengthof one beamrelative to the other. Optical interferenceoccurswhen the two beamsrecombineat the beamsplitter.The resulting modulatedIR beam is then passedthrough the samplecompartmentonto the detector.Interferometersare designedso that the "moving mirror" travelswith as little friction and tilt as possibleand at a constantvelocity. Two additional optical components are addedinside the interferometer.A low-power He-Ne laseris usedfor frequencycalibration, and in somecasesdynamic alignment. In addition, a white-light sourceor laserquadrutureis requiredto determinethe point at which the moving mirror and the fixed mirror are equidistant(called the zero point of deflectionor ZPD). A numberof other interferometerdesignsare being usedin addition to the Michelson interferometer.

FI1R & RAMAN SPECTROSCOPY

277

Optical recombinationof the two beamsresultsin a complexoptical signal that is calledan interferogram.The interferogramis the detectorresponsein volts as a function of mirror displacementfrom the ZPD position. Representative interferogramsare shownon left side of Fig. 10-5. When a Fourier transformis applied to the interferogram,the data are convertedfrom the time domain into the frequencydomain. Conversionof the interferograminto a single beam(SB) spectrumproducesa spectrumin which the x-axis is in units of wavenumbers (middle portion of Fig. 10-5; spectraC and D). Intensitiesin the SB spectracorrespondto the quantitiesof IR energyreachingthe detector. Detectors Two typesof detectorsare commonlyusedin FfIR spectrometers for midIR applications,quantumdetectorsand thermal detectors.The developmentof quantum mid-IRdetectors,suchas the mercurycadmiumtelluride (MCT) detector, hassignificantly improvedthe sensitivity of FfIR spectrometers.The MCT detector is a photoconductivedetectorthat measuresan increasein electrical conductivity when illuminated. Detectorsensitivitiesare commonly represented in termsof D* valuesthat expressthe level of responserelative to the noiseproduced in units of centimetersand Hertz squareroot per watt. Wide-bandMCT detectorscover a wavenumberrangeextendingfrom 450 to 5000 cm-I , with an optimum frequencyresponsein the 900 to 1200 cm-1 region, and have characteristic D* valuesof 109 to 1011. Narrow-bandMCT detectors,on the otherhand, cover a more limited range,about750 cm-1 to 4000 em-I, but havegreatersensitivities (D* values of 1012_1013). In general,the higher the sensitivity of an MCT detector,the more the spectralrangewill be restricted.In addition to high sensitivity, MCT detectorsare characterizedby much fasterresponsetimes than are thermal detectors.Thus, faster scanningmirror velocities can be usedwith quantum detectors,resulting in faster scan rates and shorter data acquisition times than can be obtainedwith thermaldetectors. Pyrolectric devices, such as the deuteratedtriglycine sulfate (DTGS) detector,measurechangesin temperature.They do not require cooling and provide a fairly uniform frequency responsefrom the far-IR through the mid-IR region. In comparisonto MCf detectors,they are characterizedby lower overall sensitivities(D* values of 106-1O~ and slower responsetimes. Despite these limitations, they are usedfor many applications.They havethe addedbenefitthat they do not requirecooling with liquid N2• Deuteratedtriglycine sulfatedetectors fitted with polyethylenewindows also can be usedin the far-IR region from 80 to 500 cm-1• Bolometersare anothertype of IR detectorthat are more sensitive than DTGS detectorsand also allow accessto a broaderrange of frequencies; however,thesedetectorsare considerablymore expensivethan DTGS or MCT detectors.The spectralrangesfor severaltypesof IR detectorsare listed in Table 10-2. Beamsplitters Modem FTIR spectrometersare often designedto cover more than one spectral region. Emissivity of the source, optical transmissionand reflection

I

4000

I

6000

(Retardation)

# of Data Points

2000

I

8000

I

500

Wavenumber

1000 1500 2000 2500

Spectrum

3000

Spectrum

0.2500

0.5000

0.7500

~

.~--

LI r I

.!--- .!---

Wavenumber

Surfaceacidity and reversiblefolding of palygorskite Abiotic transformationof 4-chloroanisoleon smectite Polymerizationand dechlorinationof chloroethenes Adsorption and orientationof CO2 by smectite Radicalformation and polymerizationof smectite Influence of exchangeablecationson hydration mechanisms Interlayer formation of humin in smectites Determinationof tetrahedralsubstitutionand surfaceheterogeneity Clay-organic andclay-waterinteractions IR study of reductionand oxidation in smectites

IR study of hydroxy-aluminuminterlayermaterial IR study of chargereductionin montmorillonite IR study of chargereductionin montmorillonite Surfaceacidity of smectitesas influenceby exchangeablecations Intercalation,interaction,and surfacepropertiesof organo-clay Adsorption and interactionmechanismof pesticides Sorption of water andp-xylene by montmorillonite Sorption and transformationof phenolson montmorillonite Chemicaldegradationof s-triazinesby montmorillonite Influence of exchangeablecationson clay-organicreactions Sulfur containing compoundinteractionswith montmorillonite

Hydroxy-AI Li and Na Li, Mg, Ca, and K NH3and H20 NH;t and crown ethers Organiphosphorus pesticides p-xylene and H 20 Phenols s-Triazines Sulfolane Thiolane and tetramethylenesulfoxide Pyridine 4-chloroanisole Chloroethenes CO2 Dioxin H 20 Humin NHt Crown ethers Hydrazineand dithionite

Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite Montmorillonite

Palygorskite Smectite Smectite Smectite Smectite Smectite Smectite Smectite Smectites Smectites

Description

Solute

Table 10-4. Continued.

Sorbent

(Blanco et ai., 1988) (Govindaraj et ai.,1987) (Mortland & Boyd, 1989) (McBride & Wesselink, 1988) (Boyd & Mortland, 1985) (Prost, 1975) (Cloos et ai., 1981) (Chourabi& Fripiat, 1981) (RuizHitzky & Casal,1986) (Rozenson& Heller-Kallai, 1996)

(Weismiller et ai., 1967) (Spositoet ai., 1983) (Calvet & Prost, 1971) (Mortland & Raman,1968) (Casalet ai., 1984) (Camazano& Martin, 1983) (Johnstonet ai., 1992a) (Isaacson& Sawhney,1983 (Russellet ai., 1968) (Lorprayoon& Condrate,1981) (Lorprayoon& Condrate,1981)

Reference

==

-

g

1(0

8 z

z

g

....

~

FfIR & RAMAN SPECTROSCOPY

291 Cahn Microbalilnce

Self SupportingCloy ri lm

Fig. 10-7. Schematicof the in-situ FtIRlgravimetric cell showing the electrobalanceand gas cell coupledto the samplecompartmentof the FTIR spectrometer.In this illustration, a self-supporting clay film is suspendedfrom the balanceusing a hangdownwire. The pressurewithin the cell is monitoredusing a capacitancemanometerpressuregauge.The pressurein the cell is controlled by a vacuumpumpingstation and manifold.

porting films of montmorillonite exchangedwith different metal cations. The vapor phasedesorptionisothermfor water from the surfaceof the clay is shown on the left side of Fig. 10--8. Fourier transform infrared spectrain the H-O-H bendingregion shown on the right side of the figure correspondto specific data points along the desorptionisotherm. Johnstonet al. (1992b) showedthat the position and molar absorptivity of this band are strongly influenced by the amountof water sorbedon the clay film. Fouriertransforminfrared studiesof self-supportingclay films can be coupled with other spectroscopicmethodsas well. CombinedFTIR and ultraviolet (UV) spectroscopies,for example, were used to investigate the strong color changethat accompaniesthe adsorptionof certainorganiccompoundson minerai surfaces.Sucha colorchangemay arisewhen unsaturated organic compounds undergoclay-mediatedsingle electrontransferreactions.This reactionresultsin the appearanceof a very strongelectronictransition in the UV -visible spectrum. For thesecomplexes,it is desirableto observeboth the FTIR- and UV-visible spectraof the clay-organiccomplex during the sorption process.A schematic diagramof a modified gas cell designedfor this purposeis shown in Fig. 10--9. The cell allows the use of both the IR or UV-visible optics without having to be opened.This is accomplishedby mounting the film on a rotating stage that allows orientationof the film in either the IR or UV-visible configuration.Thus, tandemFTIR and UV -visible spectracan be collectedwhile the film is in a controlled atmosphereor undervacuum(Johnstonet aI., 1991, 1992a). Self-supportingfilms of soil colloids can be preparedby drying an aliquot of an aqueoussuspensionof the colloidal materialonto a surfacethat is removed prior to analysis.A dilute aqueoussuspensionof the soil colloid is preparedwith a solids concentrationin the rangeof 0.5 to 5% (w/w). Care should be taken to ensurethat the soil colloid is completely dispersedin the suspension.The volume of aqueoussuspensionused to preparethe film, usually ranging from 1 to

I

•

•

•

•

•

••

••

•

•

•

•

•

•

•

0.02:\

\~

1500

1600

Fig. 10-8. Comparisonof the desorptionisotherm of water from a self-supportingclay film of Na-SAz-l obtainedat 24°C to the FTIR spectra(right side) of water sorbedon the Na-SAz-l clay film. Each spectrumshownon the right side correspondsto one datapoint on the desorptionisotherm.

P/Po

0'0 0.1 0.20.30.40.5 0.6 0.70.80.9

5"1'.~~

100

150j

2001

mg/g 250

300

350

400

450

V2 ,

N

... =

g

fI:o

:z

a

o

= :z

c...

is

FfIR & RAMAN SPECTROSCOPY

Top Vi ew

293

... ,

,,

, ,,

UV-v isible

UV-v isible Beam

Side View

Fig. 10-9. Schematicdiagram of controlled environmentFTIRJUV-visihle cell with rotating film mount.

25 mL, will dependon the desiredareaof the self-supportingfilm. Ideally, the self-supportingfilms should have a density of 1 to 3 mg cm-2 and a cross-sectional area>1 cm2• Thicker films are sometimesneededfor experimentsthat require handlingand mounting in a cell. Kung and McBride (1989a),for example, reporteda densityof 13 mg cm-2 for their self- supportingfilms of goethite. Materials on which self-supportingfilms can be preparedinclude Mylar, polyethylenefilm (e.g., food wrap and storagebags), and aluminum foil. The optimum materialwill depend,in part, on the particularsoil colloid. Typically, a 20- by 20-cm sectionof the substrateis attachedto a 25- by 2S-cmglassplate in such a manner that it remains flat during the evaporationstep. This can be accomplishedby taping it to the glass plate so that no wrinkles are present. Polyethylenesheetscan either be tapedor held to the glassplate by a few drops of water placedbetweenthe plate and the sheet(White & Roth, 1986). One- to two-milliliter aliquotsof the aqueoussoil suspensionare then placedon the flattened surfaceand allowed to dry. Each deposit will result in a dry deposit of approximately 4 to 8 cm2 in size. After drying, the deposited film can be removedby carefully running the sectionof sheetslowly over a knife edge.The depositwill separatefrom the sheet.Self-supportingfilms formed in this manner tend to be thicker at the edges.Thus, it is often desirableto use a smallerportion cut from the middle of the depositin order to ensurea sampleof uniform thick-

294

JOHNSTON& AOCHI

ness.The self-supportingfilm can be mountedbetweentwo magneticplatesor simply tapedto a metalor cardboardsurfacefor FTIR analysis.An exposedarea 1 cm in diametershouldbe maintainedfor sufficient throughputof infrared radiation through the sample. Another method to prepare self-supporting films for FfIR analysis involves the use of a Millipore filter apparatus(Fernandezet aI., 1970). Approximately25 mL of a dilute aqueoussuspension(solids concentration0.1% w/w) is filtered through a 0.05-,urn polycarbonatemembranefilter (47-mm diam.). The sampleis allowed to dry slowly on the polycarbonatefilter. After drying, the film is removedby carefully running the filter-deposit over a knife edge.This procedure canbe usedto prepareself-supportingfilms of soil colloids and minerals that do not form uniform dry depositsusing the first method. A third methodfor preparingself-supportingfilms of powderedsamplesis to press the samplebetweenpolishedsteelplates.This methodis describedby White and Roth (1986).

SupportedDepositson Infrared TransparentWindows For soil materialsthat cannotbe preparedas self-supportingfilms, a suspensionof the samplein aqueousor organic solvent can be evaporatedonto an IR transparentwindows and analyzedas a supporteddeposit. Kaolinite and its polymorphs,iron and aluminumoxides,and humic substances areoften analyzed in this manner. It is important to work with particles ~O

KRS-6

~5

THALLIUM CHLORIDE

0.42 .25

~

POTASSIUMBROMIDE 0.6

40

KRS-5

THALLIUM

4

25

40'>

ROM I

40 4

PO ASSIUM 10010

O.

C

201

I

POTASSIUM CHLORIDE

0.21

18>

AlUM

SODIUM CHLORIDE

1021

~C

16,

LEAO FLUORIDE

025

IUM B; OMI IUM 1010

0.1

eo

IAMOND

0.25

0.5

1.0

5.0

10

50

100iL

Fig. 10-10. Transmissionregions of optical materials.The limiting wavelengthsfor both long and short cutoff havebeenchosenas thosewavelengths(wavenumbers)at which a sample2 mm thick has 10% transmission.

ited to the OH stretchingregion, glass coverslipscan be used. Other materials may be usedif their IR absorptionbandsdo not interfere in the spectralregion of interest.Evencertaintypesof plasticwrap, for example,havea minimal number of absorptionbandsin specific regionsof the infrared spectrum.

296

JOHNSTON& AOCHI

PotassiumBromide Pellets For routine characterizationof soil colloids, the most commonly used methodfor IR or FfIR analysisis the preparationof the sampleas a KBr pellet. One of the major advantagesof the methodis that only a small amountof material is needed;1 to 4 mg for routine analysis.Using micropelletmethods,detection limits as low as 1 to 20 ",g have beenreported(Luoma et aI., 1982). Also, the spectraof KBr pellets are lesssusceptibleto anomalouslight scatteringdue to a bettermatchof the indicesof refractionfor the KBr-mineral interfacecomparedto that of the air-mineral interface.This techniquealso is amongthe most suitable samplepresentationmethodsfor quantitativeanalysisof mineral constituentsprovided that the samplehas well-defined, discretebands. Potassium bromide pellet methodshave been used extensively,for example,to determine the amountof quartz and its polymorphsin multicomponentmixtures using the threediagnosticbandsfor quartzat 695, 780, and 798 cm-1 (Tennenham& Lyon, 1960; Radulescu~D, 1976; Foster & Walker, 1984; Hlavay & Inczedy, 1985; Flehmig & Kurze, 1973; Dodgson & Whittaker, 1973; Larsen et aI., 1972; Mangia, 1975). The samplepreparationstepsrequiredto makea KBr pellet include grinding the sampleto reducethe particle size to

u

c:

'" in contact with a sampleof a lower refractive index, nz. Radiation approachingfrom the densermedium is incident upon the interface betweenthesetwo media.The fate of that radiationupon striking the interfaceis determinedby the angleof incidenceas measuredfrom the normal to the surface. If this angleexceedsthe critical angle,eo where (Eq. [7])

[7] then all of the light is reflected back into the densermedium. At the point of reflection, the incoming and reflected waves combine to establisha standing wave normal to the surface.Within the crystal,the electricfield amplitudeof this standingwave variesin a sinusoidalfashion.This electricfield continuesinto the samplemedium,where it decaysexponentially.It is the interactionof this electric field with the samplepositionedthere that gives rise to the reflectanceattenuation or light absorptionwhich in tum resultsin an IR spectrum. The most important factors in determiningthe strengthof this interaction and, therefore,the intensity of the resulting spectrum,are the magnitudeof the electric field at the interface and the depth to which it penetratesthe sample medium. Both of thesefactors decreaseas the angle of incidenceincreases.The depth of penetrationis, in addition, a function of other factors as shown by Eq. [8] [8] where A is the wavelengthof the incident radiation,eis the angle of incidence, the refractiveindex of the crystal, and nsc is the ratio of the refractive index of the sample to that of the ATRcrystal. Typical values for dp are -0.1 A (Griffiths & de Haseth,1986). From this equation,it is apparentthat the depthof penetrationis less for ATR crystals of higher refractive index. In addition, the depthwill be a function of the wavelengthof the incident radiation.The effect of this wavelengthdependencyis a spectrumwhich differs from a transmission spectrumin the relative intensity of absorptionbandsin different portionsof the spectrum.Radiation of longer wavelengthwill penetrate moredeeply into the sampleand as a result, bandsat thesewavelengthswill appearrelatively more intense. Strong bands in ATR spectra also may appear somewhat distorted becausethe refractive index of the sampleundergoessignificant changesin the vicinity of an absorptionband. The effect of dispersionin the refractive index nearabsorptionbandsis treatedextensivelyby Harrick (1979). nc is

FTIR & RAMAN SPECfROSCOPY

305

Cylindrical Internal ReflectanceCell

7

T.r

.~

-:~~\~/~ ~

Fig. 10-14a. Schematicdiagram of a cylindrical internal reflectanceATR cell.

Fig. 10-14b. Schematicdiagramof a trapezoidalATR accessory(reproducedwith permissionfrom Applied Spectroscopy).

One interesting consequenceof the standing wave phenomenonis that dipolesorientedin all directionsat the crystalsurfacewith respectto the incident radiation are able to absorbenergy from the electric field generatedthere. The amplitude of the electric field in different directions dependsnot only on the angle of incidenceand refractive indices, but also upon the polarization of the incident light. Absorption coefficients for all three directions may thus be derived by comparingspectraobtainedusing nonpolarizedlight and light polarized parallel and perpendicularto the plane of incidence.

Apparatus The most commonATR crystal shapehas been the trapezoid.A conventional design for an ATR accessoryusing this shapeis shown in Fig. 10-14 (Messerschmidt,1986). Mirrors are usedto focus the incoming beamon the end face of the crystal and direct the exiting beam to the detector.As the incident

JOHNSTON& AOCHI

306

radiationpropagatesdown the length of the crystal, it interactswith the sample in contactwith its surfacesat the points of reflection. In order to avoid spectral distortion, the beamshouldenterthe crystal normal to the face. This meansthat in the caseof this particularcrystaldesign,if a changein the angleof incidence is desired,the crystal must be replacedwith one cut at a different angleand the opticsadjustedaccordingly. Numerousvariations on this basic crystal design have been offered by manufacturerswith threebasicgoalsin mind: increasedsensitivity,accommodation to different typesof samples,and enhancedeaseof use.Crystal length can bevariedto increaseor decreasethe numberof reflections.Hemicylindricalcrystals allow the angleof incidenceto be varied over a wide rangebut allow only a single reflection. In somedesigns,selectedfacesof the crystal are mirrored in order to direct the exit beammore advantageouslyor to minimize the needfor optical alignment. One recent design enjoying considerablepopularity for the study of solutions and suspensionsis that of the CIRCLE cell, availablefrom SpectraTech. This device incorporatesa rod-shapedcrystal with conical ends. Enhancedsensitivity is claimedfor this apparatusbecausethe Cassegrainoptics usedat both endsof the crystal make maximal use of the circular beamof an FfIR instrument.It is designedto be very easyto usewith a wide variety ofsample typesand also can be usedas a flow cell. As mentionedearlier,the depthof penetrationand, therefore,the intensity of the spectrumalso is a function of the refractiveindex of the crystal. By using a materialof lower refractiveindex, the depthof penetrationinto a given sample can be increased.Common crystal materials include Ge, KRS-5 [Th(Br,I)], ZnSe,ZnS, Si, and CdTe.It shouldbe kept in mind in selectingATR crystalsthat thesematerialsdiffer not only in refractiveindex, they also differ in their useful wavelengthrange,hardness,reflective losses,chemicalinertness,and resistance to scratching.Theseall needto be consideredin selectingthe appropriateATR crystal for the desiredexperiment.Propertiesof widely availableATR crystal materialsaresummarizedin Table 10-5. Detaileddiscussionsof all of thesefactors as well as optical designconsiderationscan be found in the text by Harrick (1979). A large selection of crystal shapesand materials is available from Harrick Scientific (Harrick Scientific Corp., Ossining,NY). Table 10-5. Propertiesof ATR crystal materialscommonly usedfor mid-IR spectroscopy.

Material

Refractive index

Useful Hardness frequency (Knoop) range

Properties

cm-1 Ge Zinc selenide Zinc sulfide Si KRS-5 Cadmiumtelluride Amtir Diamond

4.0 2.42 2.22 3.42 2.35 2.65 2.5 2.4

550 150 355 1150 40 170 170 7000

5000-900 20000-700 14000-1000 9500-1500 14000-400 11 000-800 11 000-800 45000-2500 1600-FlR

Hard, brittle, inert Soft, brittle, attackedby acid Hard, brittle, inert Hard, inert Toxic, soft, attackedby acids,bases Brittle, inert Brittle, inert Very hard, very inert

FfIR & RAMAN SPECTROSCOPY

307

Sample Preparation The most importantfactor in an ATR experiment isthe degreeto which the samplecontactsthe crystal element.With a typical element,the standingwave interactswith the sampleto a depth of approximately1.5 pm (Rein & Wilks, 1982).While this interactionis replicatedat eachreflectionpoint, it is easyto see that if the sampleis not in intimatecontactwith the crystalsurface,it is very possible to get no spectrumat all. Accessorydevicesdesignedfor use with solids, therefore, include backing plates intendedto compressthe sample against the crystal element.While this works well for malleablematerialssuch as synthetic polymers,it is much more difficult to establishgood contactwith dry powders, particularly if they are of coarseparticle size and hard (characteristicsthat contribute also to the rapid degradationof the crystal surface).Some successhas been achievedwith clay films formed by the depositionand subsequentdrying of clay suspensionsonto glass plates or onto the surface of the crystal itself. Thesefilms have an additional advantage in that the clay plateletsare oriented with respect to the surface of the ATR element. Other investigators have achievedgood contactwith aqueousclay suspensions (including deuteratedsuspensions)and pastes. Surfacecontactis not as difficult to ensurewith aqueoussolutionsor colloidal suspensionsbut many other factors must be controlled in order to obtain useful spectralresults.This is due to the needto usespectralsubtraction.While spectralsubtractionis often useful in studiesof solids, it is almost universally necessaryfor investigationsof liquid systemsbecauseof the presenceof bulk solvent.The goal in sucha procedureis to subtracta referencespectrumof a system componentthat most closely reflects its spectralactivity within the sample. Water,of course,is generallythe solventof choiceand servesto illustrate the difficulties that may be encountered.The absorptionbandsof liquid water are rather broad and intense and tend to mask valuable spectral information on other speciesin the sample.The shapeof the water bandsis attributableprimarily to H-bondingeffectswhich, in tum, are greatly affectedby temperature,pH, ionic strength,solutecomposition,and the presenceof solid surfaces.All of thesefactors must be consideredbeforeconclusionscan be drawn from spectraobtained by subtraction.If spectralinformation is desiredin the immediatevicinity of the water absorptionbands,water must be less than totally absorbing(equivalentto 0% transmittance)in thosespectralregions.This may be accomplished byreducing the depth of penetration with a crystal of a higher refractive index. Unfortunately,it also resultsin lower sensitivity to the soluteof interest.

Applications Applications of ATR to problemsin soil sciencehave beenrelatively limited. In one of the earliest reportedinvestigations,polarized light was used to determinethe orientation of a lysine-vermiculitecomplex (Raupach& Janick, 1988). In this study, single crystals or clay films producedby depositionfrom aqueoussuspensionwere pressedagainstthe flat surface of a hemicylindrical ATR element.Spectrawere collectedfor both deuteratedand nondeuterated samples using parallel and perpendicularpolarizedradiation. The infrared informa-

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tion was usedin conjunctionwith x-ray diffraction data to arrive at a configuration for lysine moleculeson the silicate surface.This basic approachwas later extendedto orientationstudiesof otherchemicalspeciesand otherclay minerals (Raupachet aI., 1979, 1987; Raupach& Janick,1988). Studiessuchas theseare only possiblewith uniaxially orientedclay samples. In an unusualapplication of ATR spectroscopy,Mulla et ai. (1985) used reflectancespectraof a clay suspensionin a dilute solution of D20 in H20 to determinesurfaceareasof a numberof Na-saturatedmontmorillonites.Values obtainedusing this methodologycorrelatedwell with thosefrom ethyleneglycol monoethylether(EGME) determinations.Similar agreement,however,was not found for surfaceareasof homoionic samplesof Upton montmorillonite when the saturatingcation was not Na. In this study also, a hemicylindrical element was selected. By far the mostpopulardesignusedin recentATR investigations isthat of the CIRCLE cell (Fig. 10-14). Tejedor-Tejedorand Anderson(1986) described the application of this cell to investigationsof the goethite-aqueoussolution interface.In this and in severalsubsequentpublications(consultTejedor-Tejedor et aI., 1990; Tickanenet aI., 1991, for a discussionof methodologyand important considerations),samplecomposition withinthe cell was varied with respect to variablessuchas pH, ionic strength,and sorbatespeciesin order to characterize the surfaceof goethite, the state of water at the goethite-aqueous solution interface, and interactionsof organic moleculeswith goethite at the molecular level in the presenceof bulk water. The techniqueused is illustrated in Fig. 10-15. In this figure referenceaqueoussolution spectraof potassiumsalicylate and a Fe/salicylatecomplex are comparedto the spectrum of salicylate ion sorbedon the surfaceof goethite in aqueoussuspension.The interpretationof spectraobtainedusing this methodologyis complicatedby severalfactors. The most significantof theseis the fact that waterstrongly absorbsinfrared radiation and its spectrumis thus difficult to subtractfrom samplespectra.Subtractionalso is complicatedby variationsin the shapeof the absorptionbandsof water due to changesin the variablesmentionedabove. Even when the interactionsof interest are for componentsin true solution, problemscan arise becauserelatively high concentrations(in general,>0.1%) are neededand interactionsoften occur with the ATR elementsurfaceas well as betweenthe constituentsin solution. Theseproblemsare compoundedin the caseof suspensionsby conformational and surfaceeffects exertedby the suspendedsolid on both water and sorptive molecules.Finally, spectral interpretationcan be hindered becausevibrational assignmentsfor speciesin aqueoussolution are. notabundantin the literature. Theseaspectsalso were discussedby Morra et al. (1989) with regardto characterizationof humic acid in water. FourierTransformInfrared Microspectroscopy During the past 10 yr, IR microsamplinghas been revolutionizedby the introduction of sensitive FTIR microscopes(Griffiths & de Haseth, 1986; Chalmers& Mackenzie,1988; Messerschmidt,1988; Krishnan & Hill, 1990). The application of this method to the characterizationof soil and subsurface

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Fig. 10....15. Cylindrical internal reflectance(erR) FTIR spectraof a goethite-salicylatecomplexand related aqueoussolution referencespectra(reproducedby permissionfrom American Chemical Society).

materialsholds considerablepromisealthoughonly a limited numberof applications have been reportedin this area thus far (Johnstonet ai., 1990;Vry et ai., 1988; Pironon et aI., 1990). The principal advantageof FTIR microscopyover other methodsis the ability to obtain spatially resolvedFTIR spectraof small samples.An FTIR microscope particles or regions of complex, heterogeneous can be addedto most modernFTIR spectrometersas a samplingaccessory.The microscopecan be coupledto the externalport of a FTIR spectrometer,or it can In the latter configuration,the sambe placedin the main sample compartment. ple compartmentof the instrumentis then dedicatedto FTIR microsampling. A schematicdiagramof a FTIR microscopeis shown in Fig. 10--16. The modulatedIR beamfrom the interferometeris broughtinto the baseof the microscope and focused onto the sample using an off-axis parabolic mirror. A Cassegrainobjective (magnification is on the order of 30 to 40x for this objective) is usedto collect the IR beamthat is transmittedthrough the sample.After passingthrough an apertureand polarizer (optional), the transmittedIR beamis focusedon a small-areaMCT detector. Fouriertransforminfrared microscopesare operatedby switchingbackand forth betweenthe "visual inspection"and "IR analysis"mode. The visual mode is used to visually position the sampleso the region of interestcan be selected. Once this is accomplished,the microscopecan be switched to the IR mode to obtain the IR spectrum.The accessoryis placedin the visual modeby sliding the viewing eyepieceand folding mirror into the optical beam thus diverting the

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I side view I LN, Cooled '--r-I.~---' MCT detector

I lronl view

I

MCT detector

wire-grid polarizer iris aperature

sample stage

mirror

IR beam

Fig. 10-16. Schematicdiagramof a FTIR microscopesamplingaccessory.

image to the viewing eyepiece(consistingof a glassobjectivewith the magnification of 4x to lOx). An illuminator source(e.g., quartz halogenlamp) optically coupled to the microscope provides visible illumination of the sample. Positioningof the sampleis accomplishedthrough the use of a standardmicroscopex-y translationstage,or optionally, a motorizedmicropositioningstagecan be used. When the sampleis appropriatelypositioned,the region of interest is isolatedby meansof a rectangularor circular aperturethat servesto maskout the unwantedportionsof the sample.By this means,the IR beamis only allowed to passonly through the spatialregion of interest.In practice,the minimum dimension for the apertureis on the orderof 5 to 10fml. Severeproblemsrelatedto the limited amountof light passingthrough the sample(optical throughput)and diffraction effects are encounteredfor smallerapertures.Diffraction effects can be expectedfor aperturevaluesof lessthan 30{1m, however,they can be minimized by collecting a referenceSB spectrumusing the same aperturesettings. For a more complete discussion of diffraction effects and optical nonlinearities in FTIR microscopysee Chalmersand Mackenzie,(1988) and Krishnan and Hill (1990). Once the sampleis positionedand the aperturefixed, the visible illumination optics are then switchedout so that the transmittedIR beamis passedto the MCT-detectorand the spectrumcan then be taken. Many FTIR microscopesalso can operatein an external reflection mode which is useful when the sampleis too thick or opaquefor a microtransmission study. In this configuration,the IR beam is focusedonto the surfaceof interest and the reflectedcomponentis collected into the microscope.The band shapes observedin reflectancespectracan often be distorted becausethey are much more subject to anomalousdispersioneffects than transmissionspectra.These effects are causedby the rapid and significant variation in the samplerefractive index that occurs in the vicinity of an absorptionband. The Kramers-Kronig

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transformationcan be used to correct for these distortions (Krishnan & Hill, 1990; Chalmers& Mackenzie,1988; Gerson& Chess,1988). Fouriertransforminfrared microscopyhasbeenusedrecentlyto determine the orientationof structuralOH groupsin single crystal particlesof kaolinite and dickite (Johnstonet aI., 1990). PolarizedFfIR spectrawere obtainedfor single crystal specimensof dickite and kaolinite. The polarizationbehaviorof the OH stretching bands of dickite is illustrated in Fig. 10-17. Previous attempts to locate the positionsof the H atoms in kaolinite and dickite using various structural methodsdid not result in an unambiguousdescriptionof the OH groups. The spectral data obtained from the microscope,on the other hand, provided direct information about their crystallographicorientation.The size rangeof the kaolinite particlesusedwas 5 to 15.um acrossthe 001 face and 10 to 50.um for the dickite particles. In the caseof kaolinite, the bandsshowedless of a polarization effect than expectedon the basisof the crystal structureand indicatedthe Keokuk kaolinite crystalsexaminedwere twinned (JohnstonetaI., 1990).

Low TemperatureStudies Recentstudieshaveshownthat cooling certainclay mineralsto nearliquid He (12 K) temperaturesresultsin significantly improved resolution(Brindley et aI., 1986; Prost, 1984; Prost et aI., 1987; Prost et aI., 1989; Bish & Johnston, 1993). The OH stretchingbandsof kaolinite, for example,narrow in line width and shift in position. The room temperatureFfIR transmissionspectrumof the poorly crystalline sample is characterizedby four bands in the OH-stretching region in Fig. 10-18. Upon cooling this sampleto 12 K, however, thebandsnarrow in line width and reveal the presenceof sevendiscretebands.The additional bandsin the low temperatureFTIR spectrumof the poorly crystalline material are attributedto a dickite impurity, as well as stackingdisorders.The presence of the impurities was only evidentin the low temperaturespectrum. Cooling samplesfor IR or Ramananalysiscan be accomplishedusing several different methods.The simplestand leastexpensivemethodusesa liquid N2 cryostat in which the sample is cooled by a thermal connectionto a liquid N2 reservoir.The samplestagemust be evacuatedto a moderatevacuum( 5, the appearance of a broadfeaturelessresonanceand the diminishing of the rigid-limit spectrumrevealdifferent, possibly hydrolyzedand polymerized forms of Cuz+ on the surface. Exposure of the low-pH, gibbsitechemisorbedCuz+ to NH3 vapor does not produce desorptiondespite the displacementof OH- and HzO ligandsby NH3. The large decreasesin g-valuesand increasein ~Id-values indicatethat adsorbedCuz+ may form a complexwith NH3 while remainingrigidly bound to the oxide surface,that is at leastone Cu-O-Al bond exists. Thesefindings are confirmed in Cuz+ adsorptionexperimentsconducted on microcrystallinegibbsiteand boehmitein the presenceof the chelatingligand glycine (McBride, 1985a). Electron spin resonanceresults indicate that Cuz+ absorbsin the form of ternary complexesin which the metal coordinatessimultaneouslywith a surfacehydroxyl 0 and one (on gibbsite)or two (on boehmite) glycine molecules,with the orientation of the Cu z-axis normal to the (001) sheetsof the minerals. Electron spin resonanceanalysisrevealedthat Cuz+ is adsorbedby allophanesand imogolite as a monomeron two distinct typesof surfacecoordination sites (Clark & McBride, 1984). The preferred sites are likely adjacentAlOH binding Cuz+ by a binuclearmechanism,whereasweakertype of binding sites occur at isolatedAlOH or SiOH groups.Electron spin resonancefurther reveals that the bound ion is rigid, which is indicative of a direct bond (chemisorption) betweennonexchangeable Cuz+ and 0 atomson the surface.Also in thesecases, NH3 was able to readily displaceHzO and OH-ligandsfrom chemisorbedCuz+, leading to the formation of ternary Cuz+-ammonia-surfacecomplexes.Further evidencefor these mechanismswas obtainedfor Cuz+ adsorptionon titanium

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dioxide (Bleam & McBride, 1986). The Cuz+ chemisorbsstrongly at low pH to sites that retain it by a bidentatemechanism,whereasat higher pH a weaker complex is formed with single Ti-OH groups. The ESR techniquealso has been used to study Cuz+ adsorptionby aluminum oxidesand allophanesas affectedby phosphate (Clark & McBride, 1985; McBride, 1985b).Electronspin resonanceresultssuggestthat phosphatecoordinates with the axial position of a surface-boundCuz+, thereby producing a ternary complex. High levels of sorbedphosphateon aluminumhydroxidessuppressesCuz+ adsorption,apparentlyby blocking the coordinationof Cuz+ to surface AlOH groups.Evidenceby ESR hasbeenprovided thatVOZ+ is coadsorbed with phosphateon boehmite and aluminosilicatesas an inner-spherecomplex (McBride, 1987). The ESR spectroscopyhas been used recently to probe the changeof the ligand environmentof cr3+ upon complexationwith various inorganicanionsat the solid/waterinterfaceof hectorite and montmorillonite (Charlet & Karthein, 1990).Adsorptionof cr3+ is strongly affectedby ligandspresentin solution.The ESR spectraof the surfaceobtainedin the presenceof selenite,phosphate,and P- provided evidenceof ligand coadsorptionwith formation of ternary surface complexes.

ElectronSpin Resonance Parameters andThermodynamicConstants. Metal spin probescan be usedto estimatethermodynamicstability constantsof metal-surfacecomplexesfrom the ESR parametergil' An examplewhich illustrates this possibility is representedby Cuz+ complexeswith hydrous-Alz0 3, TiO z and somesilicasin the presenceof bidentateligands(Motschi, 1984).A linear relationshipbetweenthermodynamiccomplexformation constantsof square planarCuz+ complexesin aqueoussolution and the correspondingESR parameter gil was found. A decreaseof the gil value by 0.1 units correspondsto an increasein thermodynamicstability by about eight orders of magnitude.The potential of the model was demonstratedfor a seriesof ternary complexesof Cuz+ with bidentateligandson o-Al Z0 3, for which speciationof Cuz+ and stability constantswere obtained.On thesebases,a major revision of currentconcepts of cation adsorptionon layer silicatesis possible.

ELECTRON SPIN RESONANCE·RELATED SPECTROSCOPIES Analysis of the ESR spectrumcan provide information about the identity of paramagneticspecies,their electronic and geometricstructureand the rotational or translationalmotion. It also is particularly valuablefor the study of the free radical intermediatesor productsof chemical,photochemical,and biochemical reactionsand of valencestate and site symmetry changesof paramagnetic metal ions: The principal limiting factor of information obtainedby ESR is the resolutionof spectraltransitionsthat occursparticularlyfor paramagneticspecies in powder samples.Two methodsare potentially very useful to overcomethis limitation: (i) ENDOR spectroscopy,that is the major double resonancemethod used, basedon a combinationof ESR and nuclearmagneticresonance(NMR)

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techniques(Kevan & Kispert, 1976); and (ii) ESE spectroscopy,that is a timedomainelectronmagneticresonancemethod(Kevan & Schwartz,1979). Either ENDOR or ESE representa useful tool in determiningthe componentsof the hyperfineand superhyperfinematrices,that is, they can extendthe resolutionof the ESRexperimentby resolvinghyperfine,or superhyperfine,contributionsto inhomogeneously broadened lines in circumstanceswherethe splitting betweenthesecomponentsis lessthanthe width of the individual spin packets. For example, the hyperfine splitting may lead to line broadeningif the unpairedelectronis highly delocalizedand interactingwith many nuclei which causesa loss of details.Further,ENDOR canbe appliedto resolvebroadsingleline ESR spectrain studiesof randomly orientedsystems,such as frozen solutions, in which an ESR experimentis unableto provide the requiredresolution. Although the ESEexperimentalapparatusandspectralanalysisis more complex than for ENDOR, the former is a useful alternativeto the latter when the splittings are small and conventionalENDOR is difficult to perform. ElectronNuclearDouble ResonanceSpectroscopy Electron nuclear double resonancespectroscopymeasuresthe flip of a nuclearspin by a radiofrequencyfield, detectedby a changein the intensity of the ESR signal arising from the electronto which that nucleusis coupled.In the ENDOR experiment,the magneticfield Ho is centeredon oneof the ESRtransitions and irradiatedwith high intensity microwavepower until the signal is saturated(Eq. [5]). A varying radiofrequency(VENDOR) is then introducedinto the metal coils of the microwavecavity andthe intensity of the selectedESRtransition is measuredwhile the radiofrequency is swept over several tens of megaHertzin orderto inducenucleartransitionsaccordingto the suitableselection rule. For an effective electronspin of 1/2 and a nucleusof nuclearspin I = 1/2, two ENDOR transitionsareobservedwhenthe resonanceenergyis satisfied in the equation MENDOR = h Vo ENDOR = 1/2 ~zzl :!: gn 13n Ho

[12]

whereA zz is the z-componentof the hyperfinecoupling constantof the electron with a neighboring nucleus (see previously in section on "The Hyperfme Splitting") when the externalmagneticinductionfield Ho is appliedalongthe gzz tensordirection, which is, in tum, parallel to the z-axis of the nuclearhyperfine tensor;gn is a dimensionlessconstantcalled the nuclearg-factor; and 13n is the nuclearmagneton. For a nuclearspin I = 1 (e.g., 14N), four ENDOR transitionsshould be observedand an ENDOR spectrumconsistingof two pairs of equally separated lines should be obtained.The ENDOR spectrumcan, thus, "fingerprint" the typesof interactingnuclei, describedby gn13n. Line intensitiesare generallyonetenth to one-hundredthlessintensethan ESR lines. The ENDOR spectroscopyis a highly specializedand complextechnique. However, the sensitivity and easeof carrying out experimentsis improving rapidly, also due to the introductionof new designsof microwavecavity, called

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loop-gap resonators.They require much smaller amounts of sample, can be wound with an external radiofrequency,and can be placed inside a cryostat. Further,ENDOR providesa meansof obtainingNMR signalsfrom paramagnetic samples,e.g., the observationof 57Fe NMR in paramagneticsamples.

ElectronSpin Echo Spectroscopy The advent of powerful computersin the field of NMR spectroscopy enabledthe introductionof pulsedsourcesfollowed by Fourier transformof the nuclearechoes[nuclearspin-echoFourierTransform(Ff)-NMR] with enormous advantagesfor the NMR method.In the field of ESRspectroscopy,the spin-echo techniqueshould allow, in principle, magneticrelaxation times to be measured directly and, more importantly, very weak dipolar hyperfine interactionsto be measuredeven in powder samples.This would permit the molecularsurroundings of the paramagneticspeciesto be proven.However,a fundamentaldifficulty ariseswith electronspin echoes,namely, the speedwith which phasecoherenceof electronspinsis lost. Sincean electronis coupledmuch more strongly to its chemicalenvironmentthan a nucleusis, the electronrelaxeson a timescalean order of magnitudeshorterthan a nucleusdoes.This intrinsic difficulty can be overcomeby the use of high-powermicrowave sourcesdesignedto pulse into cavities with rapid decay times, so that detectionwithin a few tens of nanoseconds of a pulse is possible.This time is sufficient to enablethe detectionof an echo from a set of electronspins refocusingafter a 90° pulse. Therefore,a free inductivedecaycan be built up stepwiseusing varying timedelaysbetweenpulses. In practical pulsed ESR experiments,electron spin echoesare generated most commonlyby a pulsesequenceconsistingof a 90° focusingpulsefollowed by a precessiontime and then a 180° spin flip pulse which causesthe spin to refocuswithin anotherprecessiontime period. The refocusingproducesa burst of microwaveenergycalled an echo,whose intensity is measuredas a function of the time betweenthe two pulsesto generatethe echo delay envelopewith a time constantwhich gives a transversemagneticrelaxationtime. In many cases the envelopeof the train of spin echoesis not a smoothexponentialdecay,but is modulatedby weak dipolar hyperfine interactionsbetweenthe relaxing electron and neighboring nuclei with nonzero spin to which the electron is coupled. Fourier transform of the modulateddecay, called electron-spinecho envelope modulation (ESEEM or ESEM) expressesthe results in the frequency domain and the resultingpeaksgive directly the hyperfinecouplingfrequenciesof neighboring nuclei. The best spectraare yielded by nuclei which are dipolar coupled to the electronand which have nuclearquadrupolemoments. Analysis of the ESEM spectracan thus provide significant structuralinformationwhich is generallynot availablefrom the ESRspectrum alone,due to resolution limitations. Typically, the ESEM pattern can be analyzedto yield the numberof approximatelyequivalentinteractingmagneticnuclei as well as their distanceof interaction,togetherwith any small isotropic hyperfinecoupling that may be present.

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Both relaxation times that characterizethe two magneticrelaxationphenomenaof paramagneticspecies canbe measuredby the ESE techniques.These times are: (i) the spin-lattice relaxation time, Tb that characterizesthe rate of radiationlesstransitions between the spin systemand the thermal motion of the lattice or surroundings(see also in sectionon "The ResonancePhenomenon"); and (ii) the spin-relaxationtime, denotedby T2, that is due to energyexchange betweenspinsin the system.The transverserelaxation directlymeasuredin powdersby ESE is betterdescribedas a phasememory time sinceit is not generally identical to T2• However,the true T2 in solids can be obtainedby a specialESE method (Kevan & Schwartz, 1979). Although both types of relaxationshave beenusedvery little for paramagneticspeciescharacterizationor to give structural or chemicalreaction information, this is an areawith significant potential for developmentin soil chemistrystudies.

Comments The ESE techniqueis in many ways complementaryto the ENDOR technique, in that the former allows detectionof more distant nuclei with hyperfine couplingsin the low-frequencyrange,0 to 15 MHZ, where ENDOR transitions are very weak. PulsedENDOR methodsalso have beenreported.The introduction of,new designsof microwavecavities transparentto radiofrequency.radiation, will determinewith little doubt the areaof pulsedESR growth in the future. Electron nucleardouble resonanceand ESE techniqueshave not yet been applied to strictly soil chemistry studies, and it is in this area of ESR spectroscopywhere major scientific activity is expectedto occur.

REFERENCES Abragam,A, and B. B1eaney.1970. Electron paramagneticresonanceof transition ions. Clarendon Press,Oxford. Alger, R.S. 1968. Electron paramagneticresonance:Techniquesand applications.Wiley-Intersci., New York. Angel, B.R., andW.E.J. Vincent. 1978.Electronspin resonancestudiesof iron oxidesassociatedwith the surfaceof kaolins. Clays Clay Miner. 26:263--272. Atherton, N.M. 1973. Electron spinresonance.Halsted,London. Atherton, N.M., PA Cranwell, AI. Floyd, and R.D. Haworth. 1967. Humic acid I. ESR spectraof humic acids.Tetrahedron23:1653--1667. Berliner, L.J. 1976. Spin labeling: Theory and applications.Acad. Press,New York. Bleam,w.F., and M.B. McBride. 1986.The chemistryof adsorbedCu(II) and Mn(II) in aqueoustitanium dioxide suspensionsI. Colloid InterfaceSci. 110:335-346. of free radiBlois, M.S., Ir., H.W. Brown, and I.E. Maling. 1961. Precisiong-value measurements cals of biological interest.p. 117-131.In M.S. Blois, Ir. et al. (ed.) Free radicalsin biological systems.Acad. Press,New York. Carrington,A, and AD. McLachlan. 1967. Introduction to magneticresonancewith applicationsto chemistryand chemicalphysics.Harper& Row, New York. Charlet, L., and R. Karthein. 1990. Study of inorganic Iigand-chromium(III)-surfaceternary complexesby ESR spectroscopy.Aquat. Sci. 52:517-527. Clark, C.I., and M.B. McBride. 1984. Chemisorptionof Cu(II) and Co(II) on allophanesand imogolite. Clays Clay Miner. 32:300-310. Clark, C.J., and M.B. McBride. 1985. Adsorption of Cu(II) by allophaneas affectedby phosphate. Soil Sci. 139:412-421.

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Craik, OJ. 1971. Structureand propertiesof magneticmaterials.Appl. Phys. Ser. no. 1. Pion Ltd., London. Gamble, D.S., M. Schnitzer, and D.S. Skinner. 1977. Mn(II)-fulvic acid complexing equilibrium measurements by electronspin resonancespectrometry.Can. 1. Soil Sci. 57:47-53. New York. Gordy, W. 1980. Theory and applicationof electronspin resonance. Wiley-Intersci., Ingram, DJ.E. 1969. Biological and biochemical applicationsof electron spin resonance.Hilger, New York. Jameson,R.F. 1981. Coordinationchemistryof copperwith regardto biological systems.p. 1-30. In H. Siegel (ed.) Metal ions in biological systems.Vol. 12. Dekker, New York. Kevan, L., and L. Kispert. 1976. Electron spin double resonancespectroscopy.Wiley-Intersci., New York. Kevan, L., and R.N. Schwartz. 1979. Time domain electron spin resonance.Wiley-Intersci., New York. Lakatos, B., T. Tibai, and J. Meisel. 1977. ESR spectraof humic acids and their metal complexes. Geoderma19:319-338. McBride, M.B. 1976. Origin and position of exchangesites in kaolinite: An ESR study. Clays Clay Miner. 24:88-92. McBride, M.B. 1982a.Organicanion adsorptionon aluminum hydroxides:Spin probestudies.Clays Clay Miner. 30:438-444. McBride, M.B. 1982b. Electron spin resonanceinvestigationof Mn 2+ complexationin natural and syntheticmoleculesand soil organics.Soil Sci. Soc. Am. J. 46:1137-1143. McBride, M.B. 1982c. Cu2+-adsorptioncharacteristicsof aluminum hydroxide and oxi-hydroxides. Clays Clay Mineral. 30:21-28. McBride, M.B. 1985a. Influence of glycine on Cu2+ adsorption by microcrystalline gibbsite and boehmite.Clays Clay Miner. 33:397-402. McBride, M.B. 1985b.Sorptionof copper(II) of aluminumhydroxidesas affectedby phosphate.Soil Sci. Soc. Am. 1. 49:843-846. McBride, M.B. 1986. Magnetic methods.p. 219-270. In A. Klute (ed.) Methodsof Soil Analysis. SSSABook. Ser. 5., Part 1. 2nd ed. ASA, CSSA, and SSSA, Madison, WI. McBride, M.B. 1987. Ternary V02+ ligand-surfacecomplexeson boehmiteand noncrystallinealuminosilicates.J. Colloid InterfaceSci. 120:419-429. McBride, M.B. 1989. Reactionscontrolling heavy metal solubility in soils. p. 1-56. In B.A. Stewart (ed.) Advancesin soil science.Vol. 10. Springer-Verlag,New York. McBride, M.B., A.R. Fraser,and WJ. McHardy. 1984. Cu2+ interactionwith microcrystallinegibbsite. Evidencefor orientedchemisorbedcopperions. Clays Clay Miner. 32:12-18. McBride, M.B., T.J. Pinnavaia,and M.M. Mortland. 1975. Perturbationof structural Fe3+ in smectites by exchangeions. Clays Clay Miner. 23:103-107. Motschi, H. 1984. Correlation of EPR parameterswith thermodynamicstability constantsfor copper(II) complexes.Cu(II)-EPR as a probe for the surface complexationat the water/oxide interface.Colloid Surf. 9:333-347. Peisach,J., and W.E. Blumberg. 1974. Structuralimplications derived from the analysisof the electron paramagneticresonancespectraof naturaland artificial copperproteins. Arch.Biochem. Biophys. 165:691-708. Poole, C.P. 1967. Electron spin resonance.A comprehensivetreatise on experimentaltechniques. Wiley-Intersci., New York. Senesi,N. 1990a. Application of electron spin resonance(ESR) spectroscopyin soil chemistry. p. 77-130. In BA Stewart(ed.) Advancesin soil science.Vol. 14. Springer-Verlag,New York. Senesi,N. 1990b. Molecular and quantitativeaspectsof the chemistryof fulvic acid and its interactions with metal ions and organic chemicals.Part I. The electron spin resonanceapproach. Anal. Chim. Acta 232:51-75. Senesi,N., D.F. Bocian, and G. Sposito. 1985. Electron spin resonanceinvestigationof copper(II) complexationby fulvic acid extractedfrom sewagesludge.Soil Sci. Soc. Am. J. 49:119-126. Senesi,N., Y. Chen, and M. Schnitzer.1977a.Hyperfine splitting in electronspin resonancespectra of fulvic acid. Soil BioI. Biochem. 9:371-372. Senesi,N., S.M. Griffith, M. Schnitzer,and M.G. Townsend.1977b.Binding of Fe3+ by humic materials. Geochim. Cosmochim.Acta 41:969-976. Senesi,N., and C. Steelink.1989.Application of ESRspectroscopy tothe study of humic substances. p. 373-407. In M.H.B. Hayes et al. (ed.) Humic substances.Vol. 2. John Wiley & Sons, Chichester,England. Steelink, c., and G. Tollin. 1962. Stable free radicals in soil humic acid. Biochem. Biophys. Acta 59:25-34.

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Swartz,H.M., I.R. Bolton, and D.C. Borg. 1972. Biological applicationsof electronspin resonance. WiIey-Intersci., New York. Templeton,G.D., and N.D. Chasteen.1980. Vanadium-fulvic acid chemistry: Conformationaland binding studiesby electronspin probestechniques.Geochim.Cosmochim.Acta 44:741-752. Thomson,A.I. 1990. Electron paramagneticresonanceand electronnucleardouble resonancespectroscopy.p. 295-320.In D.L. Andrews (ed.) Perspectivesin modemchemicalspectroscopy. Springer-Verlag, Berlin. Vonsovskii, S.B. 1966. Magnetic resonancein ferromagnetics.p. 1-11. In S.V. Vonsovskii (ed.) Ferromagneticresonance.PergamonPress,London. Wertz, I.E., and I.R. Bolton. 1972. Electron spin resonance:Elementary,theory and practical applications.McGraw-Hili, New York.

Published 1996

Chapter12 X-Ray Photoelectron Spectroscopy R. K. VEMPATI, T. R. HESSAND D. L. COCKE, Lamar University, Beaumont, Texas

Surfacesof mineralsplaya pivotal role in the adsorption,dissolutionand precipitation of ions in the soil environment.Prior to the developmentof advancedanalytical instrumentation,surfaceprocesseswere studiedusing solution chemistry. In recentyears,the developmentof surfacesensitiveanalytical techniques,e.g., Auger electronspectroscopy(AES), ion scatteringspectroscopy(ISS), low-energy electrondiffraction (LEED), secondaryion massspectroscopy(SIMS), x-ray photoelectronspectroscopy(XPS), scanningprobemicroscopy(SPM), etc., have allowed direct probing of surfacephenomena.Of thesesurfacetechniques,XPS is the most popular amongsoil- and geochemistsbecauseit provideselemental, chemicalstateand semiquantitativecompositionalinformation. The informationgleanedfrom XPS is extremelyuseful in the determination of surfaceelementalcomposition(Bancroft et aI., 1977; Gonzalezet aI., 1988; Seyama& Soma,1988); site occupancy(Seyama& Soma,1988); oxidation and reductionchemistry(Stucki et aI., 1976); chemicalweathering(Hochella, 1988; Inskeepet aI., 1991); Lewisand Br0nstedacid sites(Boradeet aI., 1990); adsorption of cationsand anions,chemicalbondingand surfacereactivity (Koppelman et aI., 1980; Vempati et aI., 1990a, b) and differentiation of exchangeableand nonexchangeable clay components(Seyama& Soma,1988). For more examples and detail applications,the reader is advised to refer to reviews by Hochella (1988), Perry (1990) and Cocke et al. (1994). These reviews discuss elegant applicationsof XPS to geo- and soil chemistry research.For the most part, the referencescited in this chapterand thosein the reviews concernXPS studieson soil constituents,e.g., clays, Fe oxides, AI oxides, Mn oxides, carbonates,etc. However, some surfaceanalysiswork on soil colloids and sedimentshas been reported(Seyama& Soma, 1985) but the use of XPS for such studies is not a commonpractice as it is extremely difficult to trace the origin of the elemental signalsto a specific mineral. A particularelementof a certainoxidation statecan display small shifts in its binding energy signals dependingon the mineral or chemical situation.Thus for a complexmixture of materials,suchas soil and soil colloids, there are often overlappingsignalsfrom the sameelementsthat are difCopyright © 1996 Soil ScienceSociety of America and American Society of Agronomy, 677 S. SegoeRd., Madison,WI 53711,USA. Methods of Soil Analysis. Part 3. Chemical Methods-SSSA Book Seriesno. 5.

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ficult to interpret. Furthermore,XPS spectraof multielement systemscan be muddledby the variouselements'spectralpeaksoverlappingoneanother.Unlike the problemwith the small chemicalshifts of a single element,this problemmay be solvedby: (i) using a different spectralline for analysis,(ii) using a different x-ray sourceif there are problemswith an Auger peakoverlappinga core level peak,or (iii) decomposingthe overlappingpeaksinto individual componentsvia a peak-fitting routine and then subtractingthe offending peak.For thesereasons, studieson the individual soil componentsor simple mixturesare often favored. For surfaceanalysis,XPS is popularbecause:(i) elementalidentification, chemical state and semiquantitativecompositionalinformation for the all elementsin the periodictable exceptH and He becauseof their low photoionization crosssection,can be determined;(ii) depthprofiling can be performedeither by using a low take-off angle XPS method(nondestructive)or by Argon ion sputtering using an ion gun (destructivemethod); (iii) amorphousand crystalline samplesmay be investigated;(iv) conductingand nonconductingmineralsmay be analyzed,and (v) very small samplesize is needed(on the orderof 10 ng). In doing surfaceanalysisthereare somedisadvantages and theseare relatedto how the sample may be changedduring the analysis. Most surface analysis techniques,XPS included,require a high vacuumenvironmentand utilize somesort of probesuchasphotonsor electrons.This cancreateproblemsfor hydratedsurfaces becausethey are susceptibleto dehydrationdue to high vacuum requirement, althoughhydratedFe oxideshavebeensuccessfully studied by Vempati et ai. (1990a,b). In other cases,samplesmay deterioratebecauseof the vacuum environmentand by localizedheatinginducedby the photon(x-rays for XPS) or electronprobe.This is especiallytrue for clay minerals,where high vacuumcan irreversibly damagethe surface.Theseproblemscannotbe easily solved; however, they may be betterunderstoodby monitoring the surfacechemicalchanges with time. There are severalbasictexts and reviews concerningsurfaceanalysisand in particularXPS (Muilenberg, 1979; Defosse& Rouxhet,1980; Briggs & Seah, 1983; Ratner, 1983; Eland, 1984; Hochella, 1988; Payntner,1988; Perry, 1990; Cocke et aI., 1994) that the readeris encouragedto consult for greaterdetail.

X-RAY PHOTOELECTRONSPECTROSCOPYPRINCIPLES When a solid material is bombardedby a photon, the photon'senergy is transmittedto an electron.Shouldthe photonbe of sufficient energy,the electron can be ejectedfrom the atom. This processis called photoemissionand is depicted in Fig. 12-1. When the excitationoccurssufficiently closeto the surface,the photoelectroncan escapefrom the material. The photoelectronleavesthe atom in an excited state which then must lose the energygained. The excessenergy can be lost through various de-excitationprocessesincluding the ejection of a secondelectron called an Auger electron (see Fig. 12-1). All elementshave a unique set of core level electronswhose binding energiesare spreadout over a thousandelectronvolts(eV) or more. Theseare the electronsthat are of interest for XPS analysis.By determiningthe binding energiesof the photoelectons,the

X·RAY PHOTOELECTRON SPECTROSCOPY

Incident X-ray Photon

359

Photoelectron

I T Productionof Photoelectronsand Auger Electrons: 1) Absorption of X-my pholOn by core electron

2) Ejection of core-level photoelectron 3) RelMuliion or higher shell electroninto hole 4) Intern.-Ily recombinedenerg.yof relaxation

5) Ejection of Auger electron

Fig_ 12-1. Illustration of photoelectronand Auger electronprocessinducedby x-ray bombardment.

elementof origin can be determined.In fact, small changes(a few eV) in the binding energyreflectsdifferencesin the chemicalstate(suchas oxidation state) of the atom of origin and the number of photoelectronsproducedreflects the quantity of the originating atomsin the excitation region. To excite these core level electrons,photons with energiesgreater than 1000 eV should be used (x-ray photons).The most commonly used x-ray lines for XPS analysisare the AlKa (1486.6eV) and the MgKa (1253.6 eV). These sourceshave sufficient energyto excite the core level electronsand to penetrate deepinto the bulk sample.However,only thosephotoelectronsoriginating from the near-surfaceregion (top few nm) are able to escapefrom the samplewithout energy loss. It is this fact that makes XPS a surface sensitive technique.The depth from which a particular photoelectroncan escapewithout loss of kinetic energy is dependenton the attenuationlength of the electron in the sample matrix_ The attenuationlength is definedas the distancein the matrix over which an electrontravels and suffers an energyloss only 36.8% [100(e-I )] of the time. Escapedepth is simply the attenuationlength times the cosineof the angle (8 in Fig. 12-1) betweenthe surfacenormal and the direction of the escapingelectron. Aside from being dependenton the matrix, the attenuationlength shows a lopsided vee-shapeddependenceon the electron'skinetic energy and has a minimum at about50 e V (Seah& Dench,1979). Over the energyrangeof most interest (10-1200 eV), the attenuationlength varies from 2 to 10 atomic layers. Typically, the sampling depth or what Hochella (1988) calls the "information depth" is consideredto be equivalentto three times the attenuationlength. Once the photoelectronhas escapedfrom the surfacewithout collision or other energy loss, it has a certain kinetic energy(KE) that can be relatedto the binding energy for the electron (Eb) which is characteristicfor the electronic

VEMPATI ET AL.

360 Conducting Sample

Spectrometer

Non-conducting Sample

KE'

KE Vacuum Level

-

/

s Conduction Band Fermi / Level

bv

Valence \ Band Pinned Fermi Levels

} c ---.~

Uncharged Work Function

>

~--/r--Fe·'L L:: S)

Mismatchin Fermi Levels

E~

Core Level Fig. 12-2. Energy level diagram for x-ray induced photoelectronsfrom conductiveand insulating samples.

energylevel of origin. By measuringthe kinetic energyof the photoelectron,its binding energycan be determined.For a conductingsamplein electricalcontact with the spectrometer,the relationship between the measuredkinetic energy (KEsp) and binding energyis

EB

=hz -

KE - s =hv - KEsp - sp

[1]

where the EB term includesthe electronbinding energy(Eb), intra-atomicrelaxation shift (Ea) and interatomicshift (Es) (both Ea and Es are on the order of a few eV or less); hz is the energyof the incident x-ray photonand sp is the work function of the spectrometer(the work done on the photoelectronas it arrives at the electronmultiplier tube to be counted).Thesetermsmay be betterunderstood by examining Fig. 12-2. The Fermi level is defined as the highestfilled electronic state at 0 K and the binding energy, EB, is defined as zero at the Fermi level. The work function of the sample,s is the additional amountof energy requiredto move an electronfrom the Fermi level to the vacuumlevel (the point where the electron is no longer energeticallyassociatedwith the atom and is often referredto as the free electronlevel). For a conductingsample,electrically coupled withthe spectrometer,their respectiveFermi levels are pinned at the sameenergyasshownin Fig. 12-2. This meansKE + s =KEsp + sp and allows Eq. [1] to be written. Now the electron'strue kinetic energy(KE) and the sample's work function need not be known. The sp can be determinedusing standardswith well-defined binding energies(such as Au) and should remain constantif the spectrometersystemis reasonablysafe.This allows the EB to be com-

X-RAY PHOTOELECTRONSPECTROSCOPY

361

pareddirectly from sampleto sampleif measuredon the sameinstrument;however, if the conductivity of the sample'ssurfaceor a good electrical connection with the instrumentis in doubt then an internal standardshould be used. Samples,such as soils, mineralsand other geologicmaterialsare not conductive and so the sample'ssurfacewill be electrically isolatedfrom the spectrometerand their Fermi levels will not be pinned to the sameenergy (see Fig. 12-2). Equation [1] is no longer valid and a new term c, the static surface charge,must be considered.This chargeis typically positive becauseof loss of photoelectrons.The nonconducting sample'swork function, cs, now consistsof two terms: the sample'sunchargedwork function, s, and the surfacecharge,c. For the nonconductingsamplein Fig. 12-2, the kinetic energyof the photoelectron (KE') is smaller than would be expectedfor an unchargedsituation by c. Consequently,one can write EB,R = hv- KEsp - sp > E' B.

[2]

where the reported binding energy (EB,R), will be greater than the actual unchargedbinding energy (E'B)' For proper analysis, a correction between reported(EB,R) and the true binding energy (E'B) must be made. This problem can be handledby using an internalstandardthat hasa well-known binding energy. Two of the most commoncorrectionmethodsare the: (i) adventitiouscarbon method,and(ii) surfacegold method.Thesemethodsalongwith chargingeffects will be discussedlater in the text. The formation of an Auger electron (see Fig. 12-1) involves three electrons (ijk) and thereforethreeenergylevels have to be considered.The impingement of a x-ray photon or an electron on an atom leadsto the ionization of an inner shell electron(i), this leadsto the creationof a hole in the core level resulting in a transferof an electronfrom a higher atomic level (j) to fill the inner core hole. This de-exitationprocessor releaseof excessenergyresultsin emissionof a x-ray photon leading to the ejection of anotherelectron, k, called the Auger electron.The Auger electronkinetic energyis given by EK(ijk) = EB(i) - EB(j) - EB(k) - E(jk) + R(jk)

[3]

where E(jk) is the interactionbetweenthe two holes in the final state,and R is the total relaxation energy, i.e., the intra- and interatomic relaxation energy betweenthe two holes in the final state. Anotherterm that hasbeenusedto describethe chemicalstateis the Auger parameter(Wagner& Joshi, 1980)which is definedasthe differencebetweenthe kinetic energyof the most intenseAuger line, EK(ijk) and the most intensephotoelectronlines KE(P).

a = EK(ijk)- KE(P) = EB -

E~

[4]

where EB and E~ are photoelectronand apparentAuger binding energies,respectively. In order to keepthe Auger parameterpositive and independentof the photon excitationenergy,the modified Auger parameter,a', is used

VEMPATI ET AL

362

Table 12-1. Mg (Is) binding energies,Mg KLnLzJ Auger kinetic energiesand dRs valuesof Mg compounds(Seyama& Soma,1988). Compounds

Mg(ls)

Mg· F2 Exchangeable Nonexchangeable Orthopyroxene MgCI2 ·6H20 MgO MgBr2· 6H20

1306.5 1305.3 1303.8 1304.0 1304.8 1303.9 1305.3

MgKLnLzJt 1176.8 1179.0 1181.0 1180.7 1180.2 1181.3 1180.7

0.0 1.0 1.5

1.4

1.7 1.9 2.7

t As defined in Eq. [3]. :j: Extra atomic relaxationfactor.

a' =a + hv

[5]

The advantageof using the Auger parameteris that it is independentof sample chargingwhich is an importantconsiderationfor soil samples,mineralsandother geologicmaterials,many of which are insulatorsand exhibit charging. The Auger parametersare usedto understandthe chemicalenvironmentof the minerals.The Auger parametervaluesare directly relatedto the polarizability of the mineral. Studieson the structuralclay components,i.e., 0, Si, AI, and Mg showthat theseelementshavelow polarizability indicating that the electrons in silicate frameworks are delocalized (Gonzalez-Elipe et aI., 1988). Furthermore,chemicalstateplots, plots of the Auger kinetic (eV) vs. XPS binding energy(eV) are usedcommonlyto differentiatethe exchangeable and nonexchangeableions in the clay minerals(Seyama& Soma,1985; GonzalezElipe et aI., 1988). Table 12-1 shows that the Mg Auger parametervalue for the exchangeableMg is betweenMgCl2 and MgF2; whereas,the values for the nonexchangeable ion is close to MgO. The other parameterthat is commonly usedin the literatureis theextra-atomicrelaxationenergydifference(L1Rs) which is calculatedas follows: {[XPS binding energy(sample)+ Auger kinetic energy (sample)]- [XPS binding energy(standard)+ Auger kinetic energy(standard)]). Mg ion is greaterthan that of the exchangeable The L1Rs of the nonexchangeable ion indicating that the densityof electronssurroundingthe nonexchangeable Mg is greater.

X-RAY PHOTOELECTRONSPECTROSCOPY INSTRUMENTATION The basic componentsof XPS are: ultrahigh vacuum chamber, x-ray source,electronanalyzer,detector,datacollection and handling,and accessories (Fig. 12-3). The most commonly usedultrahigh vacuum(UHV) pumpsare ion pumps,vapordiffusion pumps,turbomolecularpumpsandcyropumps.The heart of the instrumentis the analyzer.Today the most commonlyusedanalyzeris the hemispherical-sector type. In this system,the electronsare subjectedto a retarding field by a constantpassenergyand resolvedby their different pathsbetween the two charged hemispheres.Slits having similar width are placed at the

X-RAY PHOTOELECTRONSPECTROSCOPY

X-ray Source

363

Electron EnergyAnalyzer

: ---------t::~::~~

l:'oow~ Incident X-rays

Sample

Electron Detector

Ein= hv

Binding Energy (eV)

Signal Processing and Display

Fig. 12-3. Schematicdiagramof the ESCA spectrometer.

entranceand exit of the analyzer.The passenergy(PE) is defined as the potential difference neededbetweenthe two chargedhemispheresso that an electron of the appropriateenergy may passthrough the analyzerwithout colliding wit~ it or the exit slit. A high PE increasesthe intensity and full width at half maximum (FWHM), and a low PE decreasesthe FWHM and intensity of the peak. There are two basic analyzermodesthat are available: (i) fixed retarding ratio (FRR), and (ii) fixed analyzertransmission(FAT). In the FRR mode, the sensitivity (S) is proportionalto the kinetic energyof the photoelectron.Thereforethe sensitivity at low kinetic energy is reducedto that of high kinetic energies.This meansthat in this mode the S for transition metal elementsis reducedcompared to Al or Si. Also, the area of sample analysisremainsconstantthroughoutthe whole kinetic range. In caseof the FAT mode, S is inversely proportionalto the kinetic energyof the photoelectron.Therefore,the S at lower kinetic energiesis improved over the FRR mode and thus provides an increasedS for transition metal elements.In the caseof FAT mode, the areaof the sampleanalyzeddoes vary slightly with the kinetic energy. The detectoris either a single- or multichanneldetector,the latter allows for increaseddatacollection speed.The x-ray sourceis usually a dual anodetype with Al and Mg as sourcemetals.Thesesourcesgive a sharp Kal,2 with a line width of 0.8 to 0.9 eV and the intensity of theselines is half of that total emitted intensities.The position of the main x-ray lines of theseelementsare MgKal = 1253.6eV Al Kal = 1486.6eV

Ka2 = 1253.4eV Ka2 = 1486.3eV

Other metalsare usedon a limited basisto provide alternativeenergies.To obtain a high resolutionXPS spectrum(-0.2 eV), a monochromaticx-ray sourceis used chargingfor insulators.A charge at the expenseof signal intensity and excessive neutralizerwhich providesa flux of low-energyelectronsis usedin someexper-

VEMPATI ET AL.

364

Electron Energy Analyzer X-ray Source

Charge Neutralizer Sample Transporter

/

"

MechanicalCleaning / and/orCleavage

Sample Transporter

Gasor Liquid........

~

Sputteror Vapor Coating

Chemical Reaction Chamber

Main Analysis Chamber

Physical Treatment Chamber

Fig. 12-4. Sketchof a typical XPS spectrophotometer.

imentsto compensatefor the chargingof nonconductingsamples(seeFig. 12-4). A sputtergun is usually presentto provide etchingof the surfaceby bombardmentof Argon ions, thus enablingdepthprofiling of samples.The sampleis generally placed on an tilting stagefor angle resolvedXPS studies.Since samples often require some form of in situ treatmentbefore or after analysis,a sample that treatmentchamberis usually a part of the instrument.Additional accessories may be present in the spectrometerare: in-vacuum fracturing devices, vapor deposition,heatingand cooling stages,gas/liquid inlet valves,etc. Most modern instrumentsmay have other surfacesensitivetechniqueslike AES, LEED, high resolutionelectronenergyloss spectroscopy(HREEL), ultraviolet photoelectron spectroscopy(UPS), secondaryion massspectroscopy(SIMS), and ion scattering spectroscopy(ISS). ReferencingBinding Energy X-ray photoelectronspectroscopyspectraof insulators,suchas soils, minerals and other geological materials,often exhibit considerablebinding energy shifts and peak broadeningbecauseof surfacecharging.This is due to the fact that thereis no electricalcontactbetweenthe sampleand instrument.In order to correctfor thesecharge-inducedshifts, standardsare often used. Oneof the common methodsto calibratethe spectraof thesematerialsis to usethe charge-shifted C(1s) XPS peak, and this C is usually referredto as an adventitiousC (element not inherentto the sample). This C(ls) peakis consideredto be at 284.6eV (some use 285.0 eV as well). Most of the samplesurfacescontain C which is

X-RAY PHOTOELECTRONSPECTROSCOPY

365

derivedasa contaminantfrom air and/orvacuumsystemcontamination.The second commonmethodfor chargecorrectionis the use of a Au(4f7/2) line at 83.8 eV. In this methoda small amountof gold is depositedon the samplesurfaceto provide a better referencematerial. The latter is the best method; however,the disadvantageis that the Au( 4f7/2) binding energy hasbeenshownto vary with the thicknessof the Au deposition due to extra-atomicrelaxation (Kohiki et aI., 1983) resulting in a compositeAu peakdue to the Au/substrateinterfaceeffects. This results in differential charging between the sample and deposited Au. However,this is not a problemif a thicker Au film is used(Kohiki & Oki, 1985). The matterof choosingAu depositionor adventitiousC is a matterof choiceand care should be taken while calculatingabsolutebinding energies.

X-ray Photoelectron SpectroscopyAnalysis X-ray photoelectronspectroscopyprobesthe core level and valencelevel electronic structure of atoms in the near surface region. The valence electron occursin the region of 0 to 20 eV. It is difficult to probethe bondinginteractions at the surfaceof multicomponentmaterialsbecauseof the overlap of the many transitions,i.e., low intensity of the peak becauseof their low photoionization crosssection.Additional peaksfrom environmentalcontaminationmake it difficult to obtain a good spectrum.For this reason,most of the bonding interactions at the surfacesof solids are studiedutilizing the coreelectronicstates.X-ray photoelectron spectroscopyspectraare usually collected as wide or survey scans (usually ~ 1000 eV) and narrow or high resolution scans (usually 20-50 eV dependingon the elementof interest). The wide scans provide information for elemental identification. Table 12-2 providesa quick referenceof binding energydataof the elementscommon to the soil environment.A goodsourceof basicelementalreferencespectrais the Handbook of X-ray Photoelectron Spectroscopy (also known as the "Phi" handbook (Wagneret aI., 1979). However, recent instrumentationhas computerized elemental identification software. The narrow high resolution scans used for determiningthe peak'sexactbinding energyand areaof the peakfor delineation of the chemicalstateand quantitativeanalysis.The peakwidth, measuredas the FWHM, also may provide additional chemicalenvironmentalinformation. When identifying and analyzingXPS data,one shouldbe awareofa number of spectralfeaturesthat can help in the analysisprocess.It is important to note that peaksfrom p, d, and f levels occur as doubletsand that transition metals from unfilled "d" shells are broad becauseof multiple splitting (seebelow). For transitionelementsin paramagneticor high spin states,an additional peakis observedat high binding energieswhich is referredto as a satellite peak. Also, all the potentialAuger lines shouldbe considered.The most importantfactor that one has to considerin insulating surfacessuch as soils is samplecharging and this should be correctedeither by adventitiousC or gold-dot method. Some of the processesthat are involved in XPS are discussedbelow. Thesephenomaare useful for identification of elements,magneticstates,oxidation states,and other chemicalinformation.

VEMPATI ET AL.

366

Table 12-2. Principal binding energiesfor the elementsin numericalorder(adaptedfrom Watneret aI., 1979). Spin orbit Principal Core splitting or electronic x-ray binding energies Element levels source t7 tI4 t19 23 t22 25 t29 30 t31 37 t40 41 t42 43 48 49 t50 t51 53 t56 t56 60 t61 61 64 67 t69 t71 t73 75 77 t84 t87 89 89 90 98 t99 tI03 103 104 109 109 t111 t112 115 117 t118 118 121 122

Lu Hf Ga 0

Ta Sn Ge F W V

Re Ne As

Cr Mn I Mg Os Fe

Li

Se Co Ir Xe Na Ni Br Pt

AI

Cu

Cs

Au Kr

Zn Mg Ba Er Si Hg La

Ga Ce Ge Rb Be Pr Ho

Tl

A1'2s Nd Ge

4f7 4f7 3d 2s 4f 4d 3d5 2s 4f7 3p 4f7 2s 3d5 3p 3p 4d5 2p 4f7 3p Is 3d5 3p 4f7 4d5 2s 3p 3d5 4f7 2p 3p3 4d5 4f7 3d5 3p3 2s 4d5 A

2p3 4f7 4d5 3p3 4d5

A

3d5 Is 4d

A

[2] [2] [2] [1]

[2] [1] [2] [3] [1] [3] [2] [1] [3] [2] [3] [4] [1] [2] [3]

[AI]

[1] [4] [3] [3] [3] [Mg] [1] [AI]

4f7

[4]

4d 3p3

[4]

Principal binding energies t128 129 t130 t134 t137 tI40 141 t146 151 t152 tI56 t157 t159 163 tI64 t167 t175 t179 181 t182 182 186 188 t189 196 t199 t202 208 211 226 228 t228 240 t242 243 t253 260 260 260 262 262 270 271 279 t280 t285 285 t294 297 297 299

(continuedon next page)

Spin orbit Core splitting or electronic x-ray Element levels source Eu Sm P Sr Pb Gd As 1b

Si Dy y

Bi Ho Se S Er Tm Zr Se Yb

Br Ga P B Lu CI Nb

Kr Hf

Ta S Mo Rb

Ar

W Tc Re Na 1b

Zn As Sr Cl Os Ru C

1b K

Dy Ir Y

4d 4d 2p3 3d5 4f7 4d 3p3 4d 2s 4d 3d5 4f7 4d 3p3 2p3 4d 4d 3d5 A

4d 3p3

A

2s Is 4d5 2p3 3d5 3p3 4d5 4d5 2s 3d5 3p3 2p3 4d5 3d5 4d5

A A A A

3p3 2s 4d5 3d5 Is 4p3 2p3 4p3 4d5 3p3

[1] [2] [5] [5]

[2] [5] [6] [1] [2]

[AI]

[7] [Mg] [10] [2] [3] [8] [11] [12] [3]

[9] [2] [13] [4] [14] [Mg]

[AI]

[Mg] [AI]

[11]

[14] [4] [37] [3] [40] [15] [12]

X-RAY PHOTOELECTRONSPECTROSCOPY

367

Table 12-2. Continued. Principal binding energies 301 t307 309 315 320 321 330 t333 333 t335 335 335 341 342 t347 360 361 361 t368 369 t377 380 385 394 t398 t399 404 t405 408 412 419 436 440 440 t444 449 t454 462 480 N85 493 495 497 499 t512 525 526 t528 t531 533 551 561

Spin orbit Core splitting or electronic x-ray levels Element source Mg Rh Ho Pt Ar Er Zr Th Tm Pd Au Cu

Yb Ge Ca Lu Hg Nb

Ag Gd U K Tl Mo N Sc Eu Cd Ni Pb Ga Ne Ca Bi In Sm Ti Ru Co Sn Na Zn Rh Sc V Nd Dy Sb 0 Pd Fe Ti

A 3d5 4p3 4d5 2s 4p3 3p3 4f7 4p3 3d5 4d5 A 4p3

A 2p3 4p3 4d5 3p3 3d5 A 4f7 2s 4d5 3p3 Is 2p3

A 3d5 A 4d5

A A 2s 4d5 3d5 A 2p3 3p3 A 3d5 A A 3p3 2s 2p3 A A 3d5 Is 3p3 A 2s

[AI] [5] [44] [17] [47] [14] [9] [51) [5] [18] [Mg] [48] [AI] [3] [53] [20] [15] [6] [Mg]

[11] [21] [17] [5] [Mg) [7) [Mg] [22] [AI] [Mg) [24) [8] [Mg) [6] [22] [Mg] [8) [AI]

[AI] [24] [8] [Mg) [AI] [9] [27] [Mg]

Principal binding energies 564 568 573 t573 t574 599 600 602 619 t619 634 637 t639 641 653 665 667 669 t670 676 682 t685 685 696 n07 709 713 715 t726 726 736 738 745 758 767 772 t778 t781 781 784 797 799 816 820 832 833 835 t836 843 t853 t863 867

(continuedon next page)

Spin orbit Core splitting or electronic x-ray Element levels source Pr Cu Ag Te Cr F Ce Gd Cd I

La Eu Mn Ni Ba In Mn Ne Xe Th Sm F Cs Cr Fe Xe Co Sn Cs Cr U I 0 Nd Sb Te Co Ba V Fe Pr Sb Sn Te F Ce Ti

La In Ni Ne

La

A A 3p3 3d5 2p3 A A A 3p3 3d5 A A 2p3 A A 3p3 A A 3d5 4d5 A Is A 2s 2p3 A A 3p3 3d5 A 4d5 A A A 3p3 A 2p3 3d5 A A A A A 3p3 A A A 3d5 A 2p3 Is A

[Mg] [AI] [31] (10] [9] [Mg] [Mg] [AI] [34] [12] [Mg] [AI]

[11] [AI] [Mg] [38] [Mg] [AI]

[13] [37] [AI) [Mg] (13] [Mg] [AI] [42) (14] [Mgl [42) [Mg) [Mg] [AI) [46) [Mg) [15) [l51 [Mgl [AI] [AI] [Mg] [Mg] [51] [AI] [AI] [Mg) [17) [Mg] (18] [AI]

VEMPATI ET AL.

368 Table 12-2. Continued. Principal binding energies 870 874 t884 886 896 900 916 918 926 t932 t933 942 952 959 964 971 971 978 979 t981 990 1005 1006 1006 1014 t1022 1032 t1034 1039 1049 1068 1071 t1072 1076 1077 t1081 1086

Spin orbit splitting or Core x-ray electronic source levels Element Cd N Ce Ba Ag Mn Sc Cs Pd Pr Cu Xe Rh Cr Ca U I

0 Ru Nd C Te K Th V Zn Sb Pm Ar Sn Ti CI Na In B Sm Nb

A A 3d5 A A A A A A 3d5 2p3 A A A A A A A A 3d5 A A A A A 2p3 A 3d5 A A A A Is A A 3d5 A

Principal binding energies 1103 1103 1107 t:j:1117 1126 1129 1149 1154 1159 1161 1168 1179 1183 1185 1186 1197 1204 1212 t:j:1217 1223 1239 1239 1241 1272 1296 1299 1303 1304 1310 1319 t:j:1324 1336 1387 1394 1401 1412 1416

[Mg] [Mg] [18] [AI] [Mg] [AI] [Mg] [AI] [Mg] [20] [20] [AI] [Mg] [AI] [Mg] [Mg] [AI] [AI] [Mg] [21] [Mg] [AI] [Mg] [Mg] [AI] [23] [AI] [26] [Mg] [AI] [AI] [Mg] [AI] [Mg] [27] [Mg]

Spin orbit Core splitting or electronic x-ray levels source Element S Cd N Oa Eu Ag Sc Bi Pd Pb TI Hg Au Rh Od Ca U Ru Oe C Th K Tb Ar Dy Mo Mg CI B Nb As

S Bi Pb TI Hg Au

A A A 2p3 3d5 A A A A A A A A A 3d5 A A A 2p3 A A A 3d5 A 3d5 A Is A A A 2p3 A A A A A A

[Mg] [AI] [AI] [27] [30] [Mg] [AI] [Mg] [AI] [Mg] [Mg] [Mg] [AI] [32] [AI] [AI] [AI] [31] [AI] [AI] [AI] [35] [AI] [37] [AI] [AI] [AI] [AI] [35] [AI] [AI] [AI] [AI] [AI] [AI]

t Thosecore level lines that are most useful for identifying chemicalstates.For core electroniclevels: 3 sublevel3/2, 5 =sublevel5/2 and 7 sublevel7/2. Spin orbit splitting (in eV) is indicated in brackets.Auger lines are designatedby A and the x-ray sourceis indicatedin brackets. :j: Core level lines that are most useful in identifying chemicalstates,and are specialfor AI source.

=

=

Spin Orbit Splitting The XPS peakfrom p, d, and f energylevelsoccuras doublets.Thesedoublets, which result from spin orbit splitting, are useful for chemicalstate interpretation.Table 12-3 showsthat for I = 1, whereI is the orbital angularmomentum quantumnumber,the electronicstatessplit into two peakswith the peakfor the higher j level occurringat a higher binding energies.The relative intensities of thesepeaksare 1:2, 2:3 and 3:4 for p, d and f energylevels, respectively.

X·RAY PHOTOELECTRONSPECTROSCOPY

369

Table 12-3. X·ray nomenclaturefor the spin-orbit splitting. Subshell s p d

f

Is

Quantumnumber (i-coupling)

0 1 2 3

1/2 1/2,3/2 3/2,5/2 5/2,7/2

Area ratio

1:2 2:3 3:4

Satellites Satellitesdue to multiple electronexcitation and ligand transferprocesses are extremelyuseful in chemicalstatecharacterization.If suchinteractionoccurs during the photoionizationprocess,the photoelectronwill lose its energyand an additional peakwill be observedin the XPS spectrum.Thesepeaksoccur at the high binding energy side of the main photoelectronpeak and are termed as shake-upor shake-offsatellitesdependingon whetherthe other electronsbeing excited are only promotedto an excited stateor to the continuum,respectively. The transitionelementshave unpairedelectrons(paramagnetic)and producethe most intensesatellite structures.

Multiplet Splitting The emissionof a core level photoelectroncan be interactively coupledto one or more valenceelectronsthrough a phenomenonknown as multiplet splitting. In multiplet splitting, the unpaired"hole" createdby photoemissionin the core level interactswith the unpairedvalenceelectrons.This phenomenonis useful for studying the paramagneticmetal ions. Although the multiplet splitting phenomenoncausesbroadeningof the 2p photoelectronspectraof transition metal ions, it also producesan useful splitting of the 3s level as well. For example, the splitting distance, referred to as AE, betweenthe Mn(3s) peak (Fig. 12-5), hasbeenusedto identify the oxidation statesof Mn (Murray et aI., 1985).

Atomic ConcentrationsCalculations Quantitative analyseswith accuracy as good as 10% are possible using XPS, but care with calibration is needed.Using the relative heightsor areasof XPS spectralpeaksalongwith suitablecalibrationfactors, it is possibleto determine the relative elementalcompositionof the near surfaceregion of a sample. Currently,the mostwidely usedapproachfor XPS quantitativeanalysisis a combination of empirically derived sensitivity factors and a basic "Three Step Model" of photoemission.The model relatesmeasuredintensitiesto basicmaterial propertiesand involves: (i) photoionization, (ii) photoelectrontransportation to the surface,and (iii) transmissionto the detector.We will presenta brief discussionof the basicaspectsof quantitativeanalysisby XPS; however,the reader may wish to refer to texts by Powell and Seah(1990), Briggs and Seah(1983), Powell (1978) and Wagner(1978) for a more detaileddiscussion.

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Mn(3s)

Region

85

90

75

80

Binding Energy (eV)

7

;;$

6

~is.

5

om

Fig. 12-5. (a) Multiplet splitting in the Mn(3s) XPS spectrum, and (b) the experimentally determinedrelationship between the splitting distance(E) and Mn oxidation state(~) Oku et at. (1975); (x) Evans and Raftery, (1982); (+) Zhao and Young, (1984); and (D) Foord et at. (1984).

tI)

~ :;:"

0

1; 0

!

•

~ 4

+ 2

3

4

5

6

Mn Oxidation State

If we considera clean, flat sample,homogeneousin the analysisvolume, the intensity of a photoelectronpeak Ix (numberof photoelectronsper second) specific to elementx is given by

[6] where nx = the numberof atomsof the elementx per cubic centimeter, J = the x-ray flux in photons percentimetersquaredper second, (J = the photoionizationcross-section in centimeterssquared/atom, 8 = an angularefficiency factor basedon the angle betweenthe x-ray sourceand the detector, A. = the attenuationlength of the photoelectronin the sample, p = the probability that the photoelectronwill undergoenergylosses other than by collision (for example,plasmonloss), A = the areaof the samplefrom which photoelectronsare detected, F = the instrumentaldetectionefficiency. For a real sample,additional factors such as attenuationof the escapingphotoelectrons by the adventitious C layer and shadowing effects due to surface roughnessand/orparticle sizecancauseadditionalcomplications.Solving for nx , Eq. [6] can be rewritten as

[7]

X-RAY PHOTOELECTRONSPECTROSCOPY

371

such that the terms in the denominatorcan be consolidatedinto a single term called the S factor, Sx' Unfortunately,the S factor involves instrumentspecific, matrix specific as well as elementalspecific terms; consequently,researchersdo not use Eq. [7] for quantitativeanalysis.Instead,the ratio of two elementsin the sampleis used. For a homogenousbinary material (AB) the following equation may be written

[8] Equation[8] may be usedif the SA/SB ratio is matrix independent.This is possible if cr, A, and F for both elementA and elementB are assumedto changeidentically or by the samefactor for different materials.Using this assumption,sets of relative S factors may be developed(Briggs & Seah, 1983; Wagner et aI., 1979). Sensitivity factors are usually determinedexperimentally in the same spectrometerfor pure elementsamplesand reportedas a relative S factor, being normalizedso the fluorine (Is) S factor is one. A generalizedform of Eq. [8] can be written as

nx

Ix/Sx

C ----x-

En - U/S· i I I I I

[9]

wherethe atomicfraction of elementx, Cx, in a samplecanbe calculatedby measuringthe intensity of a particularpeakfor eachelement(total of i elements)present in the sample. Use of S factorsas describedabovewill normally providesemiquantitative results (within 10-20%). However, problemsmay arise becauseof inhomogeneoussamples,matrix effects (such as variation in A or cr), or C contamination. To compensatefor theseproblems,additional factors such as matrix correction factors (Briggs & Seah,1983) needto be incorporatedinto Eq. [9].

Angle ResolvedX-ray PhotoelectronSpectroscopy and Depth Profile Studies Angle-resolvedXPS isa nondestructivemethodto study the changesin the chemical composition as a function of depth. Sampling depth (escapedepth times three) is a function of the anglebetweenthe axis normal to the samplesurface and the electroncollection path. Henceby changingthe sampletilt angle, a variation in the depthof analysisis achieved.The effective depth of analysis (d) is calculatedusing the equation

d = 3Acos8

[10]

whereA the attenuationlength (measuredexperimentally)and8 is the sampletilt angle (Fig. 12-1). Ion sputteringtechniquesallow one to study the chemicalcompositionof minerals along a depth profile. Mineral surfacesare erodedby bombardingthe

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surfacewith energeticions of 0.5 to 5 keV energy.Argon ions are most commonly used.To determinethe amountof material removedthe rate of material removedmust be calibrated.This is bestdone by sputteringaway a thin layer of materialsimilar to the materialof interest.To accomplishthis, thin layersof Si02 grown on silicon often are used. From the electronicsindustry, the knowledge and technologyis availableto grow Si02 layers of precisethicknesson silicon wafers. Recently, comparisonsbetween untreatedand Ar-sputtered materials have been made to interpret the chemical changes along a depth profile (Hellmannet aI., 1990; Inskeepet aI., 1991).

SAMPLE PREPARATION How one preparestheir sampleis strongly dependenton thesefactors: (i) the natureof material, (ii) the vacuumconditionswhich needto be maintained, and (iii) any desired additionalexperimentaltreatments.If UHV conditionsare desired,the sample,its holder and anymounting material must be UHV compatible. This can be a problem when the samplehas relatively high vapor pressureor hasa tendency tooutgas.To achievea reliable XPS spectruma basepressureof 1.33 x 10-6 PA (10-8 Torr) is preferred.However,short of grosssurface contamination(fingerprints, cutting oil, etc.), most mineralswith the exception of somehighly porousmaterialswith poorly boundwater of hydrationare UHV compatible(Hochella, 1988). If possible,samplesshould be relatively flat and orientedperpendicularwith respectto the spectrometer,exceptin caseof angleresolvedstudies.The samplearea should be larger than the area of the beam is anadiameter(usually a coupleof mm2) andsinceonly the near-surface region lyzed, samplesmay be very thin. Most XPS systemsare only able to handle a sampleholder in a specific way; therefore,certain designelementsfor all the samplesholdersfor a particular systemmust be the same.Once theserestrictionsare met, the sampleholder can take on any numberof different forms to meet the needsof various sample and experimentalrequirements.Usually, theseholders are made from an UHV compatible materials that is conductive such as stainlesssteel, AI or 02-free high-conductivity(OFHC) copper.The choiceof materialcan be very important if any part of the holder is "seen" by the spectrometerbecauseof the spectral interferences. Samplesmay be mountedin variety of different ways. For larger single piece samples,e.g., rocks, a mechanicalmeansof mounting often works. This usually involves a screw or clip that holds the sample in place. Mechanical mountingworks well as long asthe clampinghardwareis not "seen"by the spectrometeror doesnot interferewith the spectrallines of interest.For smallersamples,silver paint or carbonpaint is often usedto glue down a sample.Gluessuch as thesework bestbecausethey are conductiveand UHV compatible.A couple of drawbacksto thesematerialsis that the bond may not be as secureas in the mechanicalcase,and for the porousmaterialsthe glue may permeatethroughthe sampleand appearon the surface.This is particulartrue for carbonpaint. Powder samplesareperhapsthe mostdifficult to mount on to a holder. Oneof the biggest

X-RAY PHOTOELECTRONSPECTROSCOPY

373

problemsis spectralinterferencefrom the mounting material as well as preferential charging. A number of different ways of mounting powder are used. Powder may be pressedinto pellet so they can be mounted by a mechanical method.The drawbackhere is that pressingmay affect the surfacechemistryof the sample.Another methodis to sprinkle the powderonto a double stick tape. Many haveusedthis methodsuccessfullybecause the principle spectralinterference is in the C region. Recently, a conductive double stick carbon tape has appearedon the market and has proven very useful for mounting conductive samplesand may help to reducechargingproblems.Hard powdersamplesalso may be pressedinto a soft metal foil such as In. To summarize,there is no best methodfor handling and mounting samples.The best rule is to avoid materials that have interfering lines and that eachsamplehas to be handledaccordingto the natureof the sampleand the experimentproposed.

SamplingHandling andTreatment Samplehandlingis extremelyimportantbecauseone hasto protectthe surfacesfrom extraneouscontaminations.After the samplehasbeenplacedinto the spectrometer,the samplemay be treatedin a specialchamber(Fig. 12-4). The samplescan be cleanedby mechanicalmeanswhile under vacuum using metal brushes,scrappersor samplecleavers.Referencematerialssuch as Au can be depositeddirectly on the samplein va:cuumor a reactantcan be addedvia the gas phaseor by depositionin an inert atmosphere.Also, the samplecan be heatedor cooled during chemicaltreatments. RECENT ADVANCES AND DEVELOPMENTS Significant improvementshave been developedin the XPS instrumentation in recentyears.One of the more importantdevelopmentis the ability to map a region of a sample,at a particularphotoelectronenergy,with a spatial resolution of lO)lm. This allows scientistsa meansof examiningchangesin the surface composition and chemical state acrossa heterogeneoussample. Furthermore, experimentsare underway in Siegbahn'slaboratoryto developliquid phaseXPS (Barr, 1991; Siegbahn,1990). This would be a valuable tool for probing the natureof interactionsat the vapor/solidand liquid/solid interfaces.In particular, this should be extremely useful in the study of chemical interactionsbetween clay and liquid and vapor phases.One areathat needimprovementis in decreasing the analysistime so that samplesthat are sensitiveto prolongedexposureto x-rays may be analyzed.This can be achievedby improving electronanalyzers and detectors(Hochella, 1988). ACKNOWLEDGMENTS Dr. David Cocke is the Jack M. Gill Professorof Analytical Chemistry located at Lamar University, Beaumont, Texas. Drs. Vempati and Hess are researchassociatesworking with ProfessorCocke. Their contributionsare partially supportedby the TexasAdvancedTechnologicalResearchProgramof the

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Texas Higher Education Coordinating Board and the Robert A. Welch Foundation.The authorsare grateful to Dr. Micheal F. Hochella, Jr., and two anonymousreviewersfor their commentsandsuggestions.Also, part of the manuscript contributed by RKV was written when he was a National Research Council postdoctoral research associateat the National Aeronautic Space Administration-JohnsonSpace Center, Houston, TX, and he is grateful to NASA-JSC and the National Academyof Sciences,Washington,DC, for providing this fellowship. The authorsalso are grateful to Mr. StevenJ. Pytel for assistancewith the figures.

REFERENCES Bancroft, G.M., I.R. Brown, and F.W. Fyfe. 1977. Calibrationstudiesfor quantitativex-ray photoelectronspectroscopyof ions. Anal. Chern.49:1044-1048. Barr, T.L. 1991.Advancesin the applicationof x-ray photoelectronspectroscopy(ESCA). II. New methods.Crit. Rev. Anal. Chern.22:229-325. Borade,R.B., A Adnot, and S. Kaliaguine. 1990. Characterizationof acid sitesin pentasilzeolites by x-ray photoelectronspectroscopy.J. Catal. 126:26-30. Briggs, D., and M.P. Seath.1983. Practicalsurfaceanalysis.JohnWiley & Sons,Inc., New York. Cocke,D.L., R.K. Vempati, and R.L. Loeppert.1994.Analysis of soil surfacesby x-ray photoelectron spectroscopy.p. 205-235.In J. AmonetteandI.W. Stucki (ed.) Quantitativemethodsin soil mineralogy.SSSA,Madison,WI. Defosse,C.P.,andP.G. Rouxhet.1980.Introductionto x-ray photoelectronspectroscopy.p. 169-203. In I.W. Stucki and w.L. Banwart(ed.) Advancedchemicalmethodsfor soil and clay minerals research.D. Riedel Publ. Co., Boston,MA. Eland,I.H.D. 1984. Photoelectronspectroscopy.Butterworth,Boston. Evans,S., and E. Raftery. 1982. Determinationof the oxidation stateof manganesein lepidolite by x-ray photoelectronspectroscopy.Clay Miner. 17:477-481. Foord,J.S.,R.B. Jackman,andG.c. Allen. 1984.An x-ray photoelectronspectroscopic investigation Philos. Mag. A49:657-663. of the oxidationstateof manganese. Gonzalez-Elipe,A.R., I.P. Espin6s,G. Munuera,J. Sanz,and J.M. Serratosa.1988. Bonding-state characterizationof constituentelementsin phyllosilicatemineralsby XPS and NMR.I. Phys. Chern.92:3471-3476. Hellmann,R., C.M. Eggleston,M.F. Hochella,Jr., and D.A Crerart.1990.The formation of leached layers on albite surfacesduring dissolution under hydrothermal conditions. Geochem. Cosmochim.Acta 43:1267-1281. Hochella,M.F. Ir. 1988.Auger electronand x-ray photoelectronspectroscopies. p. 573-637.In EC. Hawthorne(ed.) Spectroscopicmethodsin mineralogyand geology.Vol. 18. Min. Soc.Am., Washington,DC. Inskeep,w.P., E.A Nater,D.S. Vanderwoort,P.R. Bloom, and M.S. Erich. 1991.Characterizationof laboratoryweatheredLabradoritesurfacesx-ray photoelectronspectroscopyandtransmission electronmicroscopy.GeochimCosmochim.Acta 55:787-801. Kohiki, S., T. Ohmura,and K. Kusao. 1983.Appraisalof a new chargecorrectionmethodsin x-ray photoelectronspectroscopy.1. ElectronSpectrosc.Relat. Phenom.28:229-337. Kohiki, S.T., andK. Oki. 1985.An appraisalof evaporatedgold asan energyreferencein x-ray photoelectronspectroscopy.1. ElectronSpectrosc.Relat. Phenom.31:85-90. Koppelman,M.H., AB. Emerson,and I.G. Dillard. 1980. AdsorbedCr(lII) on chlorite, illite and kaolinite: An x-ray photoelectronspectroscopicstudy. Clays Clay Miner. 28:119-124. Muilenberg,G.E. 1979.Handbookof x-ray photoelectronspectroscopy.Perkin-ElmerCorp., USA. Murray, J.W.,I.G.Dillard, R. Giovanoli, H. Moers,andW. Stumm.1985.Oxidationof Mn(I1): Initial mineralogyoxidationstateand aging. Geochim.Cosmochim.Acta 49:463-470. Oku, M., K. Hirokawa,and S. Ikeda. 1975.X-ray photoelectronspecroscopyof manganese-oxygen systems.1. ElectronSpectrosc.Relat. Phenom.7:465-473. Payntner,R.W. 1988.Introductionto x-ray photoelectronspectroscopy.In B.D. Ratner(ed.) Surface characterizationof biomaterials.Elsevier,New York.

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Perry, D.L. 1990. Applications of surfacetechniquesto chemicalbondingstudiesof minerals.Am. Chern.Soc. Symp. Ser. 323:389-402. Powell, C.J. 1978. The physical basisfor quantitativeanalysisby Auger electronspectroscopyand x-ray photoelectronspectroscopy.p. 5-30. In N.S. Mcintyre (ed.) Quantitativesurfaceanalysis of materials.ASTM STP643. Am. Soc. Test Mater., Philadelphia. Powell, C.J.,andM.P. Seah.1990.Precision,accuracy,anduncertainityin quantitativesurfaceanalysis by Auger electron spectroscopyand x-ray photoelectronspectroscopy.J. Vac. Sci. Technol.8:735-763. Ratner, B.D. 1983. Study of biomaterialsby electron spectroscopyfor chemical anlaysis. Ann. Biomed. Eng. 11:313-336. Seah,M.P., and W.A. Dench. 1979. Quantitativeelectronspectroscopyof surfaces:A standarddata basefor electroninelasticmeanfree pathsin solids. Surf.InterfaceAnal. 1:2-11. Seyama,H., and M. Soma.1985. Bonding statecharacterizationof the constituentelementsof silicate minerals by x-ray photoelectron spectroscopy.J. Chern. Soc. Faraday Trans. 1 81:485-495. Seyama,H., and M. Soma. 1988. Applications of x-ray photoelectronspectroscopyto the study of silicate minerals.Res.Rep. no. 11. Natl. Inst. Environ. Stud.,Japan. Siegbahn,K. 1990.From x-ray to electronspectroscopyand new trends.1. ElectronSpectrosc.Relat. Phenom.51:11-36. Stucki, J.W., C.B. Roth, and W.E. Baithinger. 1976. Analysis of iron-bearingclay mineralsby electron spectroscopyfor chemicalanalysis(ESCA). Clays Clay Miner. 24:289-292. Vempati, R.K., R.H. Loeppert,and D.L. Cocke. 1990a.Mineralogy and reactivity of amorphousSiferrihydrites. Solid Statelonies 38:53-61. Vempati, R.K., R.H. Loeppert, D.C. Dufner, and D.L. Cocke. 1990b. X-ray photoelectronspectroscopyto differentiatesilicon-bondingstatein amorphousiron oxides. Soil Sci. Soc. Am. J. 54:695-698. Wagner,C.D. 1978. How quantitativeis electronspectroscopyfor chemicalanalysis?An evaluation of the significant factors. p. 31-46. In N.C. Mcintyre (ed.) Quantitativesurfaceanalysisof materials.ASTM STP643. Am. Soc.TestingMater., Pheladelphia. Wagner,C.D., and A.J. Joshi. 1980. The Auger parameter,its utility and advantages.A review. 1. ElectronSpectrosc.Relat. Phenom.5:259-266. Wagner,C.D., W.M. Riggs, L.E. Davis, J.F. Moulder, and G.E. Muilenberg. 1979. Handbookof xray photoelectronspectroscopy.Phys.ElectronicsDiv., EdenPrairie, MN. Zhao, L.Z., and V. Young. 1984. XPS studiesof carbon supportedfilms formed by the resistive depositoinof manganese. J. ElectronSpectrosc.RelatedPhenom.35:45-54.

Published 1996

Chapter13 X-Ray Absorption Fine Structure Spectroscopy SCO'IT E. FENDORF,University of Idaho, Moscow, Idaho DONALD L. SPARKS,University of Delaware, Newark, Delaware

With the ever-increasingimportanceof maintainingagriculturalproductivity and environmentalquality, it is essentialfor researchersto determinethe chemical and physical propertiesof soils and waters,and to ascertainand predict the fate of many substancesin thesesystems.Determining the elementalcomposition, chemicalspeciation,and reactivity of soils directly is essentialto accuratelycharacterize these media. Such information provides one with a knowledge of the bioavailability, mobility, and toxicity of addedfertilizers, pesticidesand herbicides,and pollutants. A multitude of spectroscopicand microscopic techniquesare currently availablefor investigatingsoils and soil chemicalreactions,with the numberof techniquesand their capabilities advancingrapidly. These techniquesprovide detailed information on constituentscomposingsoils and their chemical state. Unfortunately,no single techniqueis a panaceafor investigatingthesecomplex and heterogeneoussystems,thus it is beneficial to utilize a multitude of techniquesto obtain a completeand accuratedepictionof the chemicalenvironment. One method that has recently proven to be a powerful meansfor obtaining the speciationand local structureof elementspresentin soils is x-ray absorptionfine structure(XAFS) spectroscopy. X-rayabsorptionfine structurehas a variety of advantagesfor studying soils which include: elementspecificity, in situ investigations, determination of oxidation states,and the local chemical and structural environmentof an element. X-ray absorption fine structure offers information which is somewhat unique amongthe currently availablemethods.It probesthe local chemistryand structureof a single elementthroughouta sample.The oxidation state, type of nearestneighbors,coordinationnumber,bond distances,and orbital symmetries of the x-ray absorbingelementcan be accuratelydeterminedin an array of media (Eisenberger& Lengeler, 1980). Becausethe information obtainedwith XAFS differs from that of other spectroscopiesand microscopies,when used in conCopyright © 1996 Soil ScienceSociety of America and American Society of Agronomy, 677 S. SegoeRd., Madison,WI 53711,USA. Methods of Soil Analysis. Part 3. Chemical Methods-SSSA Book Seriesno. 5.

377

378

FENDORF& SPARKS

junction with them XAFS offers a complementarymeansfor detailing the propertiesof a soil and studyingreactionstherein. Vibrational spectroscopies(infrared and Raman)give information on the molecularaspectsand interactionsof a compound.Magneticspectroscopies such as nuclear magnetic resonance(NMR) and electron paramagneticresonance (EPR) can be usedto determinethe chemicalenvironmentof a species.Both of thesespectroscopies can be conductedundernoninvasivesampleconditions,and studiescan be performedon solids, surfaces,or solutions. They are, however, subjectto the necessityof having an elementor compoundwhich is activatedby the respectiveperturbation,a magneticfield or a low-energy photon induced excitation, and vibrational spectroscopiesprovide no direct electronicinformation. Electronicspectroscopies, e.g., ultraviolet (UV), visible (VIS), or luminescent, rely on the excitation of an element'svalanceelectrons.In so doing they give oxidation stateinformation, and undersomeconditionslimited information on the structuralenvironmentof the adsorbingelement.However,detailedstructural information, such as bond distancesand the type and numberof coordinating atoms,is not derivedfrom thesespectroscopies. Scatteringtechniquescan be employedon soils to determinestructural information on a sample; however, most conventionaltechniques,e.g., x-ray diffraction, only probe the long-range order of a sampleand not the local structureof a given element.Additionally, they do not provide chemicalstateinformation. Microscopic techniquesalso can be usedto study soils and soil chemical reactions, providing a wealth of information. Scanning electron microscopy (SEM) provides three-dimensionalparticle morphology and local particle surface featuresgenerally on a micrometerscale (although high-resolutionSEMs are capableof betterthan lO-nm resolution). Higher resolutioncan be obtained with transmissionelectron microscopy(TEM) so that details about the atomic orderingof a samplecan be obtained.Both TEM and SEM can be equippedwith energydispersivespectrometersfor elementalanalysis,and electronenergyloss spectroscopy(EELS) is availableon many TEMs yielding information similar to the edgestructurein XAFS. Recently introducedsurfaceprobing microscopies(SPM), such as scanning force (SFM) and scanningtunneling microscopies(STM), yield detailed surface information. Their application should provide exciting details on the microtopologyand reactivity of soil surfaces.All of the microscopictechniques provide a meansfor obtaining the spatial proximity of surface modification; however,with the exceptionof EELS and in somecasesSTM, they do not provide electronicinformation nor the local chemicaland structuralenvironmentof a given elementor compoundthroughoutthe sample.Additionally, most of the electronmicroscopiesmust still be conductedundera vacuumenvironmentand introduce the sample to electron bombardment-whichcan severely damage many samples. X-ray absorptionfine structurecircumventsmany of theselimitations and thereforeis an extremely useful tool for investigatingsoils and their reactions, especiallywhen employedin conjunctionwith complimentarytechniques.The attributesand limitations of XAFS are the focus of this chapter.The aim is to provide the readerwith an understandingof the physical bases,experimental

X·RAY ABSORPTION FINE STRUCTURESPECTROSCOPY

379

considerations,and dataanalysisproceduresof XAFS. We give a brief overview of the principlesof XAFS followed by a summaryof dataanalysistechniquesfor XAFS that provide structural parameters.The final portion of this chapter describesthe experimentalproceduresthat are employedwith XAFS, application of XAFS to soils is addressedthroughoutthis chapter.

PHYSICAL BASIS OF X-RAY ABSORPTIONFINE STRUCTURE When x-raysare impingedupon a samplethey can be elasticallyor inelastically scattered.As they passthrougha sampleof thicknessx they are exponentially attenuated.This attenuationis proportional to the sample thickness,the incident x-ray intensity (10), and a constantof the material at a particular energy (p, the material'sabsorptioncoefficient)

It

=10 exp (-px)

[1a]

which upon rearrangementcan be expressedas,

In Ie/It

=fIX

[lb]

where It is the transmitted intensity. Photoelectricabsorption is one possible inelasticprocessaccountingfor this attenuationand is the basisfor XAFS spectroscopy. The absorptioncoefficientdescribesthe ability of the materialto absorbxrays at a given energy.This parameter,p, is a function of energyso that an elementsability to absorbx-rays at different energiesis reflectedby the value of p. Sometimesa convenientway to expressthis quantity is with a massabsorption coefficient,pip, to accountfor the quantity of matter through which the photon beamhastraveled.Simple dividing p by the densityof the material,p, yields this parameter.Absorption coefficientsare additive so that a material'stotal absorption coefficient, (pip)m, is the summationof the absorptioncoefficientof its constituents:(p/P)m =l:{p,/p k The valuesof the absorptioncoefficientsfor most elements over a wide range of energieshave been tabulatedby McMaster et a1. (1969). Measuringchangesin p with changesin x-ray energyprovidesinformation on the chemicalandstructuralstateof the absorber(Sternet aI., 1975).The information containedin XAFS results from the x-ray excitation of an atom's core electrons.With sufficient energy input the core electronscan be completely ejectedfrom the atom, the energyrequiredfor this processis the electronbinding energy (BE). Quantum mechanicalselection rules specify that for energy absorptionto occur a transition exactly equal to the incident energy must take place.Thus, if an unoccupiedvalenceorbital is present,and the transitionof the core electron to this unoccupiedstate is symmetry allowed (which is governed by dipole selectionrules), then absorptionmay take place at a defined energy below that of BE. When the incident photon energy equalsthe BE of the core electron,absorptiontakesplace ejecting the electronfrom the atomic shell into

FENDORF& SPARKS

380

O PhotoeleClrOn :}

4>

:

.. >

o

. conttnuum

;

;

:

(,,~.:~.:..:..:~:_:...:~~.~__ ~,._::,".,:< } , , -: -:« ",'

_

"bound" valencestateS

. _' .:

: .:. ::,: :: ::~~ ~:~:~:~::... :.~.'< j \~ ~;)\,",.c

T

L shell

..

: :

h"~~ "

~ ~. .~ ~: ~ ~ ~ l \.

f.

j

K shell

i. •••••••• ;.:.;.:.;.:.;.;.::.:..:.:;.::.::.:..:.::.::••::•••.• .•.••••••••••••••

Fig. 13-1. An x-ray photon inducedexcitation of an atom'score-electron.The excited electroncan either enter unoccupiedorbitals or be ejectedfrom the atomic shell into the continuum.An electron which is ejectedfrom the shell is termeda photoelectron .

the continuumcreatinga photoelectron.Figure 13-1 gives a schematicdepiction of theseprocesses. In XAFS, an energyrangeis chosento encompassthe BE for the coreelectrons of the atom of interest.Since every elementhas a specific core configuration, the binding energiesare unique to each element.Thus, XAFS is element specific. For example,if one wished to analyzeeu in a sample,which has a Is electron binding energy of 8979 eV, the incident energy would start slightly below 8979 e V and then be increasedprogressivelybeyondthis energyvalue. The energy balancefor photoelectricabsorptionis such that the kinetic energy(KE) of the photoelectroncreatedby x-ray absorptionis equalto the incident energy(Ie) minus the BE of the excitedelectron:KE = Ie - BE. The abrupt increasein photon absorptionat BE producesan absorptionedge (Fig. 13-2). BeyondBE x-ray absorptioncontinueswith the photoelectronscreatedhaving a greaterand greaterKE. Figure 13-2 depictsan absorptionedgeillustrating three general spectral regions: the pre-edge,near-edge,and extendedportion. The near-edgeregion is denotedXANES (x-ray absorptionnearedgefine structure) while the region extendinggenerallyfrom 50 eV abovethe edgeand beyondis termedEXAFS (extendedx-ray absorptionfine structure). Becausethe absorptionprocessesare due to an electronictransitionof the core electron to an unoccupiedorbital or into the continuum,XAFS inherently possesses electronic information on the absorbingelement.This information is primarily locatedin the pre-edgeand XANES spectralregions.Electronic transitions are governedby dipole selectionrules. Theserules dictate that electron transitionsfrom one orbital to another(state-to-statetransitions)can only occur

381

X-RAY ABSORPTION FINE STRUCTURESPECTROSCOPY

X-ray absorption spectral regions

NEAR EXTENDED (EXAFS) EDGE (XANES)

-1+---~----'---~-----'----~---r----~---r----~--~

5750

6000

6250

6500

6750

7000

Energy (eV) Fig_ B-2. When the x-ray energyis sufficient to eject a core electron,a dramaticincreasein absorption occurs.The threespectralregionsresultingfrom this phenomenonare depicted:the pre-edge, the nearedge (XANES),and the extendedportion (EXAFS).

with unit changesin the orbital angularmomentumquantum number(Le., 61 = 1); s~p and p~d transitionsare allowed, but s~d or p~f are not. However, mixing of orbitals often occurs,thus transitionsthat would otherwisebe forbidden are observed.The excitation of a core electronto an unoccupiedorbital is thereforegovernedby the numberof unoccupiedstatesas well as the symmetry correlation betweenthe core and empty valence orbitals. These state-to state transitionsoften result in intense,sharpabsorbances that are termedwhite lines. This is exemplified by the strong edgefeature in Cr(VI) which resultsfrom the s~d transition(for examplesee,Davenport& Isaacs,1991; Manceau& Charlet, 1992; Bidoglio et aI., 1993); the tetrahedralsymmetryof CrOl- resultsin mixed p-d orbitals thus allowing for the otherwiseforbidden transition. This intense white line is absentin Cr(lll) due to the lack of p-d orbital mixing and fewer unoccupiedd-orbitals. Furtherinformation on the oxidation stateof the absorbingelementis provided by the energyposition of the absorptionedgeand white line. The binding energyof a core electronis a function of the electronconfigurationof the atomic shell. Sincethe numberof protonsis fixed, a varying numberof electronshave to compensatefor this charge. When valence electronsare removed from an atomic shell (oxidation), the remaining electronsare drawn closer to the positively chargednucleusto increasechargeshielding.As a consequence, the BE is increasedand a greateramountof energyis necessaryfor photoelectricabsorption--shifting of the absorptionedgeand white line(s) to higher energies.Thus, the absorptionedgeor white line energyposition can be used to determinethe oxidation state of an absorbingelement.In fact, for ionic compoundsa linear relationshipbetweenthe energyof the absorptionedge,or white line, and oxidation stateresults(Kunzl, 1932).

FENDORF & SPARKS

382

a)

hv

=>

A

Energy

b)

~-----~:-\~~ mulLiple scattering

Energy

Fig. 13-3. If an atom was not coordinatedby other atomsthen a smoothmonotonicdecayin x-ray absorptionwould occur at energiesgreaterthan BE (a). However, atomsare always in a coordinated environmentwhich results in the interaction of backscattered(in-coming) photoelectron wavesin the emanatingwaves(b). This producesa fine structurein the x-ray absorptionspectraat energies greater than BE.

As can be seenfrom electronstate-tostatetransitions,the x-ray absorption processesare dependenton the fmal-state wave function (Stem, 1974; Lee & Pendry, 1975), e.g., the overlap of the core and empty valenceorbital strongly determinesthe white line intensities.A further result of the fmal statewavefunction is that when theincidentenergyis greatenoughto createa photoelectron,its interactions with the atoms coordinating the absorberaffect the absorption process.If the excited atom were in an imaginaryvacuumenvironmentwith no other atomssurroundingit, then onewould observea smoothmonotonicdecay in the amplitudeabovethe absorptionedge(Fig. 13-3a).However,this is never the casein reality. All atomshave somekind of coordinatingenvironmentand thus the photoelectronis affectedby the interactionswith its surroundings. Electrons have both a particle and wave nature. Consideringthe wave propertiesallows one to glean insight as to the interactiveprocessesof the photoelectronand the neighboringatoms that producethe oscillations beyond the edge. When a photoelectronis created,the electronwave will propagateaway from the central atom (the absorber),it may scatteroff neighboringatoms,and then finally return to its point of origin. This scenariois schematicallyillustrated in Fig. 13-3band 13-4a.Undersuchconditionsthe outgoingwavesthen will interact with incoming waves that resultedfrom the backscattering.When the

X-RAY ABSORPTION FINE STRUCTURESPECTROSCOPY

383

a)

100cractjnc WaVes

b)

c)

-+----Fig. 13-4. A schematicdepictionof (a) the outgoingand incoming wave interactions,(b) the resultant amplitudeof in-phaseinteractionand (c) out-of-phaseinteraction.

outgoing and incoming waves are in-phasewith each other, constructiveinterferenceoccursenhancingabsorption(Fig. 13-4b). As the KE changes,the photoelectron wavelength changesuntil the waves are 1800 out-of-phase.This resultsin destructiveinterferenceand a loss in absorptionintensity (Fig. 13-4c). The consequence of thesescatteringphenomenaand wave interactionsis that the intensity of the x-ray absorption,at energiesgreaterthan BE, oscillate with a dependenceon the structuralenvironmentof the absorber.Mathematicallymodeling theseoscillations,basedon physical considerations,providespreciselocal structuralinformation on the absorber. The length that the photoelectrontravels, or its survival pathlength,similar to the ion meanfree path (IMFP), is dependentupon its KE. The pathlength is greatestat low KE, decreasesto a minimum, and then graduallyincreaseswith

FENDORF& SPARKS

384

0
5.5) to neutralize hydroxy AI and Fe is reduced. The importanceof clays,AI and Fe oxides,and organic matterto buffering of soil acidity is so greatthat somelime requirementtestsare basedon soil pH and eithersoil series(as an indicatoror clay and organicmattercontent)or some rapid estimateof clay (hand texturing) or organic matter content(loss on ignition). While theseapproacheshave beensuccessfulin many situations,they are rather time-consumingand thus have usually beenreplacedby one of the rapid chemicalmethodsdescribedin more detail below (e.g., buffer solutions).

Analytical Considerations Lime RequirementMethod A variety of techniqueshavebeenusedto estimatethe lime requirementof acid soils (Adams, 1984; McLean, 1982; Soil & Plant Analysis Council, 1992; van Lierop, 1990). The most commonapproachesnow usedare summarizedin Table 17-1 and discussedin detail in "Lime RequirementMethods." In brief, lime requirementmethodsconsistof either: (i) soil-lime incubations(ii) soil-base titrations, (iii) soil-buffer eqUilibrations,(iv) exchangeableacidity (or AI), or (v) estimatesbasedon soil pH and either soil seriesor somereadily measuredsoil propertythat is well-correlatedwith soil buffer capacity. Selectionof the most appropriatetechniqueto determinelime requirement must begin with an understandingof the nature and variability in the forms of soil acidity found in the dominantsoils of the geographicareawherethe test will be used. Considerableresearchand practical experiencehas clearly shown that different lime requirementtestsmust be usedin different physiographicregions to attain the most accuraterecommendations. This is clearly shownin Fig. 17-1

~~~~u~~~~~~~~~~~~

SMP single buffer

AR-Soil water pH and CEC estimatedfrom exchangeableCa CT-Soil pH, texture and organicmatter IL-Soil water pH, soil type, and croppingsystem LA-Soil pH, incrementaltitration of soil with CaC03 MD, NJ, RI-Soil pH and estimatedtexture TX-Soil pH, crop, and texture (routine test); soil pH and 2X KCI exchangeable AI in completesoil test VA-Soil water pH and estimateof soil texturebasedon soil type

Soil pH and measuredor estimatedsoil property

Someprivate labs--Soil water pH and % Ca + % Mg saturationof CEC NY-BaCI2-TEA titration method

AZ, MT, NO, NM, NV, SO, UT, WY

Other methods

None usedroutinely

VT-Soil pH in water and ammoniumacetatereactiveAI

NC,WV 10, MS, MO-Modified Woodruff buffer

Mehlich single buffer Other single buffers

Adams-Evanssingle buffer AL, DE, FL, GA, SC, TN

many regional, private soil testinglaboratories

Stateswhere methodis used

Lime requirement method

Lime requirementdeterminedbasedon titration to pH 8.2 and relationshipbetweensoil pH and basesaturation; titrant also must be protectedfrom CO2 and water vapor Calcareoussoils predominateand lime is rarely needed,exceptionsmay include severelydisturbed soils such as minespoil revegetationprojects

Soil type or texture estimatedby individual submitting sample,county agent,or consultant Soil pH actually measuredin 0.01 M CaCI2, then equatedto water pH by adding0.6 pH units

Woodruff buffer also must be protectedfrom CO2 and water vapor

Numerouslabs stressedthe importanceof properstorage of the SMP to protectagainstCO2 and water vapor. Most midwesternstatesuse the SMP method publishedin the NCR-13 regional soil testing manual (NCR-13, 1988) DE, FL use "buffer index" basedon soil pH in water, target pH of crop, and Adams-Evansbuffer pH

Comments

Table 17-1. Summaryof lime requirementmethodscommonly usedin the USA (basedon a survey conductedof soil testing laboratoriesin 1995).

~

~

i

~

~

t"l

i

SIMS

498

.SMP

c::J

o

MEHLlCH

o

ADAMS-EVANS OTHER

NONE USED ROUTINELY

Fig. 17-1. Summaryof lime requirementmethodscommonly usedin the USA in 1995. Map based on information collected in surveysand available in regional soil testing publications(NCR-13, 1988; NEC-67, 1995; SRlEG-18,1988).

which illustrates the predominanceof the Shoemaker-McLean-Pratt(SMP) buffer solution (Shoemakeret aI., 1961) in the Northeastand north centralUSA, while the Adams-Evansbuffer solution, which was developedfor low CEC, low organic mattersoils, is most commonly usedin the Southeastand Mid-Atlantic regions.Sincebuffer solutionsvary in chemicalcompositionby design,significantover-or underestimates of the amountof lime requiredcan result if the inappropriatesoil-buffer test is selected. Practicalconsiderations,suchas the amountof time availableto conducta test and return a lime requirementrecommendation,the space available and equipmentor suppliesrequired,any safetyor health hazardsthat may arise,and the cost per sample also must be carefully evaluatedwhen selecting a lime requirementtest. Generallyspeaking,this meansthat for routine soil testingpurposesa soil-buffer solution is preferred(or required),although severallarge soil testing laboratoriesobtain acceptableresultsby basinglime requirementon soil pH and soil seriesor soil properties.

LaboratoryProtocols Strict adherenceto standardizedlaboratoryprotocolsis essentialto obtain accurate,reproducibleresults with any soil testing method and is particularly importantfor lime requirementmeasurements. Severalregional publicationsare available that provide detailed protocols for the most commonly used lime requirement methods in the USA [NEC-67, 1995 (Northeast Regional CoordinatingCommitteeon Soil Testing); NCR-13, 1988 (North Central Regional Soil Testing Committee); SERA-IEG-18 (Southern Extension and ResearchActivity Information ExchangeGroup), 1988, 1992]. Analytical factors to considerwill vary with the type of lime requirementtest. In the caseof buffer solutions,the most commonerrorsare: (i) failure to correctly prepareand

LIME REQUIREMENT

499

store the buffer solution; (ii) inconsistentor incorrect adherenceto the required soillbuffer solution ratio; (iii) inconsistentor inadequatetime of reactionof the buffer with the soil; and (iv) failure to allow adequateequilibration time for the actual buffer pH measurement. Titration, colorimetric, or spectroscopicproceduresusedto measureexchangeableacidity or Al shoulduse appropriateextraction procedures(e.g., extractant,soil/solution ratio, shaking time and method, type of filter paper),carefully standardizedtitrating solutionsand accuratelyprepared standards for atomic absorptionor inductively coupledplasmaspectrometer (ICP) analysesof extracts. Methodsused to determinelime requirementby soil-lime incubation and soil-basetitrations have varied ratherwidely and a thoroughreview of the literature on this approach(Alabi et ai., 1986; Brown & Cisco, 1984; Fox, 1980; McLean et ai., 1978; Mohebbi & Mahler, 1988; Sims & Dennis, 1989; Tran & van Lierop, 1982) is strongly recommendedprior to initiating thesetime-consuming and costly studies.For lime requirementmethodsthat are basedon soil pH and anothermeasuredsoil property (most commonly organic matter), it is essentialthat the analytical methodsselectedreflect the proceduresused in the original calibration studies that determinedthe relationship between the soil propertiesand the amountof lime required(Keeney& Corey, 1963; Tran & van Lierop, 1981). For example,if lime requirementrecommendations are basedon soil pH and organic matter content estimatedby the Walkley-Black method, it would be inappropriateto use a loss-on-ignition (LOI) method to determine organicmatterunlessadditionalcorrelationor calibrationresearchwas conducted to verify the reliability of LOI as a similar indicator of soil buffer capacity.

LIME REQUIREMENT METHODS Field Estimationof Lime Requirement The most accuratemeansto determinethe lime requirementof a soil is through a field study. Typically, field studiesinvolve adding increasingratesof the desiredliming materialto the soil using commercial-scaleapplicationequipment, allowing thelime to react foran appropriateperiodof time underthe environmentalconditionsrepresentativeof the geographicareaof interest,and then measuringthe changein soil pH at eachlime rate (Doerge& Gardner,1988).The lime requirementof the soil can then be directly determinedfrom the resulting lime response curve (plot of final soil pH vs. rate of liming materialadded).The time and expenserequiredto conductfield studiesnormally precludesthe useof a wide rangeof soils, liming materials,and liming situations(e.g.,crop rotations, tillage and soil managementvariations). Becauseof this, field studiesof lime requirementare obviously not suitable for routine soil testing programswhere the goal is to rapidly predict the lime requirementfor a large and diversenumber of soils. In general,field studiesof lime responsehave been conductedrather infrequentlyand are usually doneto evaluatethe relative efficiency of a new liming material,to quantify the effect of somesoil or crop managementpracticeon the overall effectivenessof liming, or to serve as final verification for lime

SIMS

500

requirementtestsdevelopedby someof the morerapid methodsdescribedbelow. Field studies,however, are an important componentof lime and soil pH managementand shouldbe conductedwheneverpossible,particularly as significant changesin soil managementoccur (e.g., widespreadconversionto no-tillage from conventionaltillage) or if alternativeliming materials(e.g., lime stabilized sewagesludge)begin to be usedwidely throughouta region. Soil-Lime Incubations Soil-lime incubation studiesare the most commonmethodusedto characterize the lime requirementsof soils with differing physical and chemicalproperties and, to a lesserextent, to assesshow long-term changesin soil management may affect lime requirement.The accuracy of most of the rapid lime requirementtestsdescribedbelow (e.g.,buffer solutions)was initially verified by using soil-lime incubationstudies.The methodologyusedto conducta soil-lime incubation study is similar to that in a field study, although the smaller scale (greenhouse,laboratory)involved allows for the reasonablyrapid evaluationof many more soils. In brief, thesestudiesinvolve mixing increasingratesof the liming materialwith a fixed weight or volume of soil, equilibratingthe soil-lime mixture in a moist state for severalweeks or months either in a laboratory or greenhouse,and developing a lime-responsecurve basedon the resultantpH changes.The amount of lime required to effect the desiredpH changeis then relatedback to the propertiesof the soils or to other soil measurements, suchas the depressionin pH of a chemical buffer solution to which the soil has been added,thus providing a quantitative basis for lime recommendationsby these more rapid methods. While incubationstudiesare ratherstraightforward,there are a numberof importantfactors to considerwhen designinga soil-lime incubationexperiment. Among theseare the type andfinenessofliming materialto be used(e.g.,reagent gradeCaC03 vs. standardagriculturallime), the useof wetting and drying cycles to more closely approximatefield conditions,the most suitabletemperature,and the appropriatelength of time to conductthe incubation.Additionally, because microbial activity is often stimulatedwhensoils are incubatedunderwarm, moist conditions,salts can accumulatein the incubatingsoils, particularly nitratesof Ca, Mg, and K. Sinceexcessivesolublesaltsare well-known to decreasesoil pH, it is usually desirableto leach excesssaltsfrom soils prior to determiningpH at the conclusionof the incubation. Soil-BaseTitrations Another methodto estimatethe amountof lime requiredto neutralizesufficient soil acidity to attaina desiredsoil pH is to titrate (or equilibrate)a soil suspensionwith a basic solution, such as Ca(OH)z or NaOH (Alley & Zelazny, 1987; McLean et aI., 1978). Since soils are similar in many respectsto weak acids, a properly conductedsoil-basetitration should provide an accuratemeasure of both active and potential acidity and thus be well-correlatedwith lime requirement.Severalprocedureshavebeendevelopedto conductsoil-basetitra-

UME REQUIREMENT

501

tions, but most havethe following featuresin common:(i) the soil is suspended in a relatively concentratedsalt solution, suchas 1 M KCI, to displacesomeof the less readily accessibleforms of nonexchangeable acidity associatedwith clays, oxides and organic matter; (ii) a basic solution is addedeither through direct titration or by equilibratingseparatealiquotsof the soil-salt mixture with increasingincrementsof basefor severaldays. Direct titration is often a rather slow and tediousprocedurebecauseof the time requiredfor nonexchangeable forms of soil acidity to reactwith the base,hencethe latter methodis often preferred; (iii) soil pH is measuredafter sufficient time haspassedto allow for equilibration of the soil and the basicsolution. Assessinglime requirementby soilbasetitrations also requiresthat appropriateconversionfactorsto converttitratable acidity into units of practicalvalue (e.g., Mg lime ha-1) be developedand verified, usually though soil-lime incubationstudies.Recentadvancesin titrimetric instrumentationhavemadeit possibleto assesslime requirementthrough rapid (e.g., minutes) soil-basetitrations. Few routine soil testing laboratories haveadoptedrapid titration methodsat presentbecauseof the practicaldifficulties of integratingtheseproceduresinto the laboratoryand the lack of adequate calibrationdatabetweentitratableacidity and actualfield lime requirementvalues. Soil-Buffer Equilibrations

All of the proceduresto measurelime requirementdescribedso far have significant limitations that preventtheir use by routine soil testinglaboratories. This doesnot in any way diminish the role of field lime responsestudies,soillime incubations,andsoil-basetitrationsin the overall developmentof lime managementprograms.Thesemethodshavecontributedgreatlyto our understanding of lime reactionsin soils and are the foundationof the more rapid and inexpensive proceduresneededto provideroutineliming recommendations. However,as clearly seenin Fig. 17-1, the mostwidespreadapproachto assesslime requirement in the USA is throughthe useof soil-buffer equilibrations. Soil-buffer equilibrationsare conductedby adding a buffered chemical solution to a soil sample,allowing the soil and buffer to equilibratefor a relatively short period of time (e.g., 15-30 min), and then measuringthe pH of the soil-buffer mixture (McLean, 1978).The buffer solution containsa mixture of a weakacid anda salt of the sameweakacid andthuscanneutralizeboth acidsand bases,resulting in a strong tendencyfor the buffer to resist markedchangesin pH when soil is added.The pH of most buffer solutionsdecreasesin a linear manneras the acidity in the soil reactswith the chemicalbuffering agentsin the solution.The decreasein buffer pH is a measureof the amountof soil acidity that mustbe neutralizedby liming to raisethe soil from its presentpH to the desired pH. Soil-buffer equilibrationshave the advantageof allowing for a slower neutralization of soil acidity at a lower and more constantpH than can easily be accomplishedin soil-basetitrations whererapid additionsof basicsolutionscan causetemporaryincreasesin pH well abovethat encounteredin normal liming situations.The chemicalpropertiesof buffer solutionsvary by designaccording to the propertiesof the soils wherethe buffer will be used.For example,the two

SIMS

S02

most commonbuffers now usedin the USA are: (i) the SMP buffer (Shoemaker et aI., 1961), developedand usedon soils with lime requirements>4.5 Mg ha-1, soil pH valuesof exchangeable> ftxed (nonexchangeable) > structural(Martin & Sparks,1985; Sparks& Huang,1985; Sparks,1987). Soil solution K is the form of K that is directly taken up by plants and microbesand also is the form most subject to leaching in soils. Levels of soil solution K are generallylow, unlessrecentamendmentsof K havebeenmadeto the soil. Levelsof solution K are affectedby the equilibrium andkinetic reactions that occur betweenthe forms of soil K, the soil moisturecontent,and the concentrationsof bivalent cationsin solution and on the exchangerphase(Sparks& Huang, 1985). ExchangeableK is the portion of the soil K that is electrostaticallybound as an outer-spherecomplexto the surfacesof clay mineraland humic substances. It is readily exchangedwith othercationsand also is readily availableto plants. Nonexchangeable or "ftxed" K differs from mineral K in that it is not bonded within the crystal structuresof soil mineral particles.It is held betweenadjacent tetrahedrallayersof dioctahedraland trioctahedralmicas,vermiculites,and intergradeclay mineralssuch as chloritized vermiculite (Rich, 1972; Sparks& Huang, 1985; Sparks,1987). If one equatesnonexchangeable to "ftxed" K, than also it can occur in randomgapsin the structureof x-ray amorphousclay-sized minerals(Barber, 1979). Potassiumbecomes"ftxed" becausethe binding forces betweenK+ and the clay surfacesare greaterthan the hydration forces between individual K+. This resultsin a partialcollapseof the crystalstructuresandthe K+ are physically trappedto varying degrees,making K releasea slow, diffusioncontrolled process(Sparks, 1987). NonexchangeableK also can be found in "wedgezones"of weatheredmicasandvermiculites(Rich, 1964).Only ions with

HELMKE & SPARKS

554

a sizesimilar to K+, suchas NUt and H30+, canexchangeK from wedgezones. Large hydratedcations,suchas Ca2+ and Mg2+ cannotfit into the wedgezones. Nonexchangeable K is moderatelyto sparinglyavailableto plants,depending on severalsoil parameters(Goulding & Talibudeen,1979; Sparks& Huang, 1985; Sparks,1987). Releaseof nonexchangeable K to the exchangeableform occurswhen levels of exchangeableand soil solution K are decreasedby crop removal and/or leaching and perhapsby large increasesin microbial activity (Sparks,1980). Most of the total K in mostsoils is in the structuralforms (Sparks& Huang, 1985), mainly as K-bearing primary minerals such as muscovite,biotite, and feldsparssuch as microcline and orthoclase.For example,in some Delaware soils, Parkeret al. (1989a)found that structuralK averagedabout98% of the total K (Table 19-1). Most of the structuralK was presentas K-feldsparsin the sand fractions. Sadusky et al. (1987) studied the rate of K releasefrom sandy, Delawaresoils and the sand fractions of thesesoils usinga H-saturatedresin method.large quantitiesof K were releasedwithin 30 d, and significantamounts werereleasedfrom the coarse,medium,andfme fractionsof the soils. In Kenansville, Ap and B2t horizons57 and 54% of the total K release,respectively,could be ascribedto the three sand fractions. Using scanningelectron microscopy (SEM) analyses,Saduskyet al. (1987) found that the feldsparsin the sandfractions were extensivelyweathered,as evidencedby deepetchpits. Thesefmdings suggestedthat the feldsparK in the sandfraction could be importantin the overall K balanceand K-supplyingpowerof sandy,Delawaresoils, especiallyduring severalcrop growing seasons.The large quantitiesof structuralK in theseand otherAtlantic CoastalPlain soils could help in explainingthe often observedlack of crop responseto K amendments(Liebhardt et ai., 1976; Yuan et ai., 1976; Sparkset al., 1980; Woodruff & Parks,1980; Parkeret ai., 1989b). Potassiumfixation and releaseare influencedby severalfactors including: the typesand amountsof soil minerals,levelsof K in the solutionphase,sizeand degreeof weatheringof the mineral particles,soil pH, rainfall and temperature, manuring,soil structure,wetting anddrying andfreezingandthawing,biological activity and concentrationsof complexingorganic acids, redox potential of the soil, and plant roots (Rich, 1972; Sparks & Huang, 1985; Goulding, 1987; Sparks,1987). Of thesefactors, the mineralogyof the soil and the level of Kin the soil solution havethe greatestimpacton fixation and releaseof K.

Sodium Sodiumis not an essentialelementfor plants.Thereare reportsthat some cropsbenefitfrom Na (Bear, 1953;Tinker, 1967; Coughtreyet al., 1983)but the essentialityof Na remainscontroversial(Brownell, 1979). Plantsvary in their capacityto tolerateNa and thereare only a few reportsof direct negativeeffects of Na (Pearson,1960).Thereis no standardmethodthat assesses the plant availability of soil Na. The content of exchangeableand soluble Na are important parametersin the managementof salineandsodicsoils. Excesssalinity, of which Na can be partially responsible,reducesplant growth due to the osmoticeffect and the negativeeffectsof excessNa on soil properties.High concentrationsof

ALKALI METAL PROPERTIES

555

Na tend to dispersesoil colloids and increasesoil swelling. These phenomena decreasewater infiltration, aeration,and root penetration.The electricalconductivity of a saturationextractis normally usedas an index of salinization,while the Na adsorptionratio is usedto estimatethe hazardof sodification(Rhoades,1996, seeChapter14).

Lithium, Rubidium, and Cesium Theseelementshave receivedlittle attention in soil science.Rubidium is more abundantthanLi andCs in soils, at timesexceeding1000mg kg-t and averaging 100 mg kg-to The concentrationof Rb in sites from Illinois were highest where illitic materials are presentand lowest when loess is abundant(Jones, 1989). Typicalconcentrationsof Li in soils are 20 to 30 mg kg- t and that for Cs is about5 mg kg-to Soils derived from granitic/syeniticparentmaterial have the highestconcentrationsof theseelements.Lithium is much more mobile in soils than are Rb and Cs. The latter elementsare strongly adsorbedby vermiculitic clays (Franz & Carlson, 1978). The marked preferenceof siloxane sites for Cs over Li can be exploited to estimatethe structural chargedensity of 2:1 phyllosilicates(Anderson& Sposito,1991).The strongadsorptionof 137Cs(30 yr), a radionuclidefrom atmospherictestingof nucleardevicesin the 1950sand 1960s, to soils has servedas a useful marker in soil erosion/depositionand pedoturbation studies(Longmoreet aI., 1983; Southard& Graham,1992). Of the alkali elements,Li is the mosttoxic to plants.Indigenouslevels have been reportedto be toxic to citrus (Citrus spp; Bradford, 1966). Rubidium can partially substitutefor K in plants, as their propertiesare similar (Table 19-1). This has led to the use of the radiotracer86Rb (18.7 d) in K studiesof the soilplant-watersystembecausethe only suitable K radiotracerhas too short a halflife (42K, 12.4 h) to be practical (Diest & Talibudeen,1967). There is considerable controversyover the equivalenceof the Rb and K reactionsin the soil-plantwater systemand this substitutionshould be usedwith caution (Baligar & Barber, 1978; Eckert & McLean, 1980). Plant uptakeof Cs hasbeenstudiedbecause of concernabout the fate of radioactiveisotopesof Cs from nucleartests. The propertiesof Cs are sufficiently different from thoseof K and Rb that 137Cscannot be usedas a tracerfor K and Rb (Souty et aI., 1975). ANALYTICAL METHODS FOR THE ALKALI METALS The concentrationof the alkali metals are usually determinedby instrumental techniquesno matter how the samplesare prepared.Atomic emission spectroscopyis now the most popular technique.Instrumentaltechniqueshave the important advantagesof specificity, sensitivity, rapidity of analysis,and ease of use. They have therefore almost completely displacedwet chemical proceduresfor the analysisof the alkali metals.No new wet chemicalprocedureshave beenpopularizedin recentyears,and the readeris referredto earliercompilations if such types of analysesare needed(Knudsen et aI., 1982; Hillebrand et aI., 1953).

HELMKE & SPARKS

556

Selectionof the analytical method is often dictated by the availability of instruments.The method used to preparethe sample also must be considered when selectingan analytical technique.Extracts of soils are most conveniently analyzed by. the optical techniques, inductively coupled plasma-massspectroscopy(ICP-MS) or ion chromatographybecausethey require liquid samples. Solid soil samplesare most easily analyzedby neutronactivationanalysis(NAA) or x-ray fluorescence(XRF) becausethesetechniquesdo not require the sample to be dissolved.Although, the accuracyof XRF is improvedif a glassdisk of the sampleis preparedby fusion with lithium borate. The most sensitivewavelengthsand the detectionlimits for eachelement are given in Table 19-2 for the optical methodsof analysis.The detectionlimits are for optimumconditions.Practicalworking limits are severaltimeshigherthan the valuesgiven in Table 19-2. The detectionlimits for Rb and Cs with ICP-AES are so high that they cannotbe measuredin most soil samples.Atomic absorption spec~rophotometry (AAS) has the disadvantageof being a sequential~echnique but the instrumentsare lessexpensiveand widely available.Inductively coupled plasma-atomicemissionspectrophotometryis capableof simultaneousanalysis, which greatly increasesthe samplethroughput.The compositionof the matrix affectsthe resultsfrom the optical techniques.Analystsshouldconsultthe respective chaptersin this monographand the operationmanualof their instrumentsfor recommendedionization suppressants and other potentialmatrix interferences. Flame photometrycan be usedfor determiningNa and K in solution. The most sensitivelines are the sameas thosegiven in Table 19-2 for the otheremission techniques.The detectionlimit is usually low enoughthat theseelementscan be measuredin most extractsof soils. The exact detectionlimit varies with type of instrument and type of flame. Specific analysis techniquessuitable for the availableinstrumentshouldbe used(Hossner,1996,seeChapter3). Ion chromatographyalso is suitable for measuringthe alkali elementsin solution, althoughit seemsto be not commonly usedfor soils (Nieto & Frankenberger,1985; Basta& Tabatabai,1985). The detectionlimits of modemcolumns and detectorsfor the alkali ions are estimatedto be 10 to 100 ng mL-l. The detectionlimits and information on the alkali elementsfor ICP-MS, NAA, and XRF are given in Table 19-3. The sensitivity of ICP-MS is extremely low exceptfor K. The relativeinsensitivity for K occursbecausethe dominantK isotope,39K, falls on the low masstail of the very large peak resultingfrom 40Ar Table 19-2. Approximatelimits of detectionin single elementsystemsfor flame and graphitefurnace AAS and ICP-atomicemissionspectroscopy(AES). Element

Wavelength

Flame AASt

nm Na K Rb

670.8 589.0 766.5 780.0

Cs

852.1

Li

FurnaceAAS*

ICP-AES

ng mL- 1

pg

ngmL-1

35 15 100

2.5 0.2 0.5 1

200

6

40

t Concentrationto give 1% adsorption. * Characteristicmassneededto give 1% adsorption. § NO =not determinable.

5

100

125

NO§ NO

ALKALI METAL PROPERTIES

557

in the carriergas. Inductively coupledplasma-massspectroscopyis intolerantof high concentrationsof dissolvedsolids. This limits the rangeof suitablesample preparationtechniquesunless the samplesare diluted. The easiestmethod to determinethe alkali elementsin solid samplesis NAA becausethe samplesdo not have to be dissolved.The distribution of researchreactorsunfortunatelylimits the availability of this technique,although most academicreactor facilities will analyzesamplesfor a modestfee. Very few soil researchfacilities haveXRF equipment,but t.his techniquecan determineNa and K in solid samples.

ANALYSIS FOR TOTAL ALKALI ELEMENTS Methods to determinetotal K and the other alkali elementsin soils use acids or a high temperaturefusion to decomposethe soil. The most widely employeddigestiontechniquesfor total elementsin soils and mineralshave used combinationsof HF and eitherH2S04 or HCl04 (Klesta& Bartz, 1996,seeChapter 2). The use of two acidsservesseveralpurposes.The HF decomposesthe silicate mineralsby reactionof F with Si to form SiF4, which is volatile when it is heatedwith strong acids. The secondacid should be an oxidizer to dissolveany humic substancesin the soil sample.Since the alkali elementsare not a part of any organicmolecule,the purposeof the oxidant is to preventtheir retentionon organiccolloids, removeexcessHF from the soil sample,and avoid any interferencesin the subsequentanalytical procedures. The solution resulting from HF digestionsmust be treatedwith boric acid to convertexcessF to fluoroboric acid beforethe samplesare transferredto glass containers.Even boric acid-treated solutions containenoughF to etch glasscontainerswith time and all HF-digestedsamplesmust be storedin plastic containers. Long-term analysisof solutions containing F also damagesthe quartz and glassnebulizersfound on ICP instruments.Soils that havehigh concentrationsof Ca may form so much CaF2that a portion of it remainsinsolubleafter the digesTable 19-3. Approximate limits of detectionfor ICP-MS, instrumentalneutron activation analysis, andXRF. ICP-MS Stable Element isotope

Detectiontlimit

NAA Radioactiveisotope from (n, 1) reaction Detectionlimit:j:

mgkg·1

ngmL·1 Li Na K Rb Cs

7Li 23Na 39K 85Rb \33Cs

0.1 0.06 10 0.02 0.02

XRF detectionlimit§

ND~

24Na 42K 86Rb 134Cs

ND 0.8 (0.01) 30 (0.5) 0.4 0.03

ND 15 30 1 1

t Three standarddeviationsabovebackground. :j: Three standarddeviationsabovebackgroundwith a 2-h neutronirradiation of 1 g of whole soil at 1 x 1013 n cm-2 S-1 and a single radioassayafter 5 d. The values in parentheses are the detection limits with a lO-min irradiation and radioassaythe sameday. § Three standarddeviationsabovebackgroundwith pressedpellet or glassdisk. 'II ND =not determinable.

HELMKE & SPARKS

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tion and addition of boric acid. Greaterdilution of the digestedsamplecan dissolve the CaFz but this raisesthe limit of detection. Finely groundsoil samplesalsocan be decomposedby fusion with NaZC03 or NazOz (Klesta & Bartz, 1996, seeChapter2). Obviously Na cannotbe determined by such procedures. Blanks also must be checkedbecausethe flux often containsappreciableamountsof Li, Rb, and Cs. Fusionwith NaZC03 in platinum cruciblescan damagethe expensivecruciblesbecausesomesoil elementsform alloys with Pt. Fusionwith NazOzis very effective in destroyingsoils but NazOz with sufficiently low blank concentrationsof Li, Rb, and Cs is rarely available. Zirconium cruciblesmust be usedwith NazOz fusions. Their price is reasonable of fusion techniquesare and they last for at least50 fusions. Major disadvantages the amountof analyteelementintroducedby the fusion flux, andthe largeamount of flux requiredresultsin high concentrationsof dissolvedsolids in the samples. High concentrationsof dissolvedsolidsare not toleratedby ICP techniques,especially ICP-MS. The proceduredescribedbelow employs a Teflon bomb method and is basedon those of Bernas(1968) and Buckley and Cranston(1971). It also is applicablefor microwave digestionwith the proper bomb or the bottle method describedby Klesta and Bartz (1996) in Chapter2. Hydrofluoric Acid Digestion for Total Alkali Elements Apparatus 1. 2. 3. 4. 5. 6. 7.

Agate mortar. Teflon bombsand metal containers. Oven. Volumetric flasks, 100 mL. Plasticstirring rods. Plasticbottles, 100 mL. Plasticpipettes,10 mL.

Reagents 1. Aqua regia. Mix one part of concentratedHN03 with three parts of concentratedHCl. Include one part of deionizedwater if the aquaregia is to be stored for any length of time. Without water, objectionable quantitiesof CI and othergasesare evolved. 2. ConcentratedHE 3. Boric acid. Procedure 1. Grind soil in an agatemortar to passa 0.14-mm(IOO-mesh)screen. 2. Weigh 0.5 g of soil into a Teflon bomb. 3. Add 1 mL of aquaregia and 10 mL of HF to the samplein the Teflon bomb using plastic pipettes. 4. Placethe coveredTeflon bomb in its metal container. 5. Heat the samplein an oven at 383 K for 3 h.

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6. Add 2.8 g of boric acid to a 100-mL plastic volumetric flask. 7. Pour the liquid out of the Teflon bomb into the volumetric flask. Wash the remainderof the liquid out of the bomb into the flask with deionized water and thoroughly mix the contentsof the flask. S. Dilute the solution in the flask to 100 mL with deionizedwater and store in sealedplastic bottles. 9. Analyze the solution for the alkali elementsusing AAS, ICP, or Ie.

MEASUREMENT OF SOIL POTASSIUM A numberof methodscan be employedto measurethe quantitiesof K in the various defined forms of soil K. Only the most commonly usedmethodsare detailedin this chapter. ExchangeablePotassium ExchangeableK is that K which is extractedwith a neutralnormalsalt, usually 1.0 M NH40Ac minus the water-solubleK (Knudsenet aI., 1982). In soils that are not saline, levels of water-solubleK are minimal and can be ignored. However,in salinesoils, the levelsof water-solubleK shouldbe determinedfrom a saturatedextractor somesimilar extract andsubtractedfrom the amountof K determinedusing NH 40Ac. It should be noted that in soils that contain weatheredvermiculitic and micaceousminerals"wedge zones"can be presentthat contain K. This K is not accessibleto large index cationssuchas Ca and Mg, but can be extractedby N~, which is of similar size to K. For example,in soils that contain "wedge zones," NH40Ac will extract more K than an extractantlike 1 M CaCI2. It is debatable whetherthis K is truly "exchangeable."Thus, in soils containing"wedge zones" exchangeableK could be overestimatedwith NH40Ac (Sparks & Liebhardt, 1981; Sparks& Huang, 1985). Ammonium AcetateMethcd The methodoutlined below is that of Thomas(1982).

Apparatus 1. 2. 3. 4. 5.

Erlenmeyerflask, 125 mL. Centrifugetubes,50 mL. Buchnerfunnels, 5.5 cm. Volumetric flasks, 50, 10, 250 mL. Centrifuge.

Reagents 1. Ammonium acetate(NH40Ac), 1 M: Add 70 mL of reagent-grade NH40H, sp gr 0.90, to 57 mL of 99.5% aceticacid per liter of the final solution desired.Do the mixing in about half the final volume of distilled water desired, make up to volume with additional water, mix,

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thoroughly cool, and adjust exactly to pH 7.0 with either NH 40H or aceticacid. (Only a small volume of acid or basewill be required.) BuchnerFunnelProcedure.Place10 g of 0.82t

Na20/CaO 0.82-0.27

Na2O/CaO >0.82

96.5 96.0 93.6 84.4 64.0

95.2 94.5 92.7 86.2 68.3

93.7 90.9 77.4 41.9 16.4

95.2 94.5 92.7 86.2 68.3

Microcline,

Na20/CaO 0.82-0.37

Quartz, D

93.7 90.9 77.4 41.9 16.4

99.6 99.4 99.1 98.2 96.5

t Ratio of the percentageby weight of the oxidespresentin the original sample.

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the amountof K that is neededto maximizeplant yields. Soil testsfor K usually estimatethe quantity of solution and exchangeableK, and sinceacids are usually employedas extractants,somenonexchangeable K also is extracted(Wolf & Beegle,1991).A numberof different soil testsareusedto measureextractableK. These include: Mehlich 1 and Mehlich 3 proceduresin the northeasternand southeasternUSA, the Morgan and modified Morgan proceduresin partsof the northeasternUSA, the 1 M N~OAc at pH 7 procedurein the north centralUSA, and the ammoniumbicarbonate-DTPAextractionin the westernUSA. Mehlich 1 Extraction The methodoutlined below is that of Mehlich (1953). Apparatus

1. Reciprocatingor rotary shaker,capableof at least 180 opm (oscillations per min). 2. Standardstainless-steel scoops,1 and 5 g. 3. Erlenmeyerflasks (150 mL) and filter funnels for extraction. 4. Filter funnels Reagents

1. Mehlich 1 extracting solution (0.05 M H2S04 + 0.05 M HCl): Also referredto asdilute doubleacid or the North Carolinaextractant.Using a graduatedcylinder, add 160 mL of concentratedHCl and 27 mL of concentratedH2S04 to approximately30 L of deionizedwater in a large polypropylenecarboy.Make to a final volume of 40 L by adding deionizedwater. Mix well by bubbling air into the solution for 3 h. 2. Activated e. Procedure

1. Scoop5 g of sieved,air-dried soil into a 150-mL extractionflask. 2. If it is necessaryto obtain a colorlessfiltrate, add 1 g of activatedC to eachflask. 3. Add 25 mL of the Mehlich 1 extractingsolution and shakefor 5 min on a reciprocatingshakerset at a minimum of 180 opm. 4. Filter through a medium-porosityfilter paper (Whatman no. 2 or equivalent). 5. Analyze for K using AAS, ICP, or Ie. Mehlich 3 Extraction The methodoutlined below is that of Mehlich (1984). Apparatus

1. Stainless-steel soil scoops,1 and 2.5 g.

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2. Reciprocatingshaker,Capableof 180 opm. 3. Plastic(PVC), wide-mouth,extractionbottles, 100 mL. 4. Filter funnels. Reagents

1. Mehlich 3 stock solution (3.75 M NH4F + 0.25 M EDTA): Mix 277.8 g ammoniumfluoride (NH4F) with approximately1200 mL distilled water in a 2-L volumetric flask. Add 146.1 g of ethyleneidaminetetraaceticacid {EDTA =[(HOOCCH2)2NCH2N(CH2COOHh]}.Dilute to volume andmix well. This providesenoughstocksolutionfor about 10 000 samples. 2. Mehlich 3 extractionsolution (0.2M CH3COOH + 0.25 M NH4N03 + 0.015 M NH4F + 0.013 M HN03 + 0.001 M EDTA): Dissolve 1000g ammoniumnitrate (NH4N03) in approximately40 L of distilled water in a 50-L calibrated,plastic carboy. 3. Activated carbon. Procedure

1. Scoop2.5 g of air-dried, sievedsoil into a 100-mL extractionbottle. 2. If it is necessaryto obtain a colorlessfiltrate, add 1 g of activatedC to eachflask. 3. Add 25 mL of the Mehlich 3 extractionsolution to eachbottle. 4. Shakeat 200 opm for 15 min on a reciprocatingshaker. 5. Filter through a medium-porosity filter paper (Whatman no. 2 or equivalent). 6. Analyze filtrate for K as given earlier. Ammonium AcetateExtractablePotassium The methodoutlined below is that of Brown and Warncke(1988). Apparatus

1. 2-g scoop. 2. Automatic or semiautomaticextracting solution dispenser,10 or 20 mL. 3. Extractingflasks, 50-mL Erlenmeyeror conical flasks. 4. Funnels(or filter holding devices). 5. Receivingreceptacle,20 to 30 mL beakersor test tubes [note-Most high volume soil testing laboratorieshave racks of extractingflasks, funnelsand receivingreceptaclesdesignedto handlemultiple soil samples at one time.] 6. Rotatingor reciprocatingshakercapableof 200 opm. Reagents

1. Extractingsolution (1 M NH40Ac at pH 7.0): (i) Placeapproximately 500 mL of distilled water into the mixing vessel.Add 57 mL of glacial

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acetic acid (99.5%) then add 69 mL of concentratedammonium hydroxide(note-Mix in a fume hood). Bring the volume to about900 mL with distilled water. Adjust to pH 7.0 with 3 M NH40H or 3 M acetic acid. After cooling to room temperature,bring the solution to a volume of 1 L. (ii) (Alternative) Reagent-gradeammonium acetate may be used. Add 77.1 g of NH40Ac to 900 mL of distilled water. After dissolutionof the salt adjust the pH to 7.0 as above. Dilute to a final volume of 1 L. (Checkthis solutionfor K contaminationfrom the salt.)

Procedure 1. 2. 3. 4.

Scoop1 g of preparedsoil into an extractionflask. Add 10 mL of extractingsolution to the extractionflask. Shakefor 5 min at 200 opm. Filter the suspensionsthrough Whatman no. 2 or equivalent filter paper.Refilter or repeatif the extractis cloudy. 5. Analyze for K as given earlier.

Morgan Extraction The methodoutlined below is that of Morgan (1941).

Apparatus 1. Stainless-steelsoil scoops,1 and 10 g. 2. Reciprocatingshaker,capableof 180 opm. 3. Extractionflasks, 125 mL.

Reagents 1. Morgan extractant(0.72 M NaOAc + 0.52 M CH3COOH).Add SOOO g of sodium acetatetrihydrate (CH3COONa.3HzO) to a SO-L carboy containingapproximately20 L of distilled water. Add 1450mL glacial acetic acid and mix until the sodium acetateis dissolved.Dilute to SO L with distilled water and mix well. The pH of the solution shouldbe 4.8 +/- 0.05. If necessary,adjust to 4.8 with sodium acetateor acetic acid. 2. Activated C.

Procedure 1. Scoop10 g of air-dried, sievedsoil into 12S-mL Erlenmeyerflasks. 2. If it is necessaryto obtain a colorlessfiltrate, add 1 g of activatedC to eachflask. 3. Add 50 mL of the Morgan extractantto eachflask. 4. Shakeat 180 opm for 15 min on a reciprocatingshaker. 5. Filter througha mediumporosity filter paper(Whatmanno. 2 or equivalent).

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6. Analyze filtrate for K as given earlier.

Modified Morgan Extraction The methodoutlined below is that of McIntosh (1969).

Apparatus 1. 2. 3. 4.

Stainless-steelsoil scoop,1 and 4 g. Reciprocatingshaker,capableof 180 opm. Extractionflasks, 50 mL. Filter funnels.

Reagents 1. Modified Morgan extractant(0.62 M N~OH + 1.25 M CH3COOH): Add 2874 mL glacial aceticacid to a 40-L carboycontainingapproximately 20 L of distilled water. Add 1825 mL concentratedNH40H. Dilute to 40 L with distilled water and mix well. The pH of the solution shouldbe 4.8 ± 0.05. If necessary,adjustto 4.8 with concentrated NH40H or aceticacid. 2. Activated C.

Procedure 1. Scoop4 g of air-dried, sievedsoil into 50-mL extractionflasks. 2. If it is necessaryto obtain a colorlessfiltrate, add 1 g of activatedC to eachflask. 3. Add 20 mL of the Morgan extractantto eachflask. 4. Shakeat 180 opm for 15 min on a reciprocatingshaker. 5. Filter througha mediumporosityfilter paper(Whatmanno. 2 or equivalent). 6. Analyze filtrate for K as given earlier.

Ammonium Bicarbonate-Diethylenetriamine Pentaacetic Acid Extraction The methodoutlined below is that of Soltanpourand Schwab(1977).

Apparatus 1. Reciprocatingshaker,capableof 180 opm. 2. Extraction flasks,125 mL. 3. Filter funnels.

Reagents 1. Ammonium bicarbonate[diethylenetriaminepentaaceticacid (DTPA) extractingsolution (0.005M DTPA, 7.6 pH) 1.97 g DTPA to 800 mL water. Add 2 mL of 1:1 NH40H to enhancedissolutionand to prevent

ALKALI METAL PROPERTIES

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effervescencewhen the bicarbonateis added.When most of the DTPA is dissolved,add 1 mol (79.06 g) of NH4C03 and stir gently until the mixture is dissolved. Adjust the solution pH to 7.6 with ~OH. Dilute the solution to 1 L with water, and either use immediately or store undermineral oil to maintainstablepH.

Procedure 1. Weigh 10 g of soil into a 125-mL Erlenmeyerflask. 2. Add 20 mL of ammoniumbicarbonate-DTPAextractingsolution. 3. Shakethe soil mixture on a reciprocatingshakerfor 15 min at 180 opm with the flasks open. 4. Filter the extractsand analyzefor K as given earlier.

EXCHANGEABLE AND SOLUBLE SODIUM

Introduction The highestconcentrationof soluble alkali cationsin soils is that of Na in the salt-affectedsoils that are classifiedas saline-sodie(Bresleret aI., 1982).The concentrationof exchangeableNa varies from trace amountsto a major portion of the exchangecapacity,dependingon the soil environment.The concentration of exchangeableNa is generallysmall comparedto the total concentrationof soil Na.

Procedure The concentrationof exchangeableNa in soils is usually determinedby extraction with neutral 1.0 M ammoniumacetate(see section on "Ammonium AcetateExtractablePotassium").Other extractants,such as thoseusedfor CEC measurements, also can be usedfor Na (Sumner& Miller, seeChapter40). Since the extractantalso dissolvessolubleNa, the resultsmustbe correctedfor the concentration of water-solubleNa. Water-solubleNa is measuredin a saturation extract (Rhoades,1996, see Chapter 14). Appropriate adjustmentsfor dilution factors and soil weights must be madeso that the valuesmeasuredin the ammonium acetateand saturationextractsare consistent.The concentrationof watersoluble Na is insignificant comparedto exchangeableNa for most soils of the humid region but can be importantfor soils of dry regions.

LITHIUM, RUBIDIUM, AND CESIUM

Introduction The dissolvedforms of theseelements haveattractedlittle attentionexcept of the exchangeablefraction of for Li, as discussedearlier. Few measurements theseelementshavebeenmade.Neutral 1.0M ammoniumacetateis the preferred

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extractant.ExtractableLi in 12 soils from California rangedfrom 0.1 to 0.9 mg kg-1 and averaged0.3 mg kg-1 (Bradford & Pratt, 1961). ExtractableRb in S9 soils from Illinois averaged1.18 mg kg-1 (Jones,1992).

Procedurefor ExtractableLithium or Rubidium Use the ammonium acetateextraction method given in the section on "Ammonium AcetateExtractablePotassium."

REFERENCES Anderson,S.l., and G. Sposito.1991. Cesium-adsorptionmethodfor measuringaccessiblestructural surfacecharge.Soil Sci. Soc. Am. I. 55:1569-1576. Arnold, P.W. 1958. Potassium uptake by cation exchangeresins from soils and minerals. Nature (London) 182:1594-1595. Baligar, V.C., and S.A. Barber. 1978. Potassiumand rubidium adsroptionand diffusion in soils. Soil Sci. Soc. Am. I. 42:251-254. Barber,R.G. 1979. Potassiumfixation in someKenyan soils. I. Soil Sci. 30:785-792. Basta, N.T., and M.A. Tabatabai.1985. Determinationof exchangeablebasesin soil by ion chromatography.Soil Sci. Soc. Am. I. 49:84-89. Bear, F.E. (ed.). 1953. Sodium symposium.Soil Sci. 76:1-96. Bernas,B. 1968.A new methodfor decompositionand comprehensiveanalysisof silicatesby atomic spectrophotometry. Anal. Chem.40:1682-1687. Bertsch,P.M., and G.W. Thomas.1985. Potassiumstatusof temperatureregion soils. p. 131-162.In R.E. Munson(ed.) Potassiumin agriculture.ASA, CSSA, and SSSA, Madison,WI. Bradford, G.R. 1966. Lithium. p. 218-224.In H.D. Chapman(ed.) Diagnosticcriteria for plantsand soils. Div. Agric. Sci., Univ. California, Riverside,CA Bradford,G.R., and P.F. Pratt. 1961. Separationand determinationof lithium in irrigation water,plant material,and soil extracts.Soil Sci. 91:189-193. Bresler,E., B.L. McNeal, and D.L. Carter. 1982. Salineand sodic soils. Springer-Verlag,Berlin. Brown, I.R., and D. Warncke. 1988. Recommendedcation tests and measuresof cation exchange capacity.p.15-16.In W.C. Dahnke(ed.) Recommendedchemicalsoil test proceduresfor the North Central Region. North DakotaAgric. Exp. Stn. Bull. 499. Brownell, P.F. 1979. Sodium as an essentialmicronutrientelementin plants and its role in metabolism. Adv. Bot. Res.7:117-224. Buckley, D.E., and R.E. Cranston.1971. Atomic absorptionanalysisof 18 elementsfrom a single decompositionof aluminosilicate.Chem.Geol. 7:273-284. Coughtrey,P.I., D. Jackson,and M. Thome. 1983. Sodium.p. 1-41. In Radionuclidedistribution and transportin terrestrialand aquaticecosystems.A critical review of data. Vol. 3. AA Balkema, Rotterdam. Diest, J., and O. Talibudeen.1967. Rubidium-86as a tracerfor exchangeablepotassiumin soils. Soil Sci. 104:119-122. Eckert, OJ., and E.O. McLean. 1980. Differential bondingof potassiumand rubidium-86 in soils of differing clay type and degreeof weathering.Soil Sci. Soc. Am. J. 44:425-428. Feigenbaum,S., R. Edelstein,and I. Shainberg.1981. Releaserate of potassiumand structuralcations from micasto ion exchangersin dilute solutions.Soil Sci. Soc. Am. 1. 45:501-506. Franz, G., and R.M. Carlson.1987. Effects of rubidium, cesium,and thallium on interlayer potassium releasefrom Transvaalvermiculite. Soil Sci. Soc. Am. I. 47:552-559. Goulding, K. W.T. 1987. Potassiumfixation and release.Proc. Colloq. Int. PotashInst. 20:137-154. Goulding, K.W.T., and O. Talibudeen.1963. Potassiumreservesin a sandy clay soil from the Saxmundhumexperiment:Kinetics and equilibrium thermodynamics.1. Soil Sci. 30:291-302. Haagsma,T., and M.H. Miller. 1963. The releaseof nonexchangeablesoil potassium to cation exchangeresins as influenced by temperature,moisture and exchangingion. Soil Sci. Soc. Am. Proc. 27:153-156. Helfferich, F. 1%2. Ion exchange.McGraw-Hili Book Co., New York. Hillebrand, W.F., G.E.F. Lundell, H.A. Bright, and J.I. Hoffman. 1953. Applied inorganic analysis with special referenceto the analysisof metals,mineralsand rocks. 2nd ed. John Wiley & Sons,Inc., New York.

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Hossner,L.R. 1996. Dissolution for total elementalanalysis. p. 4~. In D.L. Sparkset al. (ed.) Methodsof soil analysis.Part 3. Chemicalmethods.SSSABook Ser. 5. SSA and ASA, Madison, WI. course.Publishedby author,Madison,WI. Jackson,M.L. 1956. Soil chemicalanalysis-advanced Jones,R.L. 1989. Rubidium in surfacehorizonsof Illinois soils. Soil Sci. Soc. Am. 1. 53:588-591. Jones,R.L. 1992. Extractablerubidium in surfacehorizons of Illinois soils. Soil Sci. Soc. Am. J. 56:1453-1454. Klesta, EJ., and J.K. Bartz. 1996. Quality assuranceand quality control. p. 19-48. In D.L. Sparkset al. (ed.) Methodsof soil analysis. Part 3. Chemical methods.SSSA Book Ser. 5. SSSA and ASA, Madison,WI. Knudsen,D., G.A. Peterson,and P.E Pratt. 1982. Lithium, sodium, and potassium.p. 225-246.In A.L. Pageet al. (ed.) Methodsof soil analysis.Part 2. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison,WI. Liebhardt,w.e., L. Svec, and M.R. Teel. 1976. Yield of com as affectedby potassiumon a Coastal Plain soil. Commun.Soil Sci. Plant Anal. 7:265-277. Longmore, M.E., B.M. O'Leary, e.w. Rose, and A.L. Chandica. 1983. Mapping soil erosion and accumulationwith the fallout isotopecaesium-137.Aus!. 1. Soil Res. 21:373-385. Malavolta, E. 1985. Potassiumstatusof tropical and subtropical region soils. p. 163-200.In R.E. Munson (ed.) Potassiumin agriculture.ASA, CSSA, SSSA,Madison,WI. Martin, H.W., and D.L. Sparks. 1983. Kinetics of nonexchangeablepotassium releasefrom two CoastalPlain soils. Soil Sci. Soc. Am. J. 47:883--887. Martin, H.W., and D.L. Sparks.1985. On the behaviorof nonexchangeable potassiumin soils. Commun. Soil Sci. Plant Anal. 16:133-162. McIntosh, J.L. 1969. Bray and Morgan soil test extractantsmodified for testing acid soils from different parentmaterials.Agron. J. 61:259-265. Mehlich, A. 1953. Determinationof P, Ca, Mg, K, Na, and NH4• Mimeo 1953. North Carolina Soil Test Div., Raleigh,Ne. Mehlich, A. 1984. Mehlich 3 soil test extractant:A modification of the Mehlich 2 extractant.Commun. Soil Sci. Plant Anal. 15:1409-1416. Morgan, M.E 1941. Chemicalsoil diagnosisby the universalsoil testingsystem.ConnecticutAgric. Exp. Stn. Bull. 450. Nieto, K.E, and W.T. Frankenberger.1985. Single column ion chromatography:II. Analysis of ammonium, alkali metals, and alkaline earth cations in soils. Soil Sci. Soc. Am. J. 49:592-596. Parker,D.R., D.L. Sparks,GJ. Hendricks,and M.e. Sadusky.1989a.Potassiumin Atlantic Coastal Plain soils: II. Crop responsesand changesin soil potassiumunder intensive management. Soil Sci. Soc. Am. J. 53:397-401. Parker,D.R., GJ. Hendricks,and D.L. Sparks.1989b. Potassiumin Atlantic CoastalPlain soils: II. Crop responsesand changesin soil potassiumunder intensive management.Soil Sci. Soc. Am. J. 53:397-401. sodium.USDA Inform. Bull. 216. U.S. Gov. Pearson,G.A. 1960.Toleranceof cropsto exchangeable Print. Office, Washington,De. Pratt, P.E 1965. Potassium.p. 1023-1032.In C.A. Black et al. (ed.) Methodsof soil analysis.Part 2. Agron. Monogr. 9. ASA, Madison,WI. Rausell-Colom,J.A., T.R. Sweetman,L.B. Wells, and K. Norrish. 1965. Studies in the artificial weatheringof micas. p. 40-70. In E.G. Hallsworth and D.V. Crawford (ed.) Experimental pedology.Butterworths,London. Reitemeier,R.E 1951. The chemistryof soil potassium.Adv. Agron. 3:113-164. Rhoades,J.D. 1996. Salinity: Electrical conductivity and total dissolvedsolids. p. 417-435.In D.L. Sparkset al. (ed.) Methods of soil analysis. Part 3. Chemical methods.SSSA Book Ser. 5. SSSA and ASA, Madison,WI. Rich, e.1. 1964. Effect of cation size and pH on potassium exchange in Nason soil. Soil Sci. 98:100-106. Rich, e.1. 1968. Mineralogy of soil potassium.p. 79-96. In VJ. Kilmer et al. (ed.) The role of potassium in agriculture.ASA, CSSA, and SSSA, Madison,WI. Rich, e.1. 1972. Potassiumin minerals.Proc. Colloq. Int. PotashInst. 9:15-31. Sadusky,M.C., and D.L. Sparks. 1991. Anionic effects on potassiumreactionsin variable-charge Atlantic coastalplain soils. Soil Sci. Soc. Am. J. 55:371-375. Sadusky,M.e., D.L. Sparks,M.R. Noll, and GJ. Hendricks.1987. Kinetics and mechanismsof potassium releasefrom sandysoils. Soil Sci. Soc. Am. J. 51:1460-1465.

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Schroeder,D. 1979. Structureand weatheringof potassiumcontainingminerals. Proc. Congr. Int. PotashInst. 11:43--63. Scott, A.D., A.P. Edwards,and J.M. Bremner.1960. Removalof fixed ammoniumfrom clay minerals by cation exchangeresins.Nature(London) 185:792. Scott,A.D., and S1. Smith. 1987. Sources,amounts,and forms of alkali elementsin soils. Adv. Soil Sci. 6:101-147. Smith, S1., and A.D. Scott. 1966. Extractablepotassiumin Grundite illite: 1. Method of extraction. Soil Sci. 102:115-122. Soltanpour,P.N., and A.P. Schwab.1977.A new soil test for simultaneousextractionof macro-and micro-nutrientsin alkaline soils. Commun.Soil Sci. PlantAnal. 8:195-207. Southard,R1., andR.c. Graham.1992.Cesium-137distributionin a California pelloxerert:Evidence of pedoturbation.Soil Sci. Soc.Am. I. 56:202-207. Souty, N.R., R. Guennelon,and C. Rode. 1975. Someobservationsof potassium,rubidium-87,and caesium-137absorptionby plantsgrown on nutritive solutions.Ann. Agron. 26:41-58. Sparks,D.L. 1980. Chemistryof soil potassiumin Atlantic CoastalPlain soils: A review. Commun. Soil Sci. Plant Anal. 11:435-449. Sparks,D.L. 1987. Potassiumdynamicsin soils. Adv. Soil Sci. 6:1-63. Sparks,D.L., and P.M. Huang.1985.Physicalchemistryof soil potassium.p. 201-276.In R.E. Munson (ed.) Potassiumin agriculture.ASA, CSSA, and SSSA,Madison,WI. Sparks,D.L., andW.C. Liebhardt.1981.Effect of long-termlime andpotassiumapplicationson quantity-intensity (0/1) relationshipsin sandysoil. Soil Sci. Soc.Am. I. 45:786-790. Sparks,D.L., D.C. Martens,andL.w. Zelazny.1980.Plantuptakeand leachingof appliedandindigenouspotassiumin Dothansoils. Agron. J. 72:551-555. Sumner, M.E., and w.P. Miller. 1996. Cation-exchangecapacity and exchangecoefficients. p. 1201-1229. In D.L. Sparkset al. (ed.) Methodsof soil analysis.Part 3. Chemicalmethods. SSSABook Ser. 5. SSSAandASA, Madison,WI. Talibudeen,O. J.D. Beasley,P. Leone,and N. Rajendran.1978.Assessment of soil potassiumreserves availableto plant roots.I. Soil Sci. 29:207-218. Thomas,G.W. 1982. Exchangeablecations.p. 159-165.In A.L. Pageet al. (ed.) Methodsof soil analysis.Part 2. 2nd ed. Agron. Monogr. 9. ASA and SSSA,Madison,WI. Tinker, P.B. 1967. The relationshipof sodium in the soil to uptakeof sodium by sugarbeetsin the greenhouseand to yield responsesin the field. p. 223. In G.Y. lacks (ed.) Soil chemistryand fertility. Trans.It. Meet. Commun.2, 4, Int. Soc.Soil Sci. 1966.Univ. Press,Aberdeen,Scotland. Wells, C.B., and K. Norrish. 1968. Acceleratedratesof releaseof interlayerpotassiumfrom micas. Trans.9th Int. Congr. Soil Sci. 2:683-694. Wolf, A., and D. Beegle.1991. Recommended soil testsfor macronutrients:Phosphorus,potassium, calcium and magnesium.p. 25-34.In J.T. Sims and A. Wolf (ed.) Recommended soil testing proceduresfor the NortheasternRegion.DelawareAgric. Exp. Stn. Bull. 493. Woodruff, J.R., and C.L. Parks.1980.Topsoil and subsoilpotassiumcalibrationwith leaf K for fertility rating. Agron. J. 72:392-396. Yuan, L.L., L.w. Zelazny,and A. Ratanaprasatporn. 1976.Potassiumstatusof selectedpaleudultsin the Lower CoastalPlain. Soil Sci. Soc.Am. Proc.40:229-233.

Published 1996

Chapter20 Beryllium, Magnesium, Calcium, Strontium, and Barium D. L. SUAREZ, USDA -ARS, U.S. Salinity Laboratory, Riverside, California

PROPERTIESOF ALKALINE·EARTH METALS Beryllium, Mg, Ca, Sr, and Ba are known as the alkaline earth elements,which are characterizedby two electronsin the outermostshell. Beryllium differs in its chemical behaviorfrom the other Group II elements,in that covalent bonding predominates.In contrastto Be, the other elementsin the group readily lose their two outershell electronsand are mostly presentin ionic form as divalent cations. To date no monovalentcation compoundshave been observedfor these elements.(Cotton & Wilkinson, 1980). Table 20-1 gives generalchemicalproperties of the alkaline earthelements.A characteristicof theseelementsis the small ionic radius and high charge/radiusratio. The chemical propertiesof Mg are somewhatintermediatebetweenBe and the other Group II elementsin that Mg also can form covalentbondsand both Mg and Be can precipitateas hydroxides from aqueoussolutions. Calcium, Sr and Ba have a closer affinity, with a systematic changein their properties,related to increasingion size. Theseproperties include increasingsolubility of the sulfate,chloride and nitrate salts,increasing hydration of the crystalinesalts,and decreasingionization energy. Calcium, Sr, and Ba, form relatively insoluble precipitatesof carbonates and sulfates. In contrast, magnesiumsulfates are soluble and magnesiumcarbonatesdo not precipitateuntil Mg solution concentrationsare much greaterthan Ca. Magnesiumand Ca are abundantelementsin soil and also are amongthe most commonly analyzed,becausethey are essentialelementsfor plant growth. Calcium is essentialfor membranepermeability, solute transport and maintenanceof cell integrity (Marschner,1986). Magnesiumis requiredfor many cellular functions including productionof chlorophyll, proteinsynthesis,regulation of cellular pH and cation-anionbalance(Marschner,1986). Strontium, Ba and Be are much less abundantin soils and historically of lesserinterestto soil scientiststhan Mg and Ca. Introduction of radioactiveSr into the atmospheredue Copyright © 1996 Soil ScienceSociety of America and American Society of Agronomy, 677 S. SegoeRd., Madison,WI 53711, USA. Methods of Soil Analysis. Part 3. Chemical Methods-SSSA Book Seriesno. 5.

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576 Table 20-1. Propertiesof Group II elements.

Ionization enthalpiest Atomic weight

Ionic radiiU

Usual coordination§

3.1 7.8 1.06 1.27 1.43

Second

--klmol-1-

nm Be, 9.012 Mg,24.31 Ca,40.08 Sr,87.62 Ba,137.33

First

4 4-6 6 6-12 6-12

899 737 590 549 503

1757 1150 1146 1064 965

*

t Cotton and Wilkinson, 1980. Basedon sixfold coordination. § Threkianand Wedepahl,1961.

to nuclearweaponstesting, and subsequentfallout into terrestrialenvironments increasedinterestin Sr analysisand understandingof the biological and chemical processesassociatedwith Sr in the plant-soil environment.Plant and soil materialare not commonlyanalyzedfor Ba content.The major interestin Ba for soil systemshasbeenits useas an extractingcationfor the analysisof exchangeable ions. Its use for this purposeis not recommendedfor arid land soils where calciumcarbonateor gypsumare presentdue to the insolubility of theseBa salts. Beryllium analyseshave rarely beenperformedfor soils, due in part to generally very low concentrations.Nonetheless,Be is highly toxic to animals and heightenedenvironmentalconcernsin recentyearshas led to increasedinterest in Be chemistryand analyses.Although Be is an alkaline earth element(Group lIa), its chemicalbehavior,with regardsto hydrolysis (Baes& Mesmer, 1976), and complexation(Sillen & Martell, 1971) is closerto AI than it is to the other alkaline earth elements.Despitethis chemicalsimilarity, Be occurrencein soils has not correlatedwith AI concentration(Andersonet al., 1990), likely because of differencesin mineral source. MINERALOGY AND DISTRIBUTION OF BERYLLIUM, MAGNESIUM, CALCIUM, STRONTIUM, AND BARIUM IN ROCKSAND SOILS The averagecrustal igneousrock compositionfor Be, Mg, Ca, Sr, and Ba are 2.19,17.7x 1()3, 67.4 x 1()3, 342, and469 mg kg-I, and for sedimentaryrock 1.98, 19.7 x 1()3, 36.5 x 1()3, 338, and 525 mg kg-I, respectively(Turekian & Wedephohl,1961).Typical soil concentrationsfor Be, Sr and Ba, are given as 6, 300, and 1000 mg kg-I, respectivelyby Ure (1991). Beryllium is concentratedin dark minerals and muscovite. In granitic rocks biotite and hornblendecan contain up to 15 mg kg-I Be while for muscovite Be content ranges from 10 to 50 mg kg-I. The mineral beryl (Be2AI2Si03)6also can occur as a result of metamorphicalteration.Within mineral groupsthere is a positive relation betweenMg and Be content(Hormann, 1969).Relatively few analysesof Be in soils havebeenreported.Andersonet al. (1990) reported that water-solubleconcentrationswere below their detection

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limit (0.2 f1.g L -1) and total Be rangesfrom 0.3 to 30.5 mg kg-1 for southeastern U.S. Piedontand CoastalPlan soils. Shackletteet a!. (1971) reportedan average soil concentrationof 0.6 mg kg-1 for the conterminousUSA. Magnesiumis abundantin many silicate mineralswhere it exists in solid solution with Fe2+ (which has a similar octahedralradius). The Mg and Fe content of rocks are positively correlated,and decreasein the seriesfrom gabbroto granite. Magnesiumis relatively rare in framework silicates such as feldspars, but is an importantconstituentof many soil minerals,including olivines, pyroxenes,amphiboles,micasand clay minerals,as well as dolomite (Burns & Burns, 1974). Magnesiumalso is presentin limestone(calcite) with a substitutionof 1 to 9% for Ca. Calcium is the fifth most abundantelementand occurs predominatelyin silicates and carbonates,although phosphatesand sulfates also are important. Mineral occurrencesinclude plagioclasefeldspars,pyroxenes,amphiboles,calcite, aragonite(rare in soils), gypsumand dolomite (Faure,1978). Most Ca input to rivers is estimatedto be derivedfrom weatheringof carbonaterocks (Garrels & Perry, 1974). Minor amountsof Sr minerals are found in nature (such as strontianite, SrC03, and celestite,SrS04),however,most Sr is presentas a trace constituent of other minerals.Strontiumsubstitutesreadily for Ca as well as K (for example, in potassiumfeldspar). Most of the Sr input to rivers is from weatheringof carbonatesand sulfates(Brass,1976). Barium is presentas a substitutedion in silicates,mostly feldspars,biotite and muscovite,and rarely forms its own minerals.The most importantsubstitutions are for K and Ca, with potassiumfeldspar being the most important (Puchelt,1972). The Ba contentof most soils is in the rangeof 100 to 3000 mg kg-1 of soil.

TOTAL ELEMENTAL MAGNESIUM, CALCIUM, STRONTIUM, BARIUM, AND BERYLLIUM

Introduction Total analysisof theseelementsgenerallyrequiresa soil digestionfor complete destructionof the crystallineand organiccomponentsof the soil. The most commonmethodis still the sodium carbonatefusion (Jackson,1958), although the lithium borate fusion (Medlin et a!., 1969), the HCI-HF digestion (Isaac & Kerber, 1971) and more recently microwave digestion (Gilman & Engelhart, 1989) also are often used.Digestionwith HF shouldbe avoidedif Be analysisis to be performed,as Be is one of the elementswhich forms volatile fluorides, resulting in incomplete recovery (McLaren, 1987). Once the elementsare in solution, they can be readily analyzedby the methodsdescribedbelow, most commonly be Atomic Absorption Spectroscopy(AAS). More recentalternative methodsfor total analysisinclude high energy,destructivesampling,ion beams and Inductively CoupledPlasmaSpectroscopy(ICP) using suspendedsolids.

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DigestionMethods Detailedmethodsfor digestionof soil samplesare presentedin Chapter3 (Rossner,1996). Analysis by Atomic Absorption Spectroscopy Seesectionon "Atomic Absorption Spectrophotometry,"Tables20-2 and 20-3, and Chapter4 (Wright & Stuczynski,1996). Analysis by Inductively CoupledPlasmaAtomic EmissionSpectrometry Seesectionon "Beryllium" and Chapter 5(Soltanpouret aI., 1996). X-Ray Fluorescence Wavelengthdispersiveand energydispersivex-ray fluorescenceoffer the possibility of rapid analysiswith a minimum of samplepreparation.The spectrometer uses either a solid state Li-drifted silicon detector (wavelength) or "Bragg diffracting crystal" (dispersive).The methodis widely usedfor measuring many major and minor elementsin rock samples,including Mg, Ca, Sr, and Ba (Potts, 1987). The method is not suitablefor Be analysis(Be is usedas the window on the x-ray tube). Samplepreparationtypically consistsof preparation of powderpelletsor glassbeadsformed from the lithium boratefusion, or sodiTable 20-2. Atomic absorptionspectroscopyflame & instrumentsettings. AA setting Mg

Ca

Sr

Ba

Grating Ultraviolet Wavelength 285.2nm Oxidant Air Fuel Acetylene Flame Slightly reducing

Visible Visible Visible 553.6nm 422.7nm 460.7nm Air Air Nitrous oxide Acetylene Acetylene Acetylene Slightly reducing Slightly reducing Reducingrich

Be Ultraviolet 234.9nm Nitrous oxide Acetylene Reducingrich

Table 20-3. Atomic absorptionspectroscopyanalysesof alkali earthelements(detectionlimitt and working rangein IlgIL).

Element Be

Wavelength 234.9

Mg

285.2

Ca

422.7

Sr Ba

Flame detection limits

460.7 553.6

1.0 0.5(N-Ac) 0.3 0.5 0.5 0.3 30 10 30(N-Ac)

t 2 x SD of a blank solution.

Flame working range 50-2000 20-2000

Graphite detection limit furnace 0.02 0.2 0.01 0.004

200-20000 300-5000 1000-20000

0.04 2.0

Reference GiII,1993 Clesceriet aI., 1989 GiII,1993 Clesceriet aI., 1989 GiII,1993 Clesceriet aI., 1989 Clesceriet aI., 1989 GiII,1993 Clesceriet aI., 1989

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urn carbonatefusion (as describedin Chapter7, Karathanasis& Hajek, 1996). Cooling of the glass bead is an important considerationas it must cool sufficiently slowly to avoid crackingand sufficiently fast to avoid crystallizationand nonhomogeneity.Recentimprovementsin samplepreparationand instrumentation methodsare describedby Coutureet al. (1993). Recently total-reflection x-ray fluorescencespectrometryhas been proposedfor analysisof water and soil sampleswith increaseddetectionlimits over those of ICP, good precisions and small sample size (10 ,uL) (Mukhtar & Haswell, 1991).

Procedure Prepareglass bead (fusion) as describedin Chapter 2 (Klesta & Bartz, 1996).Typically the glassbeadis preparedusing a 5- to lO-g soil sampleground to at least 9) to preventthe formation of Si02 gel (Govett, 1961).

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2. Possibleintroduction of trace contaminantsby the flux. This could be importantif the analystintendsto use the fusion solution for trace element determinations. SamplePreparationfor Total Silicon: Acid Digestion Reagents All reagentsare reagentgradeor higher quality. 1. Aqua regia. Mix one part concentratednitric acid (HN03) with three partsconcentratedHCI. Preparefresh before eachacid decomposition. 2. Hydrofluoric acid (HF), 48%. 3. Boric acid (H3B03) solution. Dissolve 50 g crystalline H3B03 in 1 L deionizedwater. Procedure The acid digestion procedureis basedon methodspublishedby Bernas (1968), Ruch et al. (1974), and Harvey et al. (1983). Refer to the previoussection on "Procedure"for the removalof organicmatterfrom the soil. Weigh 0.100 g (to the nearest0.0001 g) of finely ground,ashedsoil sampleinto the TFE-fluorocarbonliner of a sampledigestion bomb (Parr InstrumentCo., Moline, IL 61265)1.Use plastic pipetsto add 1.5 mL of aquaregia and 2.5 mL of HF to the sample.Be careful to wet the entire sample.Closethe bomb and heat for 2 h at 373 K to 383 K. Allow the bomb to cool, then carefully open it and add 25 mL of H3B03. Heat the sampledigest(not abovethe boiling point), if necessary,for 10 to 15 min after addition of the H3B03, to aid in the dissolutionof any resulting precipitate.Somedark-colored,insolubleorganicresiduethat escapedashing may remain. Transferthe contentsof the TFE-fluorocarbonliner quantitatively to a 100-mL plastic volumetric flask, dilute to the mark with deionizedwater, then transferthe solution to an acid-washed125-mL high-densitypolyethylene bottle. Dilute 10 mL of this solution with deionizedwater to 100 mL. Store the diluted samplein an acid-washedhigh-densitypolyethylenebottle. Preparea blank solution using the samemethodas for samples.Ignite 0.2 g of spectroscopicgradeSi02 at 1273 K for 30 min. Preparea stock standardSi solution by dissolving0.1070g of ignited Si02 in an acid decompositionbomb as describedabove. Dilute the stock standardsolution to 100 mL, as described above.This solution contains500 mg Si L-1. Dilute aliquots of the stock standard solution to provide a seriesof calibration standardswith concentrationsin the rangeof 4 to 50 mg L- 1. Atomic Absorption Spectrometry DetermineSi by AAS using a fuel-rich nitrous oxide/acetyleneflame and a wavelengthof 251.65nm. Adjust the otherinstrumentparametersaccordingto I The use of trade namesin this report is for descriptivepurposesonly and does not constitute endorsementby either the University of Illinois or the Illinois StateGeologicalSurvey.

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the manufacturer'sinstructions.Follow appropriatesafetyprecautionsin using a nitrous oxide/acetyleneflame. Calculate the concentrationof Si02 in the unashed,oven-driedsoil. Inductively CoupledPlasmaEmissionSpectrometry Silicon is determinedby ICPESat 288.158nm. Spectralinterferencesfrom other elementsshould be negligible, becauseof the usually high concentrations of Si found in soils. Follow the instrumentmanufacturer'sinstructionsfor operating the instrumentand follow any recommendedsafety precautions.Calculate the concentrationof Si02 in the unashed,oven-driedsoil. Comments Hydrofluoric acid causesextremelypainful, slowly healingburnsto tissue. Exerciseextremecaution in working with HF and all work should be done in a fume hood. Always wearglovesthat are imperviousto HF, and gogglesor a face shield when working with this or any other acid. Checka chemicalcompatibility chart for properglove selection. Boric acid solution is addedto the samplesolution to dissolveprecipitated fluorides and to complexexcessfluoride as BF4. If free HF is allowed to remain in the samplesolution, it will be volatilized during analysisby either AAS or ICPES and may attack the burner or torch parts, the flame or torch ventilation system,and/or be lost to the laboratory air, where it may attack surfacesin the laboratory or harm individuals in the lab either by inhalation or by skin or eye contact. Silicon forms volatile silicon tetrafluoride(SiF4) with HF. The loss of SiF4 from solution presentsa possiblesourceof negativeerror if the bomb is opened before its contentshavecooledto room temperatureor if the soil sampleis dissolved in an open container. Tightly capped60-mL high-density polyethylenebottles have been successfully substitutedfor the TFE-fluorocarbon-linedacid decompositionvessels at temperaturesbelow 403 K. If high-densitypolyethylenebottlesare used,they should be heatedin an ordinary water bath (Langmyhr & Paus,1968) or on a steambath (Harvey et aI., 1983). Follow the sameprocedureas abovefor addition of H3B03 solution and dilution. Decomposition methods using vessels designed for microwave heating and suitable for soil materials have been describedby Kingston and lassie(1988). One type of soil that might requirespecialtreatmentin the acid dissolution methodis calcareoussoil. A weighedsampleof this soil shouldbe initially treated with dilute HCI to destroy CaC03 and CaMg(C03h- The sampleshould be takento drynesswithout baking the sample,then it may be treatedin the manner describedfor the HF-aquaregia dissolution.As part of laboratoryquality assurance, samplesof known composition should be preparedand analyzedin the samemanneras unknowns.

Advantages of Acid Digestion 1. The dissolvedsolids concentrationafter acid digestionof soil is low.

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2. Contaminationof the samplesolution by reagentsis low.

3. Solutionspreparedby acid digestionmay be usedfor determinationsof traceelementsin soils.

Disadvantages ofAcid Digestion 1. Silicon may be volatilized as SiF4 (would result in a negativeerror). 2. Somerock-forming minerals,notably zircon, are not readily solublein HF (would result in a negativeerror). 3. If all excessHF is not complexedby H3B03, the remainingfree HF may attack a glass or quartz nebulizeror torch (would result in a positive error). SILICON DETERMINATION BY LIGHT ABSORPTION SPECTROMETRY Yellow SilicomolybdicAcid Procedure Reagents All reagentsare reagentgradeor higher quality. 1. Ammonium molybdatetetrahydrate[(NH4)6 M070 24 • 4H20]. Dissolve 54 g in 750 mL of deionizedwater then adjust pH to ~7 with NaOH, dilute to 1 L and store in a high-densitypolyethylenebottle. The solution is 0.3 M with respectto MoO]-. 2. Tartaric acid (C4H60 6) solution, 20%. Dissolve 100 g tartaric acid in waterand dilute to 500 mL. Storein a high-densitypolyethylenebottle. 3. Reducingsolution. Dissolve25 g of sodiumbisulfite (NaHS03) in 200 mL water. Dissolve 2 g of sodium sulfite (Na2S03) and 0.4 g of 1amino-2-naphthol-4-sulfonic acid [NH2ClOHs(OH)S03H]in 25 mL of water. Mix solutionsand dilute to 250 L; store in a high-densitypolyethylenebottle, and refrigerate. 4. Silicon standard,50 mg L-l. Fuse 0.1070 g of spectrographicgrade Si02with NaOH. Dissolvethe fusion chargein 700 to 800 mL of deionized water (the exactamountof water is not important) and then add sufficient 0.5 M sulfuric acid (H2S04) to give a pH of 1.5. Dilute to 1 L in a volumetricflask. Storein a high-densitypolyethylenebottle. Procedure The yellow silicomolybdic acid procedure is based on the method of Govett (1961). Ash the sampleto removeorganicmatter(seethe first sectionon "Procedure").Weigh 0.100g (to the nearest0.0001 g) into a 50-mL nickel crucible. Add 3 g NaOH, mix well. Placethe crucibleover a bunsenburnerandheat to a dull rednessfor at least 10 min. After cooling, transferthe crucible and its contentsto a polyethylenebeakerand addabout 800 mL of deionizedwater. Allow to standfor at least1 h. Stir the contentsof eachbeakerwith a plasticstirring rod or a TFE-fluorocarbon-coated stirring bar until the fusion chargeis dis-

SILICON

635

solved.Removethe stirring bar andcrucibleand rinse themwith deionizedwater into the beaker.Add sufficient 0.5 M H2S04 to adjust the pH of the solution to 1.5. Quantitatively transferthe solution to a l-L volumetric flask and dilute to volume with deionizedwater. Transfera volume(~20 mL) of the preparedsolution to a 50-mL volumetric flask, so that the volume transferredcontainsless than 0.4 mg Si. Add 10 mL 0.5 M H2S04 to the volumetricflask, followed by 10 mL ammoniummolybdatesolution, and then 10 mL of tartaric acid solution. Mix thoroughly and dilute to the mark without delay. Determineabsorbanceof the samplesolution at 400 nm between2 and 10 min after additionof the ammonium molybdatesolution. Preparea standardcurve to 8 ~g Si mL-l.

Comments The gram-equivalentsof W should be kept betweenthree and five per gram-ionof MoOl- and the total ionic strengthshouldbe lessthan 0.5 to prevent formation of the a.-form of silicomolybdic acid (Strickland, 1952). The a.-form is lesssensitiveand is more stablethan the ~-form. It forms readily at a pH higher than about 2.5 or in excesselectrolyte(Strickland, 1952). Samplesprepared by sodiumhydroxide(NaOH) fusion shouldbe analyzedwithin severaldays.For purposesof quality assurance,samplesof known compositionshould be preparedand analyzedin the samemanneras unknowns.For analysesof extractsit is advisableto spike severalsampleswith Si to evaluaterecovery.

Blue SilicomolybdousAcid Procedure Procedure This procedureis basedon methodspublishedby Shapiroand Brannock (1962) and Weaveret al. (1968). Use the sameprocedureas in the yellow silicomolybdic acid procedure (seethe sectionon "Yellow SilicomolybdicAcid Procedure")for fusion and dissolution of the sample.Transfera portion of the preparedsolution to a 50-mL volumetricflask so that the volumecontainslessthan 0.02 mg Si. Add 10 mL of 0.5 M H2S04, and 10 mL of ammonium molybdate solution,and mix by swirling the contentsof the volumetric flask. After 2 min add 5 mL of tartaric acid and 5 mL of reducingsolution, mix without delay, and then dilute to the 50 mL mark. Becauseof the instability of the samplematrix, a matrix blank should be analyzed, however, standardsand samplesshould be determinedvs. a deionized water blank (Jeffery & Wilson, 1960). Correction for the absorbanceof the reagentblank should be made in the final calculations. Measure absorbance againstthe deionizedwater blank at 820 nm at least30 min after addition of the ammoniummolybdatesolution. Preparea standardcurve to 0.4 ~g Si mL-l.

Comments As in the molybdic acid procedure,it is importantto analyzesamplespreparedby NaOH fusion within 2 d and it also is advisableto spike randomsamples with known quantitiesof Si to evaluaterecovery.A numberof reductants

JONES & DREHER

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have beenusedincluding stannouschloride, hydroxylamine,and ascorbicacid; however, the mixed reagent with l-amino-2-napthol-4-sulfonicacid, listed amongthe reagents,producesthe moststablecolor. Interlaboratorystandardsfor Si shouldbe carriedthroughthe procedurefor quality assurance.

REFERENCES American Society for Testing and Materials. 1991. Standardtest methodfor major and minor elementsin coal and coke ash by atomic absorption,Method D 3682-87.p. 369-374.In RA Storer(ed.) Annual Book of ASTM Standards,GaseousFuels,Coal and Coke. Version 5.05. ASTM, Philadelphia,PA. Bernas,B. 1968.A new methodfor decompositionand comprehensiveanalysisof silicatesby atomic absorptionspectrometry.Anal. Chern.40:1682-1686. Bowen, HJ.M. 1966. Traceelementsin biochemistry. Acad.Press,New York. Brown, D.F.G., AM. MacKay, and A. Turek. 1969. Preparationof stablesilica standardsolutionsin rock analysesusing lithium tetraborate.Anal. Chern.41:2091. Condie, K.C. 1974. Abundancein igneous rocks and in the crust. p. 14-E-1 to 14-E-4. In K.H. Wedepohl(ed.) Handbookgeochemistry.Vol. 2. Springer-Verlag,Berlin. Drees,R, L.P. Wilding, N.E. Smeck,and AL. Senkayi.1989.Silica in soils: Quartzand disordered polymorphs.p. 913-976.In I.B. Dixon and S.B. Weed (ed.) Minerals in soil environments. SSSABook Ser. 1. SSSA,Madison,WI. Gladney, E.S., and C.E. Bums. 1983. Compilation of elementalconcentrationsin eleven United StatesGeologicalSurvey rock standards.Geostand.Newsl. 7:3-226. Govett, GJ.S. 1961. Critical factors in the determinationof silica. Anal. Chim. Acta 25:69-80. Harvey, RD., RA Cahill, C.-L. Chou, and I.D. Steele.1983. Mineral matterand trace elementsin the Herrin and Springfield coals, Illinois Basin coal field. Illinois State Geol.Surv. Contract/GrantRep. 1983-4. Iackson, M.L. 1974. Soil chemical analysis-advancedcourse. 2nd ed. Dept. Soil Sci., Univ. Wisconsin,Madison. Jeffery, P.G., and AD. Wilson. 1960.A combinedgravimetricand photometricprocedurefor determining silica in silicate rocks and minerals.Analyst (London) 85:478-486. Karathanasis,AD., and B.F. Hajek. 1996. Elementalanalysisby x-ray fluorescencespectroscopy.p. 161-223. In D.L. Sparkset al. (ed.) Methods of soil analysis. Part 3. Chemical methods. SSSABook Ser. 5. SSSAand ASA, Madison,WI. Kingston, H.M., and L.B. lassie(ed.). 1988. Introduction to microwavesamplepreparation:Theory and Practice.Am. Chern.Soc. Prof. ReferenceBook. ACS, Washington,DC. Kodama, H., and GJ. Ross. 1991. Tiron dissolutionmethodusedto removeand characterizeinorganic componentsin soils. Soil Sci. Soc. Am. I. 55:1180-1187. Langmyhr,FJ., and P.E. Paus.1968. The analysisof inorganicsiliceousmaterialsby atomic absorption spectrophotometry andthe hydrofluoric acid decompositiontechnique.Anal. Chim. Acta 43:397-408. Piperno,D.M. 1988. Phytolith analysis.Acad. Press,San Francisco,CA Robinson,W.O. 1930. Method and procedureof soil analysisusedin the Division of Soil Chemistry and Physics.USDA Circ. 139. USDA, Washington,DC. Ruch, RR, HJ. Gluskoter, andN.F. Shimp. 1974.Occurrenceand distribution of potentially volatile traceelementsin coal: A final report. Illinois StateGeol. Surv. Environ. Geol. Note No. 72. Shacklette,H.T., and I.G. Boerngen.1984. Elementconcentrationsin soils and other surficialmaterials of the conterminousUnited States.U.S. Geol. Surv. Prof. Pap. 1270. U.S. Gov. Print. Office, Washington,DC. Shapiro, L. 1975. Rapid analysisof silicate, carbonate,and phosphaterocks. Rev. ed. U.S. Geol. Surv. Bull. 1401. U.S. Gov. Print. Office, Washington,DC. Shapiro,L., and W.W. Brannock. 1962. Rapid analysisof silicate, carbonateand phosphaterocks. U.S. Geol. Surv. Bull. 1144-A U.S. Gov. Print. Office, Washington,DC. Strickland,I.D.H.1952.The preparationand propertiesof silicomolybdicacid. III. The combination of silicate and molybdate.I. Am. Chern.Soc. 74:872-876. Yuan, T.L., and H.L. Breland. 1969. Evaluationof atomic absorptionmethodsfor determinationsof aluminum,iron, and silicon in clay and soil extracts.Soil Sci. Soc. Am. Proc. 33:868-872.

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Wang,C., andP.A. Schuppli.1986. Determiningammoniumoxalate-extractable Si in soils. Can.1. Soil Sci. 66:751-755. Weaver,R.M., J.K. Syers,and M.L. Jackson.1968. Determinationof silica in citrate-bicarbonatedithionite extractsof soils. Soil Sci. Soc.Am. Proc. 32:497-501.

Published 1996

Chapter23 Iron RICHARD H. LOEPPERT,Texas A&M University, College Station, Texas WILLIAM P. INSKEEP,Montana State University, Bozeman, Montana

Total soil Fe concentrationsvary widely and rangefrom 20%; the median concentrationis approximately3% (Murad & Fischer,1988). Under earthsurface conditions,Fe may exist in either the Fe2+ (ferrous)or Fe3+ (ferric) oxidation state.Although Fe is an abundantelementin primary and secondarymineralsin soils, it's low availability frequently limits plant growth, especiallyin alkaline and calcareoussoils, becauseof the low solubilities of the Fe-containingsecondaryminerals(Lindsay & Schwab,1982; Lindsay, 1984). In low pH soils and/ or reducingconditions,soluble forms of Fe can be presentin sufficient concentrationsto be toxic to plants. The objective of this chapteris to summarizemethodologiesfor the determination of total soil Fe (fusion and digestion procedures),forms of soil Fe (selectiveextraction procedures)and Fe bioavailability, and for the analysisof total dissolvedFe, Fe2+ and Fe3+ in soil digestsand extracts.Thesediscussions will be precededby a summaryof the principal forms of soil Fe and their properties,especiallywith regardto thosefactorswhich influencebioavailability. The researcheralso is referredto previousreviews by Olson (1965), Olson and Ellis (1982), and Loveland(1988).

PRINCIPAL FORMS OF SOIL IRON The predominantiron oxides in the soil are hematite,goethite, lepidocrocite, magnetite,maghemiteand ferrihydrite (Schwertmann,1988; Schwertmann & Taylor, 1989). Each of theseoxides is an Fe3+-containingmineral, exceptfor magnetitewhich containsboth Fe2+ and Fe3+, and eachexists predominantlyin the clay-sizefraction of the soil (exceptfor magnetitewhich can occur predominantly in the silt- and sand-sizefractions).The soil iron oxide mineralscanexhibit considerablesubstitutionof other ions, e.g., Al or Mn, for Fe in the structure. Goethiteand hematiteare the dominantmineralsin well-oxidizedor arid or semiarid environments,and lepidocrocite is found predominantlyin hydromorphic Copyright © 1996 Soil ScienceSociety of America and American Society of Agronomy, 677 S. SegoeRd., Madison,WI 53711,USA. Methods of Soil Analysis. Part 3. Chemical Methods-SSSA Book Seriesno. 5. 639

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environments.Magnetiteis most commonin reducedor slightly-weatheredsoils. Ferrihydriteis likely to exist in most soils, thoughusually in small quantities,but is frequently a major componentin soils exhibiting fluctuating redox potentials. Each of the soil iron oxide mineralsexists in relatively well-orderedcrystalline forms in the soil, exceptfor ferrihydrite which is more highly hydrated,with only short-rangecrystallineorderand with particle sizesof 10 nm or less.Ferrihydrite often exists in associationwith layer silicates, amorphoussilicates and organic matter. Positive identification of the predominantFe oxides is accomplishedby powder x-ray diffraction, infrared spectroscopyand mossbauerspectroscopy (Schwertmann& Taylor, 1989).Identification is facilitated by isolation of the clay-sizecomponentsby particle-sizefractionationprocedures,followed by magnetic separationof the iron-containingminerals(Schulze& Dixon, 1979). Positive identification of ferrihydrite in the soil is especiallydifficult due to its usually low concentrationand poor crystallinity, though selective-extractionproceduresindicatethat ferrihydrite or a ferrihydritelike phasemay be presentin most soils. The relative stabilities of the soil iron oxide mineralsdecreasein the following order (Schwertmann,1991) hematite= goethite:>lepidocrocite= magnetite:>ferrihydrite. The major factor which influencesthe bioavailability of a soil iron oxide is its relative easeof dissolution,which is influencedby the particlesizeand surfacereactivity of the mineral phase.Kinetic studieshaveindicatedthat ferrihydrite, due to its small particle size, high surfaceareaand high surfacereactivity, is considerably more reactivethan equivalentquantitiesof goethiteor hematite.Ferrihydrite and similar poorly crystallineiron oxidesplaya dominantrole in influencing the availability of Fe to plants in oxidized calcareoussoils (Loeppert & Hallmark, 1985; Morris et aI., 1990). Iron is presentas a structuralcomponentof layer silicatesin the soil (2~50 g kg-1 is common),but this Fe is largely unavailablefor plant growth. Iron also may exist as an exchangeion on layer-silicatesurfaces,but in appreciablequantities only in low-pH «4.5) or low-redox conditions. Iron is a componentof other primary or secondaryminerals, e.g., pyroxenes,amphiboles,pyrite and siderite.The occurrenceof eachof thesemineralsis usually restrictedto reducedor relatively less-weathered soils. Pyrite (FeS2)is an importantsourceof Fe in acid mine drainage,wherethe oxidation of Fe2+ and Sresultsin high concentrationsof dissolvedFe3+ and sOi-. The Fe3+ releasedduring FeS2oxidationin theseenvironmentsoften forms secondaryminerals,includingjarosite[KFe3(S04h(OH)6]andferrihydrite (Nordstrom,1982; van Breeman, 1988). Siderite (FeC03) is sometimesimportant in reducedenvironments(e.g., submergedsedimentsor flooded soils) where Fe2+ is stablerelative to Fe3+ and concentrationsof CO2(g) are high (Lindsay, 1979). Complexationof Fe by organic matter, e.g., humic substances,in soils is highly influencedby pH and redox potential.At low pH «5.0), stablecomplexesof organicmatterwith Fe, especiallyFe3+, are likely to exist (Goodman,1987). At pH valuesgreaterthan 6.0, Fe3+-humic complexesbecomelessimportantdue to hydrolysisof Fe and precipitationof iron oxides.In calcareoussoils, wherethe

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pH of the bulk soil solution is usually within the rangeof 7.5 to 8.5, Fe3+ is not readily retainedby soil humatesagainsthydrolysis; however,in microenvironmentsof reducedpH and redox potential,e.g.,in zonesof active microbial activity, complexationof Fe by organic mattercan be appreciable.Also, in the rhizosphere,complexationof Fe by soluble organicscan be appreciabledue to the action of plant-andmicrobial-siderophores (chelators),someof which will retain Fe against hydrolysis at pH values >7.5. In addition, high concentrationsof organic acidssuchas oxalateand citrate in the rhizosphereof someplant species can be importantin the complexationof Fe3+ (Inskeep& Comfort, 1986). These observationsall point to the fact that Fe concentrationscanbe considerablyhigher in the rhizospherethan in the bulk soil solution. The concentrationof solution-phaseFe is controlled by pH, redox potential, the concentrationof water-solubleiron-col11plexingagents,the solubility of soil iron-oxide phases,and the kinetics of dissolutionand precipitationof these phases(Lindsay, 1979). In the pH rangeof most calcareoussoils (approximately 7.5-8.5),the concentrationof solution-phaseFe3+ is at an approximateminimum (Lindsay, 1979). In the absenceof organic complexing ligands, the total dissolvedFe3+ concentrationis approximately10-10 M, which is considerably less than the approximately 10-7.7 M concentrationthat is required for plant growth in nutrient culture (Lindsay & Schwab,1982; Lindsay, 1984). The equilibrium total Fe concentrationof the soil solution increaseswith decreasingredox potential, due to the increasedconcentrationof Fe2+ speciesin solution. Lindsay and Schwab(1982) calculatedthat for Fe in solution to exceedthe criticallevel for plantsthe redox potentialmust drop below a pe + pH level of 9.75. Most calcareoussoils havebulk soil solutionpe + pH valuesconsiderablygreater than 9.75; therefore,except under conditions of flooding or saturationof soil poresfor extendedperiods,it is likely that the total dissolvedFe concentrationof a bulk soil is insufficient to meet the immediatenutritional needsof most plant species.Plantsand microorganismshave evolved mechanismsof increasingthe solubility and availability of Fe in the rhizosphere,by exudationof H+, organic acids, reducingagentsand phytosiderophores.

TOTAL IRON

Introduction Sincesoil Fe is presentpredominantlyin the form of iron oxidesor a componentof solid silicate, phosphate,carbonateor sulfide phases,the quantitative determinationof total soil Fe usually involves decompositionof the soil matrix followed by dissolutionof the Fe, prior to chemicalanalysis.The most commonly usedproceduresfor decompositionof the soil matrix are Na2C03or Li metaborate(LiB0 2) fusion or acid digestionwith either HF or an HFIHCIOJH2S04 mixture (Loveland, 1988). When only Fe is to be determined,the acid digestion proceduresare usually preferredsince they are simpler and errors due to loss of sampleare lesslikely to occur. The disadvantageof acid dissolutionprocedures is that concentratedacidsare used,for which the researchershouldbe thorough-

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ly familiar with hazardsand necessaryprecautions(Hossner,1996, seeChapter 3). Iron solubilizedby either methodcan then be determinedby atomic absorption spectroscopy(AAS) or inductively coupledplasmaatomic emissionspectroscopy (ICPAES), or colorimetrically (most commonly by formation of the Fez+-phenanthroline complex).The final choiceof procedurefor decomposition of the soil matrix and Fe analysismay dependprimarily on availability of equipment. Soils for total elementalanalysisshouldbe finely ground, to passa 0.15mm pore size (approximately100 mesh in.-1) sieve, to ensureadequaterate of reactionduring the digestionor fusion process. Iron may exist in either the 2+ or 3+ oxidation states;therefore,a modification of the procedurefor total analysiscan involve determinationof Fe2+ and Fe3+. This modification usually involves the determinationsof Fe2+ and total Fe; concentrationof Fe3+ is then determinedby difference.Specialprecautionsare usually required,becausesome samplepretreatment(e.g., storage,drying) and fusion or digestion proceduresmay precludean accuratedeterminationof the original concentrationsof Fez+ and Fe3+ (seeLoveland, 1988).

Decompositionof Sample(SodiumCarbonateFusion) Principles Fusion with NazC03 is useful when the concentrationsof other elements (e.g., Al, Ca, Mg, Mn and Si) in addition to Fe are being determined.The NazC03 fusion reaction will result in the transformationof Fe to ionic forms which are totally solublein hydrochloricacid (HCI). The experimentershouldbe thoroughlyfamiliar with the careand maintenanceof platinum crucibles,which are utilized in this procedure(Jackson,1958; Lim & Jackson,1982). Graphite cruciblesare often preferredsincethe melts do not adhereto the crucible walls. Although they are cheaperthan platinum crucibles,graphitecruciblesmay last for only 6 to 10 digestions.

SpecialApparatus 1. Platinumcrucible, 30-mL capacity,with lid.

Reagents 1. Sodiumcarbonate,anhydrous. 2. Hydrochloric acid, 6 M. 3. Hydrochloric acid, concentrated,12 M.

Procedure Place1 g of soil, which has beengroundto passthrough a 100 meshin.-1 (0.15-mmnominal pore size) sieve, into a 15- to 30-mL capacityplatinum crucible (Jackson,1958). Cover the crucibleloosely, with the platinum coverslightly ajar. Heatthe crucibleslowly, to preventa suddenignition of soil organicmatter, with a Mekker burner to about 900°C, and maintain the heat for approximately 30 min. Cover the crucible, and allow it to cool in a desiccator.

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Add approximately5 g of anhydrousNazC03 to the previouslyignited soil, and mix the contentsthoroughly. Cover the crucible loosely and heat it slowly, to preventunnecessarysplattering,for 10 min; then increasethe intensity of the heat until the crucible bottom is red, and maintain this temperaturefor 15 to 20 min. Remove the cover and continue heating for a few additional minutes to completethe fusion. Discontinueheating, and while the sampleis still molten, swirl the crucible to distribute the cooling material along the sides of the container. Allow the sampleto cool completely,and then placeit in a Pyrex beaker. The formation of bubbles or small cracks during the initial cooling suggests incompletefusion. If this occurs,remelt the sampleand continueheatingat a sufficient temperatureto achieve a red crucible bottom for an additional 15 min. Allow the crucible to cool to room temperature. Removalfrom the crucible of a samplecontaininga high amountof silica can be facilitated by addinga sufficient quantity of hot deionizedwater to loosen the cooled sampleprior to placing it in the beaker.Add water to the crucible to facilitate the completetransferof sampleto the beaker.Add approximately5 mL of 6 M HCI to the crucible, and heatslowly to disintegratethe remainingsample. Samplesthat are difficult to remove may be transferredby placing the cooled crucibleon its side in the beakerand addingsufficient water to cover the sample; the crucible is then heatedslowly to loosenthe sampleand rinsed with water to remove the remaining sample.The whole sample is dissolvedby adding concentratedHCI slowly to the beaker,while using care to preventexcessiveeffervescence.The dissolvedFe is then brought to a known volume and analyzedby AAS, ICPAES or a colorimetric procedure. Decompositionof Sample(Hydrofluoric AcidlPerchloricAcid/ Sulfuric Acid Digestion) Principles

Digestion involves reaction of the soil with a mixture of concentrated acids, usually hydrofluoric acid (HF), perchloric acid (HCI04) and sulfuric acid (H ZS04). Since concentratedacids are involved in the procedure,the researcher mustbe cognizantof the hazardsand precautionsin their use(Hossner,1996,see Chapter3). Eachacid in the digestionmixture playsa specific role in the process. Hydrofluoric acid promotesdecompositionof the silicate matrix by formation of gaseousSiF4, HCI04 oxidizesorganicmatter,HZS0 4 moderatesthe violent reaction betweenHF and the sample(Loveland,1988)and promotes volatilizationof HF and HCI04. Soils that are high in organic matter should be pretreatedwith nitric acid (HN03) to minimize the possibility of explosionduring the reaction of organicmatterwith HCl04. The mixed acid systemdescribedbelow (Jackson, 1958; Lim & Jackson,1982) cannot be used to differentiate betweenFe z+ and Fe3+, since Fe z+ is readily oxidized in the presenceof HCI04 or HN03. Jeffery and Hutchison(1981) and Kiss (1984) describemixed acid systems,involving the useof H ZS04 and HF, which havebeenusedto differentiateFe z+ and Fe3+. A procedureinvolving the digestion of soil with HF/HzS04 in the presenceof excess1,1O-phenanthrolinein the dark, to prevent the photoreductionof Fe3+, hasbeenusedto determineFez+spectrophotometrically asthe Fe2+-1,1O-phenan-

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throline complex (Stucki, 1981b).Total Fe in the digest is then determinedfollowing the photoreductionof Fe3+ upon exposureto a mercury vapor lamp (Komadel & Stucki, 1988). SpecialApparatus 1. Platinumcrucible, 30-mL capacity,with lid, or a 50-mL PTFE (polyte-

trafluoroethylene)beaker. Reagents 1. Hydrofluoric acid, concentrated,48 to 50%.

2. 3. 4. 5.

Perchloricacid, concentrated,69 to 72%. Nitric acid (HN03), concentrated,69 to 71%. Sulfuric acid, concentrated,95 to 98%. Sulfuric acid, 3 M.

DigestionProcedure This digestion procedure(Jackson,1958; Lim & Jackson,1982) must be conductedin a HCI04 hood. The proceduredescribedbelow will result in the loss of Si; therefore,modificationsmust be employedif Si also is to be determined (Lim & Jackson,1982). Place 0.5 g of soil, ground to 0.15-mmparticle size (to passa 100 mesh in.-1 sieve) with a nonmetalgrinder, into a 15 to 30-mL capacityplatinum crucible. The PTFE beakeris heat resistantto 260°C; therefore,if the PTFE beaker is usedin place of the platinum crucible, specialprecautionsmust be taken that the beakeris not exposedto temperaturesover 240°C. Glassmust not be used sinceit will reactwith HE Carry a reagentblank throughthe entire digestionprocedure.With organic soils or soils high in organic matter, pretreatthe sample overnightat room temperaturewith 3 mL concentratedHN03. Add a few drops of H2S04, 5 mL of HF, and 0.5 mL of HCI04• Acids should always be added to room temperaturesamples;they should never be addedto hot samples,to minimize the dangerof explosion.The digestioncan be convenientlycarriedout on a sandbath. During digestion,the sampleshouldbe loosely covered,with the cover slightly ajar. Heat the crucible slowly to prevent a suddenignition of organicmatter, and then maintain the temperatureat 200 to 225°C until the sampleis dry. Cool the crucible, and add3 mL of water and a few dropsof HCI04• Heat on the sandbath until dry. Removeheat,and allow the crucible to cool. Add 5 mL of 3 M H2S04 and approximately5 mL of water, and heat the crucible to a gentleboil. Repeatthe procedureuntil the residueis completely dissolved.Dilute the samplewith deionizedwater for the determination of Fe by AAS, ICPAES or colorimetry. Decompositionof Sample(Hydrofluoric Acid Digestion) Digestionwith HF aloneis sufficient to dissolvemost soil minerals,especially at high temperatureand pressure(Bernas,1968). Hydrofluoric acid diges-

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tions are usually performed in sealedcontainers(usually PTFE or polypropylene), comparedto mixed acid digestionswhich are usually performedin open systems.The HF digestion procedureis summarizedin this book by Hossner (1996, see Chapter3). This procedureis not suitable for the differentiation of Fe2+ and Fe3+ due to the presenceof aqua regia. For a discussionof possible digestionproceduresfor the determinationof Fe2+ and Fe3+, the reader isreferred to Loveland(1988), Stucki (1981a,b)and Komadel and Stucki (1988).

SELECTIVE EXTRACTION PROCEDURES Introduction A large numberof selectivedissolutionproceduresdesignedto targetspecific fractions of soil Fe havebeendevelopedover the last 30 yr. Sincetotal soil Fe is of limited use in understandingbioavailability or pedogenicprocesses,it is often helpful to estimatethe amountof Fe presentin different solid-phaseforms. The principal fractions of soil Fe which are targetedfor selectivedissolution (extraction) include total or "free" iron oxide, "amorphous"or "active" iron oxide, organically bound Fe, and exchangeableor solution-phaseFe. The most commonlyusedselectivedissolutionproceduresare summarizedin Table 23-1, and the detailed proceduresare presentedin the sectionswhich follow. It is appropriateto considerthese selective-dissolutionproceduresas operationally defined basedon the specific extractionprocedure,rather than to think of each extractionas an accuratemeasureof a specific fraction of soil Fe, since none of the proceduresis absolutelyselectivefor the specific phasesfor which they are intended.

Total "Free" Iron Oxide Introduction The largestproportion of the total Fe of most soils is in the form of oxide minerals such as hematite,goethite, lepidocrociteand ferrihydrite. The proceTable 23-1. A summaryof commonly usedselective-dissolutionproceduresfor specific fractions of soil Fe. Targetphases

Procedure/reagent

References

Total "free" iron oxides

Holmgren, 1967 Mehra & Jackson,1960

Poorly crystalline ("active") iron oxides

Citrate-dithionite(unbuffered) Citrate-dithionite-bicarbonate (buffered at pH 7) NH4-oxalate-oxalicacid (pH 3) (in the dark)

Organically boundFe

Pyrophosphate,pH 10t

ExchangeableFe Availability indices

MgCl z Lindsay & Norvell, 1978 DTPA, buffered at pH 7.3 Ammonium bicarbonate-DTPA Soltanpour& Schwab,1977 pH 7.6

t This procedureis not specific for organicallybound Fe.

Schwertmann,1964 McKeague& Day, 1966 Jacksonet aI., 1986 McKeague,1967 Bascomb,1968

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dure for extractionand determinationof free iron oxide involves reductivedissolution, e.g., with zinc powder and ammonium tartrate (Haldane, 1956) or Na dithionite (Deb, 1950; Kilmer, 1960). During the latter reaction,sodiumdithionite decomposes to hydrogensulfite, resultingin an acid solution (pH 2.6-3.5)and the possible precipitationof FeS and elementalS. To prevent this precipitation reaction, Mehra and Jackson(1960) used citrate to chelatedissolved Fe2+ and NaHC03 to buffer the solutionnearpH 7. Holmgren (1967)concludedthat either NaHC03 or citrate in adequatequantitieswill result in sufficient pH buffering to prevent reagentdecompositionand precipitation of reaction products and that bicarbonateis not necessaryif the citrate/dithioniteratio is greaterthan 20:1. The bicarbonatemethodrequirestwo sampletreatmentsto ensurethoroughextraction of Fe and careful control of temperature(75-80°C) to: (i) ensureadequatereaction rate and (ii) minimize the decompositionof dithionite that occursat higher temperatures(Loveland, 1988).The citrate buffered system requires a single overnight extraction at room temperature;consequently,it is simpler than the HC03"-bufferedmethodand is more suitablefor large numbersof samples.Neither of the dithionite proceduresis suitable for determinationof the iron oxide Fe2+/Fe3+ mole ratio, since dithionite resultsin the reductionof Fe3+. Dithionite is a strong reductant and theoretically should be capable of reducing allFe3+ in iron-containingoxidesto Fe2+ at pH valuesbelow 10 (Borggaard,1988);however, thedissolutionefficiency is affectedby particlesize. With insufficient grinding, large crystalsof magnetite,goethiteand hematitemay not be totally dissolved during the dithionite procedure(McKeague & Day, 1966; McKeagueet aI., 1971; Walker, 1983). Although the dithionite proceduresare targetedfor "free" iron oxides,the extractswill include small contributionsfrom water-soluble,exchangeableand organically bound Fe. In addition, dithionite procedureshave been shown to attack a fraction of the Fe containedin layersilicate minerals, especially nontronite (Dudas & Harward, 1971; Ryan & Gschwend,1991) and montmorillonite and vermiculite containinghydroxy-iron interlayers (Carsteaet aI., 1970). Nevertheless,the amountsof pedogenically formed iron oxidesextractedusingdithionite procedureshavebeenshownto correlatewell with the amountsof iron oxidesdeterminedby x-ray diffraction. A relatively new procedure, outlined by Ryan and Gschwend(1991), utilizing a acid (EDTA) ternary complex of TI(III), citrate and ethylenediaminetetraacetic showssomepromisefor extractinglessstructuralFe from the layer silicates, than the dithionite procedures. Citrate-dithioniteExtractableIron

Reagents 1. Sodiumdithionite (Na2S204)' 2. Sodiumcitrate (Na3C6Hs07• 2 H20). 3. Superfloc16 polyacrylamideflocculating agent(Cytec Industries,Inc., West Paterson,NJ), 2 g L-l in water.

Procedure.The procedurebelow is a modification of the Holmgren(1967) procedure.Place0.5 g of soil previously ground to passa 100 mesh in- l (0.15mm nominal pore size) sieve into a 50-mL polypropylenecentrifugetube. Add

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0.5 g of Na2S204and 6 g of Na3CJfs07• 2 H20. Add 30 mL of deionizedwater, cover the tube, and shakeovernight(16 h) on a reciprocatingshaker.Transferthe suspensionto a 50-mL volumetric flask, and add one to two drops of Superfloc polyacrylamideflocculating agent. Shakevigorously for 15 s, dilute to volume, and shakeagain.A black or dark-graycolor of the suspensionis an indication of precipitationof FeS.In this case,increasethe quantity of Na3C6Hs07in the original suspensionand begin the procedureover again.Allow the suspensionto settle for 1 h, and centrifugeif necessary.Dilute the supernatewith deionizedwater as appropriatefor Fe analysis,and measureFe by AAS or ICPAES. During AAS analysis,the citrate dithionite reagenttendsto clog the burnerhead;therefore,it is helpful to aspiratedeionizedwater for at least 20 s betweensamples.

Citrate-bicarbonate-dithionite Method The procedurebelow is a modification of that describedby Mehra & Jackson (1960) and Jacksonet al. (1986).

Special Apparatus 1. Heatingwater bath capableof attaining80°C.

Reagents 1. Sodium citrate (Na3C6Hs07• 2 H20), 0.3 M. Add 88.2 g of solid Na3C6Hs07• 2 H20 to approximately200 mL deionizedwater in a l-L volumetric flask, swirl until dissolved,and bring to volume. 2. Sodiumbicarbonate(NaHC03), 1 M. Add 84 g of NaHC03 to a l-L volumetric flask, and dilute to volume. 3. Sodiumdithionite (NaZS204)' 4. Sodiumchloride (NaCl), saturatedsolution. 5. Acetone,reagentgrade. 6. Superfloc16 polyacrylamideflocculating agent(Cytec Industries,Inc., West Paterson,NJ), 2 g L-I in water.

Procedure.Transfer5 g of soil, containing less than 0.5 g of Fez03 and groundto passa 250-meshin.-l (0.06-mmnominal pore size) sieve,to a 100-mL polypropylenecentrifuge tube. The samplesize can be adjustedin accordance with the expectedamountof extractableFe. Add 40 mL of 0.3 M sodium citrate and 5 mL of 1 M NaHC03. Shakethe tube to mix the contents,and heat the tube in a water bath at 75 to 80°C for severalminuteswhile stirring the suspension with a glassrod. Care must be taken to avoid temperatureshigher than 80°C to preventdecompositionof dithionite and the possibleformation of FeS.When the temperatureof the soil suspensionhas risen to 75 to 80°C, add about 1 g of NaZS204powderwith a calibratedspoon;immediatelystir for 1 min, then intermittently for 5 min. Add a secondI-g portion of Na2SZ04,and continuestirring intermittently for an additional 10 min. After the digestion,add 10 mL of saturated NaCI to promote flocculation, and centrifuge. If the suspensionfails to flocculate with the addition of NaCI, add 10 mL of acetone(C3H60), mix the contents,warm in a water bath, and centrifuge for 5 min at 1600 to 2200 rotations per minute (rpm). Decantthe supernateinto a 500-mL volumetric flask. An

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alternativeflocculation procedureinvolves the addition of two to four drops of Superflocsolution, thorough mixing and centrifugationfor 5 min at :WOO rpm (Holmgren, 1967). Repeatthe extractionprocedurefor samplesin which a brown or red coloration persistsor thosecontainingmore than 5% Fez03'Washthe sampleswith sodiumcitratesolution(andwith NaCl if necessaryfor flocculation), combinethe washingswith the previousdecantate,dilute with deionizedwaterto volume, and mix. Iron can be determinedby AAS by direct aspirationof the appropriately diluted citrate-bicarbonate-dithionite solution (Jenneet aI., 1974). During AAS analysis,the citrate dithionite reagenttendsto clog the burnerhead;therefore,it is helpful to aspiratedeionizedwater for at least20 s betweensamples.Alternatively, Fe may be determinedby the Fez+-1,1O-phenanthroline procedure.A reagent blank should be utilized throughoutthe procedureand subtractedfrom the Fe determinedin the sample.

"Active" or "Amorphous"Iron Oxide Introduction The "active" iron oxides(often called noncrystalline,poorly ordered,poor1y crystalline,short-rangeorderedor amorphousiron oxides) are the most reactive iron oxides in the soil due to their small size and consequentlyhigh surface area.This classof oxidesincludesferrihydrite and the ferrihydritelike minerals. The most commonlyusedprocedurefor obtainingquantitativeestimatesof the "active" iron oxide componentis pH 3.0, 0.2 M ammoniumoxalateextraction in the dark for either 2 h (Schwertmann,1964) or 4 h (McKeague& Day, 1966). Although eachof the iron oxide mineralswill react with ammoniumoxalate to someextent (Schwertmann,1991), the reactionsproceedat considerablydifferent rateswhich are dependenton particle size and surfacereactivity. The preferential dissolutionof poorly crystallineiron oxide (including ferrihydrite) hasbeen confirmedby differential x-ray diffraction (Schwertmannet aI., 1982); however, considerabledissolutionof lepidocrocite andmagnetitealso may occur(Schwertmann, 1973). Acid ammoniumoxalate has little effect on kaolinite, montmorillonite, vermiculite or illite (McKeague& Day, 1966; Hodges& Zelazny, 1980); however,Arshad et aI. (1972) observedconsiderabledissolutionof trioctahedral layer silicates,e.g., biotite and chlorite. The ammoniumoxalate extraction is a kinetically controlledprocedure;therefore,the quantity of Fe extractedis strongly influencedby reactiontime and temperature,as well as shakingintensity. The proceduremust be performedin the dark to preventphotoreductionand retard rate of dissolutionof the crystalline iron oxides.Ammonium oxalateextractable Fe will include water-solubleFe, exchangeableFe and a fraction of the organically boundFe. Most of the variationsof the ammoniumoxalateprocedureinvolve preparation of pH 3 ammoniumoxalatefrom a mixture of ammoniumoxalateandoxalic acid (Tamm, 1922; Schwertmann,1964; McKeague & Day, 1966; Chao & Zhou, 1983).Jacksonet ai. (1986) preparedthe reagentby acidification of 0.2 M

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ammoniumoxalate to pH 3.0 with HCl. Calcareoussoils must be pretreatedto removeCaC03, sinceoxalic acid will reactwith CaC03 to changethe pH of the oxalate/oxalicacid buffer; also, oxalatewill be precipitatedas the Ca salt. Calcium carbonatecan be removedby reactionwith ammoniumacetateat pH 5.5. In a comparisonof a rangeof soils and sediments,Chao and Zhou (1983) observedthat extraction with a combined solution of 0.2 M hydroxylamine hydrochlorideand 0.2 M HCI at 50°C for 30 min gave results similar to those obtainedwith the ammoniumoxalateextractionin the dark. Alkaline EDTA also has beenused to dissolve noncrystallineiron oxides (Borggaard,1988); however, reactiontimes of 90 d or more are requiredfor quantitativeextraction.Results by the acid ammonium oxalate and EDTA proceduresare highly correlated (Borggaard,1988). Reagent Acid Ammonium Oxalatein Darkness-Thmm's The procedure described below is a modification of the procedureof Schwertmann(1964) and McKeagueand Day (1966).

Reagents 1. Acidified ammoniumoxalate,pH 3.0. Preparea solutionof 0.175M ammonium oxalate[(NH4)2C204] + 0.1 M oxalic acid (H 2C20 4). Add 24.87 g of (NH4)2C204• H20 and 12.61 g of H2C20 4 • 2H20 to approximately 800 mL deionizedwater, adjust to pH 3.0 by addition of NH40H or HCI, and dilute to 1 L final volume. 2. Ammonium acetate,1.0 M, pH 5.5. Add 60 g of glacial acetic acid to 600 mL of deionizedwater, adjustto pH 5.5 with NH40H, and dilute to 1 L final volume. Procedure.Place500 mg of soil, which hasbeenpreviouslygroundto pass a 100 meshin.- I (0.15-mmnominal poresize) sieve,into a 50-mL polypropylene centrifugetube. Calcareoussoils must be pretreatedto remove CaC03. Add 30 mL of pH 5.5 1.0 M ammoniumacetate,allow to react for 1 h with intermittent stirring of the ventedcontainer,measurethe pH with a combinationpH electrode, and readjustthe pH to 5.5 by the dropwise addition of acetic acid. Repeatthe addition of aceticacid hourly until the pH remainsapproximately constant. Centrifuge, decant, wash with deionizedwater twice to remove dissolved Ca and acetate,and allow the sampleto air dry. Before the ammoniumoxalate is added in the stepbelow, care must be taken that the sampleis crushedto a suitableparticle size. Add 30 mL of pH 3.0 ammoniumoxalatesolution, stopperthe tube, and immediately place the tube in a light-proof container.Immediately begin agitation of the sample,and continuefor exactly 2 h on a reciprocatingshaker.Centrifuge the sample, decant, dilute the supernateas appropriatewith deionized water, and analyzefor Fe by AAS or ICPAES. If the samplecannotbe analyzed immediatelyit should be storedin the dark to preventphotoinduceddecomposition of oxalate,which could result in precipitationof Fe (Borggaard,1988). High concentrationsof oxalatecan result in cloggingof the burnerheadduring the nebulization process;therefore,samplesshould be diluted with deionizedwater to

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the extent possibleprior to analysis.Also, to minimize clogging, it is helpful to aspiratedeionizedwater for at least 20 s betweensamples.High concentrations of oxalatewill interfere with AAS analysis;therefore,it might be necessaryto decomposethe oxalateprior to analysis.This stepis accomplishedby evaporating a known volume of the ammonium oxalate extract to dryness,ashing at 500°C in a porcelaincrucible for 1 h, and redissolvingthe sedimentin a known volume of 1 M HCl, prior to AAS analysis.Samplescan alternativelybe treated by digesting1 mL of the ammoniumoxalate extractwith lO mL of concentrated RN03 to drynessand dissolvingthe sedimentin a knownvolume of 1 M HCI. As an alternativeto AAS analysis,the samplecan be analyzedspectrophotometrically by the Fe2+-l,lO-phenanthrolinemethod.

Organically Bound Iron Introduction Potassium-or Na-pyrophosphate extractions,usually at pH lO, have been utilized for the estimationof organicallyboundFe (McKeague,1967; Bascomb, 1968).Thereareseveralvery severeproblemswith this procedure,especiallythe peptization and dispersion of microcrystalline iron oxide by pyrophosphate (Jeanroy& Guillet, 1981).Becauseof this dispersionphenomenon,results are highly dependenton the centrifugationand ftltration procedure(Schuppli et aI., 1983; Loveland & Digby, 1984). The quantity of Fe extractedwith pyrophosphate decreaseswith increasingcentrifugation(McKeague& Schuppli, 1982); therefore,uniform high-speedcentrifugationor micropore filtration treatments are required (Schuppli et aI., 1983; Loveland & Digby, 1984).Even following high-speedcentrifugation, microcrystalline clays and oxides may remain suspended(Borggaard,1988); however,high-speedcentrifugationmay be preferred to ftltration becauseof the problemof cloggedfilters during microporeftltration procedures(Loveland, 1988). Clariftcation of the extractalso hasbeenachieved by treatmentwith Superflocor Na2S04(Schuppli et aI., 1983). The dispersion Fe also is influenced phenomenon and the quantity of pyrophosphate-extractable by pH and whetherthe Na or K salt is utilized (Loveland& Digby, 1984). Loveland and Digby (1984)concludedthat a techniqueusing 0.1 M Na pyrophosphate at pH 10 andcentrifugationat 20 000 x G providedthe most reproducibleresults. Studieshave indicatedthat the pyrophosphateextract obtainedfollowing high-speedcentrifugation includesmicrocrystalline iron oxide particles which are mixed or possibly coatedwith organicmatter,though it is not clear whether theseiron-oxide/organic-mattercomplexeswere originally presentin the soil or formed during the extraction procedure(McKeague & Schuppli, 1982). Pyrophosphate-extractable Fe cannotbe designatedas organically bound Fe, due to uncertaintyabout the actual sourceof the extractedFe (Schuppli et aI., 1983). Despite these problems, the procedureis presentedbelow due to its frequent occurrencein the literature.The researchershouldpreciselystatethe centrifugation and clariftcation procedureand must take care not to overextendthe interpretationof results. Acetylacetone (Bascomb & Thanigasalam,1978) and 0.05 M Na2B407at pH 9.7 have beenusedas alternativesto the pyrophosphateextraction.Sodium

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tetraboratewas found to be a poor extractantfor organically bound Fe (McKeague& Sheldrick, 1977). Bascomband Thanigasalam(1978) concludedthat acetylacetoneis lesssuitablethan pyrophosphate for extractingorganicallybound Fe, sinceacetylacetonedissolvesmore of the mineral forms of Fe than pyrophosphate. SodiumPyrophosphateExtractableIron

Reagents 1. Sodiumpyrophosphate(Na4P207),0.1 M, pH 10.0. Dissolve 44.61 g of Na4P207.10 H20 in approximately800 mL of deionizedwater. Adjust to pH 10 with NaOH, and dilute to volume (1 L) with deionizedwater. Storethe pyrophosphatesolution in the refrigeratorand in a sealedcontainer with a CO2 trap to preventcontactwith atmosphericCO2, Procedure. Place 250 mg of soil «2-mm particle size) into a 50-mL polypropylenecentrifugetube. Add 25 mL of 0.1 M Na4P207(pH 10), and shake 16 h at 23°C on a reciprocatingshaker. Centrifuge at 20 000 x G for 30 min. Decantthe supernate,and analyzefor Fe using AAS or ICPAES. Water-Solubleand ExchangeableIron Introduction Water-solubleFe in the soil is usually presentin such low concentrations that it is detectableonly in soils of very low pH «4.5; e.g., exposedlignite beds or soils exposedto acid mine drainage),or very low redox potential (e.g., tidal marshsedimentsor flooded soils). It is possibleto directly extractsuitablequantities of soil pore waterfor subsequentelementalanalysisonly from wet or watersaturatedsoils. Immiscible displacementproceduresare required for the displacementof pore water from drier soils (Kinniburgh & Miles, 1983; Kittrick, 1983).Otherproceduresfor obtainingsoil waterextractsincludeextractionof the soil with deionizedwater (as is discussedin the section below)or preparationof a water-saturatedsoil paste. Extractionof soil with neutral(e.g.,CaCl2, MgCl 2or KCl) or buffered(e.g., NH4C2H30 2) salt solutionswill result in the displacementof both dissolvedFe and exchangeable Fe. Acid-bufferedextractantsmay not be suitablefor neutralor calcareoussoils, sincethey could result in considerabledissolutionof solid-phase carbonatesand oxideswith the possiblereleaseof Fe from the mineral structures. Becauseof the usually low concentrationsin the soil, detectionof exchangeable Fe extractedwith neutral or buffered salts only will be possiblefor soils of low pH and/or redox potential. Samplesfor the determinationof water-solubleor salt-extractableFe must not be air or oven dried or evenexposedto air, sincetheseprocedurescould result in changesin the forms and concentrationof extractableFe. Samplesextracted with either deionizedwater or neutral salt solutionsmust be protectedfrom the atmosphereafter sampling to prevent changesin redox potential, oxidation of Fe2+ to Fe3+ and precipitationof dissolvedFe as iron oxides.

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Microcrystalline iron oxides are readily dispersedduring soil extraction, especially during extraction with deionized water and the alkali metal salts. Therefore,extractsshouldbe passedthrough filters with pore sizes EDTA. The quantity of Fe extractedgenerallydecreasesin the sameorder. Lindsay and Norvell (1978) concluded that DTPA was the most useful extractantfor the simultaneousextraction of Fe, Zn, Mn and Cu. A modification of the DTPA procedure,in which the extractingsolution is bufferedby 1 M NH4HC03 at pH 7.6, hasbeenproposedby Soltanpourand Schwab(1977) for the simultaneousextractionof NO), K+ and phosphate,as weltas Fe, Mn, Zn, and Cu.

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Diethylenetriaminepentaacetic Acid (DTPA) Soil Test

Theory The DTPA solution (0.005 M) is buffered at pH 7.3 with 0.1 M triethanolamine(TEA) in the presenceof 0.01 M CaCl2 (Lindsay & Norvell, 1978). The rate of releaseof Fe by DTPA is highly dependenton pH. A pH of 7.3 was adoptedto preventthe dissolutionof CaC03 and the possiblereleaseof occluded Fe which could occurat lower pH values,and to avoid the reductionin Fe release that is observedat higher pH values.Triethanolaminewas selectedas the buffer becauseof its pKa (7.8) and becauseit burnscleanly with little interferenceduring AAS analysis.At pH 7.3 and 0.01 M Ca2+, the DTPA is fully complexedwith Ca; in the presenceof Fe3+, the Fe competeseffectively with Ca for complexation by DTPA. Approximately 50% of the added Ca is complexedby DTPA, while the remaining50% is in solution as Ca2+. At pH 7.3, approximately75% of the total TEA is presentin the protonatedtriethanolammonium(HTEA+) form. The HTEA+ can competewith Ca2+ for cation exchangesites, thereby further increasingthe Ca2+ concentrationin solution and retarding the dissolution of CaC03· Diethylenetriaminepentaacetic acid will form complexeswith soluble and exchangeableFe, as well as with Fe which is mobilized from the Fe containing solid phases,predominantlythe iron oxides. The releaseof Fe from iron oxide occursas a two-stepreaction:(1) a rapid chemisorptionof ligand at the mineral surfaceand a concomitantweakeningof internal O-Fe-O bonds,and (2) a slow (rate-limiting) abstractionand chelation of Fe by the surface adsorbedligand (Stumm & Furrer, 1987; Schwertmann,1991). Steps(1) and (2) are each influencedby the propertiesof the ligand and by the mineralogyand surfaceareaof the oxide. The rate of reactionof DTPA with soil iron oxidesgenerallydecreases in the following order: ferrihydrite :> lepidocrocite= magnetite:> goethite > hematite. The quantity of DTPA-extractableFe is highly dependenton reactiontime (Lindsay & Norvell, 1978; Geiger & Loeppert,1988).The rate of dissolutionof Fe from soil decreasescurvilinearly, due to the initial more rapid reaction with high-energysurfacesitesand a gradualreductionin reactionrate asthe active surface sitesare consumed.In the normal2-hextractionof soil with DTPA, the actual final concentrationof the Fe-DTPA complex (usually within the range of approximately0.25-20mg L- 1) is significantly less than the theoreticalequilibrium concentration(approximately 279 mg L- 1) (Lindsay & Norvell, 1978). Therefore, the quantity of DTPA-extractableFe is highly dependenton any experimentalfactor that influencesreaction rate, e.g., grinding intensity, extraction temperature,shaking time and shaking intensity (Soltanpouret aI., 1976). Standardizationof the procedureis very important to obtain uniform and reproducible results in the DTPA soil test. Since DTPA is presentin excessof the micronutrientmetal cationsthat are normally solubilizedduring the extractionof most agriculturalsoils, the extractionof one micronutrientwill probably not significantly affect the amounts of other metals extracted (Lindsay & Norvell, 1978). The DTPA soil test developedby Lindsay and Norvell (1978) and the NH4HC03 DTPA test proposedby Soltanpourand Schwab(1977) were primari-

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ly intendedfor determiningFe bioavailability in neutraland calcareoussoils. The useof theseextractantshasbeenquestionedfor acid soils below pH 6, for which it may not be appropriateto usea bufferedextractantat pH 7.3 or 7.6 (O'Connor, 1988). Furthermore,highly acid soils may exhaustthe buffering capacity of the TEA presentin the DTPA. extractingsolution. If the final pH of the extracting solution varies for different soils, one can expect significant variability in the amountof metal extracteddue to changesin reactionkinetics as a function of pH. Haynesand Swift (1983) evaluatedthe use of DTPA extraction in moderately acid soils and found significant variability in extractableFe as a function of the pH of the extracting solutions. Generally, the amounts of extractablemetals, including Fe, increasewith decreasingpH of the extractingsolution. At very low pH values,as may be encounteredin somemine spoil soils, the concentrationof DTPA may not be sufficient at 0.005 M to avoid secondaryinteractionsamong metal ions. In summary,althoughit may be more appropriateto usean unbuffered DTPA for soils with pH valuesbelow six, the concentrationof DTPA may be too low, resulting in competitionamongmetalsfor the DTPA. O'Connor(1988) has summarizedseveral limitations and inappropriateuses of the DTPA soil test, especiallywith regard to soils with high metal loadings. Users should be cognizant that the DTPA soil test in its current form was designedto extract labile forms of Fe for the purposeof distinguishingbetweenFe deficient and nondeficient neutraland calcareoussoils.

Influenceof Soil Factorson the DTPA Extraction Procedure The amountof Fe extractedby DTPA (Loeppert& Hallmark, 1985; Geiger & Loeppert, 1988; Morris et aI., 1990), as well as by EDTA (Borggaard,1976; Borggaard,1982) and EDDHA (Loeppert & Hallmark, 1985), is positively correlatedwith the "active" iron-oxidecontentof the soil as assessed by pH 3 ammonium oxalateextractionin the dark, though lower quantitiesof Fe are extracted in a given time by the chelateextractantsthan by ammoniumoxalate.Dissolution of soil iron oxide by DTPA occursas a surfacereactionwhich is strongly influencedby the quantity, particle-sizedistribution, surfaceareaand surfacereactivity of the soil iron oxides. The evidencestrongly suggeststhat the predominant sourceof DTPA-extractableFe in oxidized calcareoussoils is the highly reactive poorly crystalline iron-oxide component,which often exists in associationwith soil organic matter and layer silicates. A much smaller proportion is usually attributable directly to organically bound Fe2+ and Fe3+ (Geiger & Loeppert, 1988). In acid soils, a significantly higher proportion of DTPA-extractableFe may be attributableto organicallybound Fe, due to the strongpH dependencyof the stability of the Fe3+-humic complex.

Influenceof SamplePreparationVariableson the DTPA Extraction Procedure The air drying of oxidized field-moist samplesusually resultsin increases in the level of DTPA-extractableFe (Shuman,1980; Leggett & Argyle, 1983; Cihacek,1988; Geiger& Loeppert,1994); however,air drying of highly reduced or flooded soils may result in decreasesin DTPA-extractableFe due to oxidation

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of Fe2+ andprecipitationas iron oxides.LeggettandArgyle (1983) observedthat the increasedextraction of Fe due to air drying was only partially reversedby reincubationwith water. Ovendrying of the soil prior to extractionusually results in substantial increasesin DTPA-extractable Fe (Khan & Soltanpour, 1978; Leggett & Argyle, 1983; Geiger & Loeppert, 1994). This latter phenomenonis attributable to the formation of surface defects on the poorly crystalline iron oxidesand increasesin surfacefree energyand reactivity.

Correb.tionof DTPA·ExtractableIron with Iron Chlorosis Correlationstudiesrelating DTPA-extractableFe with crop growth are necessaryto give meaningful interpretationsto the soil test. Lindsay and Norvell (1978) observedthat DTPA-extractableFe was highly correlatedwith the incidenceof Fe chlorosisof sorghum(Sorghum x alum L. Parodi) and reportedthe following Fe soil-testcorrelationvaluesfor sorghum:4.5 mg kg-1, sufficient. Correlationof DTPA-extractableFe with the incidenceof Fe chlorosisgenerally has beenmost successfulfor grasses(StrategyII plants; Marschneret aI., 1986; Romheld& Marschner,1986) in well-drainedcalcareoussoils (Loeppert& Hallmark, 1985). For Strategy II plants the primary mechanismsof Fe-deficiency stressresponseare phytosiderophore(chelator)releaseand subsequentFe mobilization anduptake.The strengthof the DTPA soil test lies in the ability of DTPA to extractthe samelabile forms of soil Fe as the plant (Geiger& Loeppert,1988). The DTPA soil test has beenconsiderablyless successfulwith StrategyI plantsin which the primary Fe-deficiencystress-response mechanismsare acidification of the rhizosphereand increasedroot plasma-membrane Fe3+-reductase activity (Marschneret al., 1986; Romheld & Marschner,1986). Diethylenetriaminepentaacetic acid-extractableFe predictedonly 14 to 20% of the observed variation in chlorophyll concentrationof soybean(Glycine max L. Merr.) grown in 23 calcareoussoils in a growth room (Morris et al., 1990). For the StrategyI plants, solution HCO)" concentration(Fleming et aI., 1984; Inskeep & Bloom, 1986; Marschneret al., 1986) and soil CaC03 reactivity (Morris et aI., 1990) are importantsoil factorswhich influence the effectivenessof the plant Fe-deficiency stress-response mechanism.Also, in wet or poorly drained soils, the DTPA of Fe availability to soil test is usually of limited usefulnessin the assessment plantsbecauseof the importantinfluenceof soil aerationand root respirationon Fe mobilization and uptake.Other indigenoussoil factors (suchas soil carbonate reactivity) and soil environmental factors(such as soil water, aerationand compaction), as well as cultivar Fe-deficiencystresstolerance,also must be consideredto adequatelypredict Fe chlorosisof soybean(Morris et aI., 1990). Multiparametermodels(e.g., DTPA-extractableFe, soil-waterstatus,and CaC03 activity) may be much more generallyuseful, but havenot beenextensivelyutilized.

Procedurefor DTPA·ExtractableIron Reagents 1. Triethanolamine. 2. Diethylenetriaminepentaacetic acid.

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3. Calcium chloride (CaCI2 • 2 H20). 4. Hydrochloric acid, 6 M.

Preparationof DTPA ExtractingSolution Dissolve 149.1 g reagentgradeTEA, 19.67 g DTPA, and 14.7 g CaCl2 • 2 H 20 in approximately200 mL of deionizedwater. After the DTPA hasdissolved, dilute to 9 L with deionizedwater. Adjust the pH to 7.3 ± 0.05 with 6 M HCI (approximately83 mL), and dilute to 10 L final volume.

Procedure The samplepretreatmentprocedureshould be precisely defined, since it will influence the quantityof DTPA-extractableFe. Most researchersprefer air drying at room temperature,since the exact conditionsof pretreatmentare easier to reproduce;results also are usually reproducible.More intensedrying procedures, e.g., oven drying, should be avoided. The proceduresummarizedbelow was originally proposedby Lindsay and Norvell (1978). Add 20 mL of extracting solution to 10 g of air-dried soil (previously ground to pass a 2-mm pore-sizesieve) in a SO-mL polypropylenecentrifuge tube. Shakeon a reciprocatingshakerfor exactly 2 h at room temperature(230 C). Since this is a kinetically controlled procedure,temperature,shaking time and shaking intensity are critical experimentalvariables.Centrifuge immediately at 3000 x G, decantthe supernate,and filter through a O.4S-~m pore-sizemembranefilter. Following appropriatedilution, the extractshouldbe analyzedfor Fe by AAS or ICPAES.

Procedurefor Ammonium Bicarbonate-DTPA Extractable Iron Theory The ammonium bicarbonate-DTPA(AB-DTPA) soil test was originally developedby Soltanpourand Schwab(1977) as a modification of the DTPA soil test. The primary goal was to develop a soil test which would simultaneously extractN03, P, K, Zn, Fe, Mn and Cu from neutraland calcareoussoils. This test also has beenusedto extractother elementsincluding Pb, Cd, Ni, Se, As, B, Mo and S from mine spoils and soils treatedwith sewagesludge. The relationship betweenbioavailability and AB-DTPA extractableamountsof severalof these elementswas reviewedby Soltanpour(1991). The AB-DTPA extracting solution is composedof 1 M NH4HC03 and 0.005 M DTPA and is adjustedto an initial pH of 7.6. The extractingsolutionwill releaseCO2(g) when exposedto the atmosphereor during soil extraction,with the subsequentincreasein pH to valuesas high as 8.5. At 1 M NH 4HC03, the ABDTPA extracting solution contains similar concentrationsof NUt and HC03 used in traditional extraction methodologiesfor cations including K+ and for anionsincluding pol-. The presenceof DTPA allows for extractionof tracemetals which complex with DTPA (Lindsay & Norvell, 1978). The solutionmay be slightly colored due to the solubilization of organic matter; however, carbon black shouldnot be usedas a clarifying agent,sinceit can result in adsorptionof DTPA and Fe-DTPA from the extract(Soltanpour& Workman, 1979).

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The AB-DTPA soil test hasbeenshownto successfullydistinguishFe deficient from nondeficientsoils for sorghum (Havlin& Soltanpour, 1981, 1982, 1984). Soils with AB-DTPA extractable-Fevalues5% as much F as the initial 400 mL, continuetaking 100-mL samplesof distillate until complete recovery is indicatedby lack of measurableamountsof F. In practice,the amountof F releasedafter 400 mL is collectedwill seldomexceed1 or 2% of the total F containedin the sample. SteamDistillation for Separationof Fluorine from Sulfuric AcidS

Special Apparatus 1. Steamdistillation apparatus(see "Special Apparatus" under "Steam Distillation Methodfor Separationof Fluorinefrom PerchloricAcid").

Reagents 1. Sulfuric acid, 70%: Add slowly 1 L of reagent-grade H2S04 to 790 mL of deionizedwater. 2. Silver sulfate(Ag2S04), reagent-grade. 3. p-nitrophenolindicator: Dissolve 0.5 g of p-nitrophenolin 100 mL of deionizedwater. 4. Sodium hydroxide,50%: Dissolve50 g of reagent-gradeNaOH in 50 mL of deionizedwater. Procedure. Transfer the condenseddistillate from "Procedure"under "Sulfuric Acid Method for Total Fluorine" to a distillation flask, rinse the Erlenmeyerflask twice with 25 mL of 70% H2S04, and add 0.1 g of Ag2S04 (a small measuringscoopcan be madeto containapproximatelythis amount)and 8 to 10 glassbeads.When the temperaturereaches130°C,introducesteamand continue

8 Hoskins and Ferris (1936), Armstrong (1936), Hillebrand and Lundell (1953, p. 737-748),as adaptedfrom Brewer (1965a).

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heatinguntil the temperaturereaches13SoC.Adjust the heatand steamto give S to 6 mL of distillate per minute at a distillation temperatureof 13S ± 2°e. Distill approximately37S mL into a SOO-mLErlenmeyerflask calibratedat 400 mL and containingone drop of p-nitrophenoland one drop of SO% NaOH. If the yellow color disappears,add additional NaOH to maintain the distillate alkaline. Comments.Under certainconditions,particularly when the stills are new, mild to violent bumpingof the acid solutionmay be experiencedbetween11S to 130°e. To avoid such an occurrence,a trickle of steam should be bubbled through the acid solution beginningat about lOSoC. Only a minimum of steam should be admittedbelow the prescribedtemperatureof 130°C, however. SolubleFluorine Principles Soil type, pH, Ca, P, and AI contentsappearto be the predominantfactors controlling F availability to plants (Brewer, 1965b). Prince et al. (1949) and Hurd-Karrer (19S0) showedthat only under very special conditions (e.g., very acid, sandysoils low in P and Ca) was plant uptakeof F from soil relatedto total soil F. Water-solubleF is expectedto be the most readily available form for plants. However, uptake by plants will not be controlled completely by F concentration in water extracts. Fluoride is adsorbedon soil surfacesand forms insoluble compoundsmuch like phosphate. For water-solubleF, Brewer (196Sa)recommendeda 1:1 Vol. soil extract. Distillation of the extractfrom HCI04 (see"SteamDistillation Method for Separation of Fluorine from Perchloric Acid") or H2S04 (see "Steam Distillation Method for Separationof Fluorine from Sulfuric Acid"), followed by titration with Th(N03)4, was recommendedfor accurateresults.A colorimetricprocedure was outlined when high precisionwas not necessary(Brewer, 1965a).Use of the SIE offersanotherrapid, accuratemethodfor determiningwater solution F (see "Fluorine DeterminationUsing Direct PotentiometricMethod"). Larsenand Widdowson(1971) useda CaCl2 solution to obtain water soluble and weakly adsorbedF. They recommendedusing an anion exchangeresin for labile F. Oncethe extractis obtained,as before,selectionof an analytical procedurefor F dependson the precisionneededand equipmentavailability. Water-SolubleFluorine Procedure.Mix SOO mL of deionizedwater with SOO mL of weighed,SOmesh(300-/lm), air-dried soil, and allow the mixture to stand2 to 3 h. Transfer the soil suspensionto a IS-cm Buchnerfunnel fitted with Whatmanno. 1 filter paper,and apply vacuum.Fluorine in the extractcan be determinedby a potentiometric method (see "Fluorine Determination Using Direct Potentiometric Method"), by a volumetricmethod(see"Titrimetric FluorineDeterminationwith Thorium Nitrate in Presenceof ChromeAzurol Sulfur" or "Titrimetric Fluorine Determinationwith Thorium Nitrate in Presenceof Alizarin Red Sulfur"), after distillation (see"SteamDistillation Method for Separationof Fluorinefrom Per-

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chloric Acid" or "Steam Distillation for Separationof Fluorine from Sulfuric Acid"), or colorimetrically (see"Fluorine DeterminationUsing Sodium2-(parasulfophenylazo)-1,8-dihydroxy-3,6-Naphthalene disulfateMethod"). The method describedis adaptedfrom thosereportedby Brewer (1965a).

0.01 M Calcium Chloride-ExtractableFluorine Special Apparatus 1. End-over-endshakeror a suitablereciprocatingshaker.

Reagents 1. Calciumchloridesolution,0.01 M: Weigh 1.47g of CaCl2• 2H20, and dilute to 1 L with deionizedwater. Procedure.To 16 g of air-dried andsieved«0.42mm) soil, add50 mL of 0.01 M CaCl2 solution. Using an end-over-endshaker,shakethe mixture for 16

h at 25°C. Analyze the solution for F by direct SIE measurement after filtration. Fluorine can be analyzedby another method as outlined in "Fluorine Determination Using Sodium2-(parasulfophenylazo)-1,8-dihydroxy-3,6-Naphthalene disulfate Method." The method describedis adaptedfrom those reported by Larsenand Widdowson(1971). Comments.The possibility of fluoride (CaF2) controlling the solubility of F shouldbe acknowledged.However,Larsenand Widdowson(1971) found that F concentrationswere too low to allow CaF2 formation. In soils they tested,F was commonly in the 0- to 0.2 mg kg-1 rangewith the 0.01 M CaCl2 extractant. By an anion exchangemethod,they found F in the 20-mg kg-1 range. Measurementof Fluorine in Solution Fluorine DeterminationUsing Direct PotentiometricMethod Principles.The direct potentiometricmethodfor F determinationby solidstateSIE hasreceivedwide application.This electrode,whoseuniqueproperties arisefrom a Eu-dopedLaF3 crystal, is sensitiveto P-- andvirtually no otheranion or cation.It is one of the few truly specific ion-selectiveelectrodes.It gives a Nernstianresponseto P-- from >1 M to 2.0 mL of Th(N03)4solution B, titrate the duplicatesamplewith Th(N03)4 solution A using the samecolor blank as with solution B, and multiply the titration value by 5. Make up a new color blank every 15 min. Preparetwo 20-llg F standardsby pipettingexactly 2.0 mL of 10 mg L-1 of NaF solution into Nesslertubesand making the solution up to 50 mL with pH 3.3 water. Add 1.0 mL of indicator, and titrate each standardwith Th(N03)4 solution B until the color matchesthe color blank. Subtract0.1 mL (color blank) from the volumesof Th(N03)4 required,and divide by 20 to obtain the titer. From the titration valuesfor individual soil samples,subtractthe volumeof Th(N03)4 solutionB requiredto titrate a reagentblank. For each12 determinations,carry at leastone blank (reagentsonly-no soil samples)throughthe completedecompositionand separationprocesses.Multiply the correctedtitration valueby the titer to obtain the milligrams of F in the 50 mL that were titrated. Multiply this value by eight to obtainthe total milligrams of F in the original sample.To obtainthe F concentrationin milligrams perkilogram, divide the total milligrams of F found by the weight of the soil sampleexpressedin grams.

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Titrimetric Fluorine Determinationwith Thorium Nitrate in Presence of Alizarin Red SulfurlO

Reagents 1. Sodiumalizarin sulfonate:Dissolve0.5 g of sodiumalizarin sulfonate in 1 L of deionizedwater. 2. Hydrochloric acid, approximately1 M solution. 3. Chloroacetatebuffer: Dissolve9.45 g of monochloroaceticacid and 2 g of NaOH in 100 mL of deionizedwater. 4. Reagents2, 3, 4, 5, 8, 9, and 10 described in "Reagents"under "Titrimetric Fluorine Determinationwith Thorium Nitrate in Presenceof ChromeAzurol Sulfur." Procedure.Pipette 100 mL of distillate from "Procedure"under "Steam Distillation Method for Separationof Fluorine from PerchloricAcid" or "Procedure" under "SteamDistillation for Separationof Fluorine from Sulfuric Acid" into a beaker.Add 2 mL of sodium alizarin sulfonate. Neutralize the solution with 1 M NaOH addeddropwiseuntil a pink color appears,and then add 2.0 mL of buffer solution. Checkthe solution with a pH meterto makesurethat the pH is between2.9 and 3.1. Divide the buffereddistillate evenly betweentwo 50-mL Nesslertubes,and titrate eachtube with 0.002N (0.0005M) Th(N03)4 titrating solution until a pink color that matchesthat of a color blank is obtained.Prepare the color blank by adding 1 mL of indicator, 1 mL of buffer, and 0.1 mL of Th(N03)4 Solution B to 50 mL of deionizedwater in a Nesslertube. The color blank shouldbe a light peach-blossompink. For distillatescontaining>10 Ilg of F/50 mL of solution,usetitrating Solution A, and multiply the titration by 5 to obtain the correspondingvolume of Solution B. A color blank is not necessaryat higher concentrations unlessvery high accuracyis desired. To find the F equivalenceof the Th(N03)4, add 2 mL of 10 mg L-1 of NaF solution to eachof two Nesslertubes.Add 1 mL of indicator and 1 mL of buffer solution to eachtube,and titrate the solutionswith Th(N03)4 SolutionB until the color matchesthe color blank. Subtract0.1 mL of Th(N03)4 from eachtitration, and divide the remainderby 20 to determinethe milligrams of F equivalentto 1 mL of Th(N03)4 Solution B. Subtractthe distillation blank (indicator of F in water, acid, and reagents) from the titration value for eachunknown sample,and calculatethe milligrams of F in the 50 mL of distillate. Averagethe valuesfrom at leasttwo titrations,and multiply the result by eight to calculatethe total F contentof the original sample. Divide the total microgramsof F in the sampleby the weight of the soil sample in gramsto calculatethe F concentrationin milligrams per kilogram. Comments.McClure (1939) statesthat the precision of the Th titration end-point is equivalentto approximately±0.25 Ilg of F. The precision can be

\0 Hoskins and Ferris (1936), Annstrong (1936), MacIntire et al. (1951), and McHenry and Charles(1960),as adaptedfrom Brewer (1965a).

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improved considerablyby titrating to a specific end-point, i.e., matching the color of the titrated solution with a previously preparedcolor blank as indicated above.For very low F concentrations,the back titration proceduredevelopedby Dahle et aI. (1938) offers someadvantages.

Ion ChromatographicMethods For information aboutthe use of ion chromatographyfor determinationof

P- in soil solution, seeTabatabaiand Frankenberger(1996, seeChapter8). OTHER AVAILABLE METHODS FOR DETERMINATION OF BROMINE, CHLORINE, AND FLUORINE The methodsjust describedare simple andinexpensiveand do not require large facilities. There are other methodsthat can be usedfor sensitiveand accurate analysisof Br, CI, and F in soil and other agriculturalsamples.Bromine can be determinedwith x-ray fluorescence(Getzendaner et aI., 1968; Van Cauwenberge & Gordts, 1977; Reuter et aI., 1976; Yamada, 1968), neutron activation (Castro & Schmitt, 1962; Guinn & Potter, 1962; Yamada, 1968; Nadkami & Ehmann, 1969;Kline & Brar, 1969; Morrison et aI., 1969; Lag & Steinnes,1973; Van Wambeke, 1974; Koons & Helmke, 1978), and stripping voltammetry (Denniset aI., 1976).Chlorine can be determinedwith neutronactivation(Yamada, 1968; Morrison et aI., 1969; Lag & Steinnes,1973; Castro& Schmitt, 1962) and stripping voltammetry (Dennis et aI., 1976), and F can be determinedwith neutronactivation(Damset aI., 1975) and fluorometry (Guyon et aI., 1968; Willard & Horton, 1952). Very comparableBr resultswere obtainedwith x-ray fluorescenceand neutronactivation analysisof soil samples(Yamada,1968). The paperby MacDonald(1976) summarizedthe various most commonly used methodsfor P- provided by 13 laboratoriesthroughout the world. It outlined the various decompositionproceduresand analytical determinations and included various titrimetric, spectrometric,and potentiometrictechniques.The Th(N03)4 titration outlined in this chapterwas included, and direct potentiometry with SIE appearsto be popular, as is with Be and CI-.

REFERENCES Abdalla, N.A., and B. Lear. 1975. Determinationof inorganicbromide in soils and plant tissueswith a bromide selective-ionelectrode.Soil Sci. Plant Anal. 6:489-494. Adriano, D.C., and H.E. Doner. 1982. Bromine, chlorine, and fluorine. p. 449-483.In A.L. Pageet al. (ed.) Methods of soil analysis. Part 2. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison,WI. Adriano, D.C., P.E Pratt, and K.M. Holtclaw. 1973. Comparisonof two simple methodsof chlorine analysisin plant materials.Agron. J. 65:130--134. Adriano, D.C., EH. Takatori, P.E Pratt, and O.A. Lorenz. 1972. Soil nitrogen balancein selected row-crop sites in southernCalifornia. 1. Environ. Qual. 1:279-283. American InstrumentCo. 1969. Chloride determination-automatic titrator method(instruction manual). Am. Instrum. Co., Silver Springs,MD. American Public Health Association. 1980. Standardmethodsfor the examinationof water and wastewater.15th ed. Am. Public Health Assoc.,Washington,DC.

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Armstrong,W.D. 1936. Microdeterminationof fluorine. Ind. Eng. Chern.Anal. Ed. 8:384-387. Associationof Official Analytical Chemists.1980.Official methodsof analysis.13th ed. Assoc.Off. Anal. Chern.,Washington,DC. Baker, R.L. 1972. Determinationof fluoride in vegetationusing the specific ion electrode.Anal. Chern.44:1326-1327. Bellack, E. 1972. Methodsand materialsfor fluoride analysis.J. Am. Water Works Assoc.64:62-66. Bergmann,J.G., and I. Sanik,Jr. 1957.Determinationof traceamountsof chlorine in naphtha.Anal. Chern. 29:241-243. Brewer, R.E 1965a.Fluorine. p. 1135-1148.In C.A. Black et al. (ed.) Methodsof soil analysis.Part 2. Agron. Monogr. 9. ASA and SSSA,Madison,WI. Brewer, R.E 1965b.Fluorine. p. 180-196.In H.D. Chapman(ed.) Diagnosticcriteria for plants and soils. Qual. Print. Co., Inc., Abilene, TX. Brewer, R.E, and G.E Liebig. 1960. Improved multiple all-glassdistillation apparatusfor determination of fluorine in plant samples.Anal. Chern. 32:1373. Broyer,T.C., A.B. Carlton,C.M. Johnson,and P.R. Stout. 1954.Chlorine-amicronutrientelementfor higher plants. Plant Physiol. 29:526-532. Butler, IN. 1969.Thermodynamicstudies.p. 143-189.In R.A. Durst (ed.) Ion-selectiveelectrodes. Natl. Bur. Stand.,Spec.Publ. 314. U.S. Gov. Print. Office, Washington,DC. Canelli, E. 1976. Simultaneousautomateddeterminationof chloride, nitrite, nitrate, and ammoniain water and wastewater.WaterAir Soil Pollut. 5:339-348. Carlson,R.M., and D.R. Keeney. 1971.Specific ion electrodes: Techniques and usesin soil, plant, and water analysis.p. 39-65. In L.M. Walsh (ed.) Instrumentalmethodsfor analysisof soils and plant tissue.SSSA, Madison,WI. Castro,C.E., and R.A. Schmitt. 1962. Direct elementalanalysisof citrus cropsby instrumentalneutron activation:A rapid methodfor total bromide,chloride, manganese,sodium and potassium residues.J. Agric. Food Chern. 10:236-239. Chapman,H.D., and P.E Pratt. 1961. Methodsof analysisfor soils, plantsand water. Div. Agric. Sci., Univ. California, Riverside. Churchill, H. V. 1945. A multiple still for use in the Willard-Winter separationof fluorine. Ind. Eng. Chern.Anal. Ed. 17:720-721. Conway,E.J. 1950. Microdiffusion analysisand volumetricerror. 3rd ed. D. Van NostrandCo., New York.

Cotlove, E., H.V. Trantham,and R.L. Bowman. 1958. An instrumentand method for automatic, rapid, accurate,and sensitivetitration of chloride in biological samples.I. Lab. Clin. Med. 51:461-468. Crenshaw,G.L., and EN. Ward. 1975. Determinationof fluorine in soils and rocks by known-increment addition and selective-ionelectrode.U.S. Geol. Surv. Bull. 1408:77-84. Crosby,N.T. 1969. Equilibria of fluorosilicate solutionswith specialreferenceto the fluoridation of public water supplies.J. Appl. Chern. 19:100-102. Crosby,N.T., A.L. Dennis,and J.G. Stevens.1968.An evaluationof somemethodsfor the determination of fluoride in potable waters and other aqueous solutions. Analyst (London) 93:643-652. Dahle, D., R.U. Bonnar,and H.I. Wickman. 1938.Titration of small quantitiesof fluorine with thorium nitrate. I. Assoc. Off. Agric. Chern. 21:459-474. Dahle,D., and HJ. Wickman. 1936.A quantitativestudy of fluoride distillation. J. Assoc.Off. Agric. Chern. 19:313-320. Dahle, D., and HJ. Wickman. 1937. Further studieson fluorine distillation. J. Assoc. Off. Agric. Chern.20:297-303. Dams, R., J. Billiet, and I. Hoste. 1975. Neutron activation analysisof F, Sc, Se,Ag and Hf in aerosolsusing short-lived isotopes.Int. I. Environ. Anal. Chern.4:141-153. Dennis,B.L., G.S. Wilson, and J.L. Moyers. 1976.The determinationof bromide,chloride and lead in airborneparticulatematterby stripping voltammetry.Anal. Chim. Acta 86:27-34. Durst, R.A. 1969. Analytical techniquesand applicationof ion-selectiveelectrodes.p. 375-414.In R.A. Durst (ed.) Ion-selectiveelectrodes.Natl. Bur. Stand.Spec.Publ. 314. Washington,DC. Endelman,FJ., D.R. Keeney,J.T. Gilmour, and P.G. Saffigna. 1974. Nitrate and chloride movement in the Plainfield loamy sandunderintensiveirrigation. J. Environ. Qual. 3:295-298. Evans,L., R.D. Hoyle, and J.B. Macaskill. 1970. An accurateand rapid methodof analysisfor fluorine in phosphaterocks. N.Z. I. Sci. 13:143-148. Farrel,B.L. 1974. Fluorine,a direct indicatorof fluorite mineralizationin local and regionalsoil geochemicalsurveys.J. Geochem.Explor. 3:227-244.

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Ficklin, W.H. 1970.A rapid methodfor the determinationof fluoride in rocks and soils, using an ionselectiveelectrode.U.S. Geol. Surv. Prof. Pap. 700-e. Fleischer,M., and W.O. Robinson. 1963. Some problemsof the geochemistryof fluorine. R. Soc. Can. Spec.Publ. 6:58-75. Frant, M.S., and 1.w. Ross,Jr. 1966. Electrodefor sensingfluoride ion activity in solution. Science (Washington,DC) 154:1553-1554. Fuge,R. 1976.The automatedcolorimetricdeterminationof fluorine and chlorine in geologicalsamples. Chern. Geol. 17:37-43. Getzendaner,M.E., A.E. Doty, E.L. Mclaughlin, and D.L. Lindgren. 1968. Bromide residuesfrom methyl bromide fumigation of food commodities.1. Agric. Food Chern. 16:265-271. Gilliam, 1.w. 1971. Rapid measurementof chlorine in plant materials. Soil Sci. Soc. Am. Proc. 35:512-513. Gran, G. 1952. Determination of the equivalence point in potentiometric titrations. Part 2. Analyst(London)77:661-671. Greenhill, N.B., and K.I. Peverill. 1977. Determinationof cation exchangecapacity of soils using ammoniaand chloride electrodes.Soil Sci. Plant Anal. 8:579-589. Guinn, V.P., and J.e. Potter. 1962. Determinationof total bromine residuesin agricultural crops by instrumentalneutronactivation analysis.1. Agric. Food Chern. 10:232-236. Guyon,J.e.,B.E. Jones,and D.A Britton. 1968.A f1uorometricmethodfor determiningtrace quantities of fluoride. Mikrochim. Acta 1:1180-1184. Hardin, LJ. 1952. Report on the determinationof fluorine-contentof soils. J. Assoc. Off. Agric. Chern.35:621-633. Hillebrand, W.F., and G.E.F. Lundell. 1953. Applied inorganicanalysis.2nd ed. JohnWiley & Sons, Inc., New York. Hipp, B.w., and G.w. Langdale.1971. Use of a solid-statechloride electrodefor chloride deteminations in soil extracts.Soil Sci. Plant Anal. 2:237-240. Hoffman, G.M., and H.P. Malkomes.1974. Bromide residuesin vegetablecropsafter soil fumigation with methyl bromide.Agric. Environ. 1:321-328. Hopkins, D.M. 1977.An improvedion-selectiveelectrodemethod forthe rapid determinationof fluorine in rocks and soils. J. Res. U.S. Geol. Surv. 5:589-593. Hoskins,W.M., and e.A Ferris. 1936.A methodof analysisfor fluoride. Ind. Eng. Chern.Anal. Ed. 8:6-9. Huang,W.H., and W.D. Johns.1967. Simultaneousdeterminationof fluorine and chlorine in silicate rocks by a rapid spectrophotometricmethod.Anal. Chim. Acta 37:508-515. Huckabay,W.B., E.T. Welch, and A.V. Metler. 1947. Constanttemperaturesteamdistillation apparatus for isolation of fluorine. Anal. Chern. 19:154-156. Hurd-Karrer,AM. 1950. Comparativefluorine uptakeby plants in limed and unlimed soil. Soil Sci. 70:153-160. Kline, J.R.,and S.S. Brar. 1969. Instrumentalanalysisof neutronirradicatedsoils. Soil Sci. Soc. Am. Proc. 33:234-238. Koons, R.D., and P.A Helmke. 1978. Neutron activation analysisof standardsoils. Soil Sci. Soc. Am. J. 42:237-240. laCroix, R.L., D.R. Keeney,and L.M. Walsh. 1970. Potentiometrictitration of chloride in plant tissueextractsusing the chloride ion electrode.Soil Sci. Plant Anal. 1:1-6. Lag, J., and E. Steinnes.1973. Distribution of chlorine,bromineand iodine in Norwegianforest soils studiedby neutronactivation analysis.p. 383-395.In Proc. of Symp. on the Use of Isotopes and Radiation in Researchon Soil-Plant Relationshipsincluding Application in Forestry. Vienna. Larsen,S., and AE. Widdowson.1971. Soil fluorine. J. Soil Sci. 22:210-211. MacDonald,AM.G. 1976. The presentstate of methodsfor the microdeterminationof fluorine in organiccompounds.Pure Appl. Chern.45:31-37. MacIntire, W.H. 1945. Soil contentof fluorine and its determination.Soil Sci. 59:105-109. MacIntire, W.H., LJ. Hardin, and L.S. Jones.1951. Report on fluorine-the direct double distillation for the determinationof fluorine contentof soils. J. Assoc. Off. Agric. Chern. 34:597-603. Martin, J.P.,G.K. Helmkamp,andJ.O. Ervin. 1956. Effect of bromidefrom a soil fumigant and from CaBr on growth and chemical composition of citrus plants. Soil Sci. Soc. Am. Proc. 20:209-212. Mavrodineanu,R., and J. Gwirtsman. 1954. Improved apparatusfor the distillation of fluorine as hydrofluosilicic acid. Contrib. Boyce ThompsonInst. 17:489-494. McClenahen,J.R., and E.R. Schulz. 1976. Total soil fluoride determinationby a single distillation selectiveion electrodeprocedure.Soil Sci. 122:267-270.

FRANKENBERGER ET AL. McClure, FJ. 1939. Microdeterminationof fluorine by thorium nitrate titration. Ind. Eng. Chern. Anal. Ed. 11:171-173. McHenry, C.R., and H. Charles.1960.Monitoring fluoride contentof air, waterand vegetation.Farm Chern. 123:58-62. Mcleod,S., andB. Zarcinas.1976.The determinationof ammoniumandchloride by an autoanalyser for the measurementof cation exchangecapacityof soils. Soil Sci. Plant Anal. 7:743-750. McQuaker,N.R., and M. Gurney. 1977. Determinationof total fluoride in soil and vegetationusing an alkali fusion-selectiveion electrodetechnique.Anal. Chern.49:53-56. Milton, R.F. 1949. Titrimetricestimationof fluorine. Analyst (London) 74:54. Morrison, G.H., J.T. Gerard, A. Travesi, R.L. Currie, S.F. Peterson,and N.M. Potter. 1969. Multielement neutronactivation analysisof rock using chemicalgroup separationsand high resolutiongammaspectrometry.Anal. Chern.41:1633-1637. Nadkarni,R.A., andW.D. Ehmann.1%9. Determinationof traceelementsin biological standardkale by neutronactivation analysis.1 Radioanal.Chern. 3:175-185. Nommik, H. 1952.Fluorine in Swedishagriculturalproducts,soil and drinking water.Acta Polytech. Incl. MetaJl. Ser. 105:1-121. Omueti,J.A.!., and R.L. Jones.1977a.Regionaldistribution of fluorine in Illinois soils. Soil Sci. Soc. Am. J. 41:771-774. Omueti, lA.I., and R.L. Jones.1977b.Fluorine contentof soil from Morrow plots over a period of 67 years.Soil Sci. Soc. Am. J. 41:1023-1024. Onken,A.B., R.S. Hargrove,c.w. Wendt, and O.C. Wilke. 1975.The useof a specific ion electrode for determinationof bromide in soils. Soil Sci. Soc. Am. Proc. 39:1223-1225. Orion Research,Inc. 1976. Orion ionalyzerand electrodes. OrionRes. Inc., Cambridge,MA. Ozanne,P.G. 1958. Chlorine deficiency in soils. Nature(London) 182:1172-1172. Peck, L.c., and V.C. Smith. 1964. Spectrophotometricdeterminationof fluorine in silicate rocks. Talanta11:1343-1347. Pluger,W.L., and G.H. Friedrich. 1973. Determinationof total and cold-extractablefluoride in soils andstreamsedimentswith an ion-sensitivefluoride electrode.p. 421-427.In MJ. Jones(ed.) Geochemicalexploration.V.K. Inst. Min. MetaJl., London. Prince, A.L., F.E. Bear, E.G. Brennan,I.A. Leone, and R.H. Daines. 1949. Fluorine: Its toxicity to plantsand its control in soils. Soil Sci. 67:269--277. Reeves,R.D., and R.R. Brooks. 1978.Traceelementanalysisof geologicalmaterials.p. 304-309.In Chemicalanalysis.Vol. 51. JohnWiley & Sons,Inc., New York. Reuter,F.W., G.E. Secor,and M. Friedman.1976. A methodfor brominedeterminationin wool fabric by X-ray fluorescencespectrometry.Text. Res.J. 46:463-465. Ross, J.W., Jr. 1969. Solid-stateand liquid membraneion-selectiveelectrodes.p. 57-88. In R.A. Durst (ed.) Ion-selectiveelectrodes.Natl. Bur. Stand. Publ. 314. U.S. Gov. Print. Office, Washington,DC. Rowe, R.D. 1965. Wickbold combustion and spectrophotometricanalysis procedure for trace amountsof organicchlorine in viscouspolybutenepolymers.Anal. Chern.37:368-370. Rowley, R.I., J.G. Grier, and R.L. Parsons.1953. Determinationof fluoride in vegetation.Anal. Chern. 25:1061-1065. Saffigna, P.G., D.R. Keeney,and L.L. Hendrickson.1976. Halide analysisin soils with a chloride titrator and a bromideelectrode.Soil Sci. Plant Anal. 7:691~99. Selmer-Olsen,A.R., and A. 0ien. 1973. Determinationof chloride in aqueoussoil extractsand water samplesby meansof a chloride-selectiveelectrode.Analyst (London) 98:412-415. Shergold,H.L., and F.L. Selfe. 1974. Determinationof fluorine content of ores with fluoride-ion selectiveelectrode.Inst. Min. MetaJl. Trans. 83:256-257. of chloSmart,R.St.C.,A.D. Thomas,and D.P. Drover. 1974. Selectiveion electrode measurements ride concentrationsin the determinationof cation exchangecapacitiesof soils. Soil Sci. Plant Anal. 5:1-11. Steinkoenig,L.A. 1919. The relation of fluorine in soils, plants and animals. J. Indus. Eng. Chern. 11:463-465. Stout, P.R., and C.M. Johnson.1965. Chlorine and bromine.p. 1124-1134.In C.A. Black et al. (ed.) Methodsof soil analysis.Part 2. Agron. Monogr. 9. ASA and SSSA, Madison,WI. Tabatabai,M.A., and W.T. Frankenberger,Jr. 1996. Liquid chromatography.p. 225-245. In D.L. Sparkset al. (ed.) Methodsof soil analysis.Part 3. Chemical methods.SSSABook Ser. 5. SSSAand ASA, Madison,WI. Thomas,1, Jr., and HJ. Gluskoter. 1974. Determinationof fluoride in coal with the fluoride ionselectiveelectrode.Anal. Chern.46:1321-1323.

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Troll, G., A. Farzaneh,and K. Cammann.1977.Rapid determinationof fluoride in mineral and rock samplesusingan ion-selectiveelectrode.Chern.Geol. 20:295-305. u.s. EnvironmentalProtectionAgency Staff. 1974. Methods for chemical analysisof water and wastes.EPA 625/6-74-003.USEPA, Office Technol.Transfer,Washington,DC. Van Cauwenberge,P.P., and L.A. Gordts. 1977. X-ray fluorescencespectroscopicdeterminationof Br residuesin crops after soil treatment with methyl bromide. J. Agric. Food Chern. 25:1000-1002. Van Loon, J.C. 1968a.Determinationof chloride in chloride-containingmaterialswith a chloride membraneelectrode.Analyst (London)93:788-791. Van Loon, J.C. 1968b.The rapid determinationof fluoride in mineral fluorides using a specific ion electrode.Anal. Lett. 1:393-398. Van Wambeke,E. 1974. Bromide residuesin lettuceafter soil fumigation with methyl bromide,and somefactorsinvolved. Agric. Environ. 1:277-282. Vinogradov,A.P. 1959. The geochemistryof rare and dispersedchemical elementsin soils. 2 ed. ConsultantsBur., New York. Willard, H.H. 1912.The preparationof perchloricacid. J. Am. Chern.Soc. 34:1480-1485. Willard, H.H., and C.A. Horton. 1952. Fluorometric determinationsof traces of fluoride. Anal. Chern.24:862-865. Willard, H.H., and O.B. Winter. 1933. Volumetric methodfor determinationof fluorine. Ind. Eng. Chern.Anal. Ed. 5:7-10. Yamada,Y. 1968.Occurrenceof brominein plantsand soils. Talanta15:1135-1141. Zall, D.M., D. Fisher,andM.O. Garner.1956.Photometricdeterminationof chloridesin water.Anal. Chern.28:1665-1668.

Published 1996

Chapter 32 Phosphorus s. KUO, Washington State University, Pullman, Washington The total P concentrationin soils is generally in the rangefrom 200 to 5000 mg P kg-I with an averageof 600 mg P kg-I (Lindsay, 1979). Physicochemicaland biological reactionsin soils and sedimentsact In concertto regulateP solubility which in turn affectsboth agronomicproductionas well as eutrophicationof surface water. Phosphorusexists in soil as organic and inorganic P forms. Oxidation of organicconstituentsand acid dissolutionof mineralsare necessaryfor total P determination.This is generally accomplishedby Na2C03 (sodium carbonate)fusion, aciddigestion,H20 2 (hydrogenperoxide)or NaOBr (sodium hypobromite) oxidation. The soil organic P fraction may be derived from plant residuesand from soil flora and fauna tissueand residuesthat resist rapid hydrolysis. While a large proportion of soil organic P remains uncharacterized,inositol phosphate,phospholipids, nucleic acids and their derivatives, and polyphosphateshave been identified. Chromatographicand nuclear magnetic resonancetechniqueshave beenusedto quantify various organic P fractions. Quantificationof organic P is necessaryto better understandthe mineralization-immobilizationturnover of P under particular environmentsand cropping systemsin soils. Dalal (1977), Anderson(1967, 1980) and Stevenson(1982) have provided detaileddiscussionof organic P and its transformationsin soil. To understandthe inorganic P statusand availability in soil, diverse fractionation schemesand soil testshave beendeveloped.The P fractions have been usedto study the transformationof applied P fertilizer in soils and interpretation of P soil test values.The fractionation schemeproposedby Changand Jackson (1957), which is intendedto separateCa-P, AI-P, and Fe-P fractions, has been modified to considersoil type effects. When the P concentrationin soil solution, or P intensity, is diminishedby P removal, it is replenishedby labile P which in turn is replenishedmuch more slowly by nonlabile P.

Copyright © 1996 Soil ScienceSociety of America and American Society of Agronomy, 677 S. SegoeRd., Madison,WI 53711,USA. Methods of Soil Analysis. Part 3. Chemical Methods-SSSA Book Seriesno. 5.

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Nonlabile soil P ... labile soil P ... soil solution P

[1]

The quantity of labile P, the concentrationof P in soil solution, as well as the P buffering capacitythat governsthe distribution of P betweenthe solution and solid phases,are the primary factors characterizingsoil P availability. The basic function underlying P soil tests is to determinethe quantity and intensity factorsof soil P. The soil test P levels, however,are dependenton both test methods and soil characteristics.Thus, in many respects,P soil test levels are operationally defined unlessthe relatedP buffering capacityof the soil also is included in the interpretationof soil test results(Kuo, 1991). A wide variety of soil test schemeshave evolvedover the years,reflecting regional preference,considerationof soil types and efficiency of operation.The newer methods that include chelates [e.g., ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepeI1taacetic acid (DTPA)] are designedto test, in addition to P, other anionsas well as cations.The recentlydevelopediron oxideimpregnatedfilter papermethod,which removesP in the samemanneras anion exchangeresin does,can overcomethe tediousnessof separatingresin from soils requiredby the latter method.This methodwill deservegreaterconsiderationand acceptanceif a uniform quality of oxide-impregnatedpaperbecomesavailable. Phosphorusin soil extractsand plant ashcan be determinedby instrumentation or by wet chemistry.Both inductively coupledplasma spectrometry (ICPS) and ion chromatography (IC)are convenientparticularly for multielementanalysis. For wet chemistry,four colorimetric methodsfor P determinationare included in this chaptervarying in sensitivity, color stability, and toleranceto interfering cations,anions and labile organic P. Users have the choice of selectingone that bestsuits the conditionsof their test solution and sensitivity desired.

TOTAL PHOSPHORUS Principles

Four methodshavebeenusedto determinetotal P in soils: sodiumcarbonate fusion (Jackson,1958), perchloric acid (HCI04) digestion (Jackson,1958), H2S04 (sulfuric acid)-H20z-HF (hydrogenfluoride) digestion(Bowman, 1988), andsodiumhypobromiteoxidationfollowed by dissolutionin dilute H2S04 (Dick & Tabatabai,1977a).All four methodsconvertorganicP to inorganicP to facilitate total P determination. The HCI04 and NaOBr digestions,however,do not readily dissolveP imbedded in the matrix of silicate minerals (e.g., quartz). Thus, the HCI04 or NaOBr digestionsmay underestimatetotal P in proportionto the quantity of the P imbedded(Syers et aI., 1967; Sommers& Nelson, 1972). Sodium carbonate fusion extractsmore P than HCI04 or NaOBr, providedthat the melt is extracted with dilute H2S04 (Sherrell & Saunders,1966). Differencesbetweenthe total P determinedby fusion and that by HCI04 digestionbecomewider as the proportion of fme and coarsesandsin the soil increases(Mattingly, 1970).

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SpecialApparatus 1. 2. 3. 4.

Platinumcrucibles,30-mL capacity,with lids. l00-mL Teflon beakerswith lids. Stainless steelperchloricacid fume hood. Hot plate. Methods

SodiumCarbonateFusion

Reagents 1. Sodiumcarbonate,anhydrous. 2. Sulfuric acid, 4.5 M: Slowly add 250 mL of concentratedH2S04 (18 M) to 500 mL of deionizedwater in a l-L volumetric flask. Cool the solution to room temperatureand dilute to 1 L. 3. Sulfuric acid, 1 M: Add 56 mL of concentratedH2S04 to 800 mL of deionizedwater in a l-L volumetric flask. Cool the solution to room temperatureand dilute to 1 L. 4. Sodiumhydroxide (NaOH),5 M: Dissolve200 g of NaOH in deionized water and dilute to 1 L. 5. p-nitrophenol(C6HsN03), 0.25%: Dissolve 0.25 g of p-nitrophenolin 100 mL of deionizedwater. Procedure.Mix 1.0 g (or 0.5 g of soil containinghigh Fe) of air-dried soil «0.15 mm) and 4 g of reagentgrade anhydrousNa2C03 in a 30-mL platinum crucible. Placean additional 1 g of Na2C03on top of the mixture. Cover the crucible with a lid but leave 10% opento allow gasesto escape.Place a Meeker burnerwith a low flame underone side oppositeto the side with the openingfor 10 min to remove moistureand fuse the massgently. Continuethe heatingwith a full flame of the Meekerburnerfor 10 or more min to allow the massto fuse completely and form a liquid melt. Adjust the lid to provide a larger openingduring the heating. Incline the crucible to insure the complete fusion of soil particles sticking on the uppersidesof the crucible. When the bubblesof gas ceaseto come off, tum off the flame, and rotate the crucible to spreadthe contentsover the sides of the crucible to expedite removalof the melt. Quantitativelytransferthe melt with 30 mL of 4.S M H 2S04 with care to avoid loss by effervescence,to a 2S0-mL volumetric flask. Boil the crucible and the lid in 25 mL of 1 M H2S04 in a beaker.Removethe crucible and lid, and transferthe solution to the 250-mL volumetric flask. Add five drops of 0.2S%p-nitrophenolindicator and adjust thesolution pH with S M NaOH until the color of the indicator just changesfrom colorlessto yellow. Determine the P concentrationby the ascorbic acid method outlined in "Ascorbic Acid Method" or by any othermethodin "PhosphorusDetermination." The quantity of the total P presentin the soil is calculatedas follows. SO 2S0 Total P =P Concentration(llglmL) • - • . VI g soIl used where VI

=samplevolume usedfor the determinationof P concentration.

[2]

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Digestionwith PerchloricAcid

Reagents

1. Perchloricacid, 70%. 2. Nitric acid (HN03), concentrated,15.8M. 3. Sodium hydroxide,SM: Dissolve 200 g of NaOH in deionizedwater and dilute to 1 L. 4. p-nitrophenol,0.25%: Dissolve 0.25 g of p-nitrophenolin 100 mL of deionizedwater. Procedure.Add 2.0 g of air-dried soil «0.5 mm) and 30 mL of 70% HCI04 in a 250-mL volumetric flask or Erlenmeyerflask that is coveredwith a pyrex funnel to ensurethe reflux of the HCI04. Digest the mixture in a preheated sandbathon a hot plate at 130°Cin a well-ventilated,stainlesssteelhood until the dark color due to organicmatterdisappears.Continueheatingat 203°Cfor 20 min. Heavy white fumes of HCI04 appearwhen the digestionis completed,and the silica becomeswhite. Add 1 or 2 mL of HCI04 to washdown any black particles sticking to the sidesof the flask. If the soil containsa high organic matter content, add 20 mL of concentratedHN03 and heatto oxidize the sample(or alternativelyallow the oxidation to continue overnight at room temperature)in a well-ventilated hood prior to digestionwith HCI04 as previously described. When the digestion is complete,remove the flask and cool the mixture. Dilute with deionizedwater to 250 mL and mix well. Allow the solid materialto settle. Transferan aliquot containing2 to 40 )lg P to a 50-mL volumetric flask. Add five dropsof 0.25%p-nitrophenolindicator, and adjustthe solution pH with dropwiseadditionof 5 M NaOH until the color of the indicatorjust changesfrom colorlessto yellow. Determinethe P concentrationby the ascorbicacid method outlined in "Ascorbic Acid Method." Use Eq. [2] to calculatethe total P concentrationin the soil. Digestionwith Sulfuric Acid-HydrogenPeroxide-HydrofluoricAcid

Reagents

1. Sulfuric acid, concentrated,18 M. 2. Hydrogenperoxide,30%. 3. Hydrogenfluoride, concentrated,24 M. 4. Sodium hydroxide,SM: Dissolve 200 g of NaOH in deionizedwater and dilute to 1 L. 5. p-nitrophenol,0.25%: Dissolve 0.25 g of p-nitrophenolin 100 mL of deionizedwater. Procedure.Add 0.5 g of air-dried soil «0.15 mm), or 0.25 g for soils that are sandyor have a high organicC content,to a 100-mL fluoropolymer(Teflon) beaker.Add 5 mL of concentratedH2S04 and swirl to suspendthe soil particles

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adheringto the bottom of the beaker.Placethe beakerin a well-ventilatedhood and slowly add 3 mL of 30% H20 2, 0.5 mL at a time. Mix well to facilitate the oxidation following each addition of H20 2• Care should be taken to avoid the overflow of foam, particularly for soil with high organicmattercontent.After the reaction subsides,add 1 mL of concentratedHF, 0.5 mL at a time, using a polypropylenepipette,mix well, and placethe beakeron a preheatedhot plate at 150°C for 10 to 12 min to eliminate excessH20 2• Removeand cool the beaker. Quantitatively transferthe contentsto a 50-mL volumetric flask with deionized water and dilute to volume. Transferan aliquot containing2 to 40 flg P to a 50-mL volumetric flask. Add five drops of p-nitrophenolindicator, and adjust the solution pH with 5 M NaOH until the indicatorcolor just changesfrom colorlessto yellow. Add 15 mL of 0.8 M H3B03 (boric acid) to eliminate the interferenceof r if necessary. Determinethe P concentrationby the ascorbicacid methodoutlined in "Ascorbic Acid Method." Calculatethe total P concentrationas follows. 50 Total P = P Concentration(flg/mL)· - . VI

50 g soil used

[3]

where VI = samplevolume usedfor the determinationof P concentration. SodiumHypobromiteOxidation

Reagents 1. Sodium hydroxide,SM: Dissolve 200 g of NaOH in deionizedwater

and dilute to 1 L. 2. Sodiumhypobromite:Add 3 mL of Br2 slowly (about0.5 mL per min) to 100 mL of 2 M NaOH with constantstirring. Preparethis reagent immediatelybefore use. 3. Formic acid (HCOOH), 90%. 4. Sulfuric acid, 0.5 M: Dilute 27.8 mL of concentratedH2S04 to 1 L with deionizedwater. Procedure.Add 0.2 or less of air-dried soil «0.15 mm) to a 50-mL boiling flask. Add 3 mL of NaOBr solution and swirl to mix. Allow the suspension to standfor 5 min and mix again. Placethe flask in a preheatedsandbath at 260 to 280°C on a hot plate situatedin a well-ventilatedhood. Heat the flask until its contentsare evaporatedto dryness,which takes 10 to 15 min, and continueto heat for an additional 30 min. Removeand cool the flask. Add 4 mL of deionizedwater and 1 mL of HCOOH. Swirl to mix and add 25 mL of 0.5 M H2S04. Mix well and quantitativelytransferthe mixture to a 50mL volumetric flask with about 15 mL of deionizedwater and dilute to volume. Filter the contentsor allow the solid material to settleto the bottom of the volumetric flask. Transferan aliquot containing2 to 40 flg to a 50-mL volumetric flask. Add five drops of p-nitrophenolindicator and adjust the solution pH with 5 M NaOH

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until the indicator color just changesfrom colorlessto yellow. Determinethe P concentrationby the ascorbicacid methodoutlinedin "AscorbicAcid Method." Calculatethe total P concentrationin the soil usingEq. [3]. Comments The amountsof P determinedby the NaOBr andHCl04 methodsare about the same(Dick & Tabatabai,1977a)but less than that by the Na2C03 fusion (Sherrell & Saunders,1966; Syderset aI., 1968) or H2S04-H20 r HF method (Bowman,1988).This is becausethe HCl04 or 0.5 M H2S04 addedafter NaOBr oxidation does not dissolve silicate mineralscontaining imbeddedP minerals (e.g.,apatite). Sommersand Nelson(1972) modified the HCl04 digestionprocedureso that digestioncould be carriedout in an aluminum digestionblock to facilitate routine analysisof large numbersof samples.The NaOBr methodis useful for routine analysisas well. Heatedmixture of HCl04 andorganicmattermay explodeviolently. Avoid adding HCl04 to heatedsampleswith high organic matter contents.For such samples,initiate HN03 pretreatmentfirst. The digestionwith HCl04 shouldbe done using a specially constructedstainlesssteel hood and the hood shouldbe washedafter eachuseto removeperchlorates.

TOTAL ORGANIC PHOSPHORUS Principles The total organic P (Po) is generallydeterminedby the ignition method (Saunders& Williams, 1955; Walker & Adams, 1958) or by extraction using concentratedHCl (hydrogenchloride)(Mehtaet al., 1954),concentratedH2S04 (Bowman,1988)or acetylacetone (CH3COCH2COCH3; 2,4-pentanedione) (Halsteadet aI., 1966)as a primary extractant.In the ignition method,Po is converted to inorganicP (Pi) by high temperatureoxidation. The Po is then determined by the differencebetweenthe amountsof H2S04-extractablePi for the ignited and unignited soils. However, high temperaturecan alter the solubility of Pi in soils, affecting the accuracyof Po measurements. Strongmineral acidsdissolve iron and aluminumoxides,and removeAI, Fe and other polyvalentcationsthat precipitatewith Po. This, coupledwith heatfrom the addition of water to concentratedH2S04, facilitates Po extraction.However, the acids can hydrolyze somePo [e.g., glucose-I-phosphate, ribonucleicacid (RNA), and deoxyribonucleic acid (DNA)] and lower the estimateof Po. Acetylacetone(PH 8.0) is used as a complexing agent for polyvalent cationsto facilitate extractionof organicmatteror Po. Although this extractionis lesseffectivethanthe ignition or acid extractionmethod,it is lessdestructiveof the organicstructureandovercomesthe problemof the hydrolysisof Po inherent in the acid extractions.Pretreatmentof soil with dilute mineral acid, however,is neededto removeCa that precipitateswith Po underalkalineconditions.

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The oxidation of Po in alkaline extractsby HCI04 digestion requiresthe additionof magnesiumchloride(MgCI2) to preventP lossthroughthe conversion of P to pyrophosphate(Na2H2P207)and then to metaphosphate(Na30309) at temperaturesgreaterthan 240°C (Brookes& Powlson,1981).

SpecialApparatus

1. Muffle furnace. 2. Centrifuge. 3. Aluminum blockfor digestion. 4. Hot plate. 5. Spectrophotometer. 6. pH meterequippedwith a combinationelectrode. 7. Stainlesssteelperchloricacid fume hood. 8. Mechanicalshaker. 9. Ultrasonicdisperser. 10. Hot water bath. Methods

Ignition Method

Reagents 1. Sulfuric acid, 0.5 M: Dilute 27.8 mL of concentratedH2S04 (18 M) to 1 L with deionizedwater. 2. Sodiumhydroxide,5 M: Dissolve200 g NaOH in deionizedwater and dilute to 1 L. 3. p-nitrophenol,0.25%: Dissolve 0.25 g of p-nitrophenolin 100 mL of deionizedwater. Procedure.Add 2.0 g of soil «2 mm) in a porcelaincrucible and placethe crucible in a cool muffle furnace. Increasethe temperatureof muffle furnace to 550°C and maintainit for 1 h. Allow the crucible to cool and transferthe ignited soil to a lOO-mL centrifugetube. Place2.0 g of unignitedsoil into another100mL centrifuge tube. Add 50 mL of 0.5 M H2S04 to eachcentrifuge tubeand shakethe tubesfor 16 h. Centrifuge or filter through a 0.45-~m membranefilter to obtain a clear solution. Transferan aliquot containing 2to 40 ~g of P into a 50-mL volumetric flask, add five dropsof 0.25%p-nitrophenolindicatorand adjustthe solution pH with 5 M NaOH until the indicator color just changesfrom colorlessto yellow. Determinethe P concentrationusingthe ascorbicacid methodoutlinedin "Ascorbic Acid Method." The total P extracted(Pex) for the ignited or unignitedsoil is calculatedas follows. Total Pex

50

=P concentration(~glmL)· -VI .

50 g soil used

[4]

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where VI

=samplevolume usedfor the determinationof P concentration. Po =total Pignited- total Punignited.

[5]

Extractionwith Dilute SodiumHydroxide, ConcentratedHydrochloric Acid, and Dilute SodiumHydroxide

Reagents 1. Sodium hydroxide,SM: Dissolve 200 g of NaOH in deionizedwater and dilute to 1 L. 2. Sodiumhydroxide,0.5 M: Dilute 100 mL of 5 M NaOH solution to 1 L with deionizedwater. 3. Sodium hydroxide,0.3 M: Dilute 60 mL of 5 M NaOH solution to 1 L with deionizedwater. 4. Hydrochloric acid, concentrated,12 M. 5. Hydrochloric acid, 2 M: Dilute 167 mL of concentratedHCI (12 M) to 1 L with deionizedwater. 6. Sulfuric acid, concentrated18 M. 7. Perchloricacid, 70%. 8. Magnesiumchloride (MgCI 2) saturated:Suspend600 g of MgCl2 in 1 L of deionizedwater. 9. p-nitrophenol,0.25%: Dissolve 0.25 g of p-nitrophenol in 100 mL of deionizedwater. Procedure.Weigh 1 g of soil «0.15 mm) in a 100-mL polypropylenecentrifuge tube. Add 70 mL of 0.3 M NaOH. Shakethe suspensionfor 16 h and centrifuge. Decantthe supernatantliquid into a 100-mL volumetric flask. Add five dropsof p-nitrophenolindicatorand adjustthe pH with dropwiseaddition of concentratedH2S04 with constantstirring until the color just changesfrom yellow to colorless.Bring up to 100 mL with deionizedwater. Add 10 mL of concentratedHCI to the soil residueand heat at 70°C in a water bath for 10 min. Removethe tube and addan additional 10 mL of concentratedHCl. Swirl to mix and allow the suspensionto cool at room temperaturefor 1 h. Add 50 mL of deionizedwater, and mix well. Centrifuge, and decantthe supernatantliquid into a 200-mL volumetric flask. Add 30 mL of 0.5 M NaOH to the centrifugetube, mix well, and allow to standat room temperaturefor 1 h. Centrifuge,and decantthe supernatantinto the 200-mL volumetricflask containingthe acid extract.Add 60 mL of 0.5 M NaOH to the tube and mix thoroughly. Cover the tube loosely with a beaker andheatin an oven at 90°C for 8 h. Cool the tube, centrifuge,and decantthe supernatantliquid into the 200-mL volumetricflask. Bring up to 200-mL volumewith deionized water, and mix well. Transferaliquotscontaining2 to 40 )lg P of the baseextractin the 100-mL volumetric flask and the acid and baseextractsin the 200-mL volumetric flask to Folin-Wu (or comparable)digestiontubesfor total P determination.Add 0.5 mL of saturatedMgCl 2 solution, one drop of concentratedH2S04 and 1 mL of 70% HCI04 to each tube and digest in an AI digestion block at 205°C for 30 min. Removethe tube, and let cool. Quantitatively transfer the digest with about 30

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mL of deionizedwater to a 50-mL volumetric flask for P determination.Adjust the solution pH in a similar manneras describedbelow, prior to P determination by the ascorbicacid methodoutlined in "Ascorbic Acid Method." Allow the suspendedmaterials in the 100-mL and 200-mL volumetric flasks to flocculate. Pipettean aliquot containing2 to 40 J..lg P to a 50-mL volumetric flask for inorganicP determination.For the acid and baseextractadd five dropsof p-nitrophenolindicatorand adjustthe solution pH with 5 M NaOH until the indicator color just changesfrom colorlessto yellow. Determinethe P concentrationby the ascorbicacid methodoutlinedin "Ascorbic Acid Method" or by the modified ascorbicacid methodin "Modified Ascorbic Acid Method" for the determinationof inorganic P in the first base extract if sufficient acid-labile organicP is present.A blank containingall reagentsusedin the extractionshould be included. InorganicP in the first baseextract(Pf) or the acid plus baseextract(pY) is calculatedas follows:

Pf or pY

50

= P concentration(J..lglmL)· -

Vl

V2

•. g soli used

[6]

where Vl =volume of extractsusedfor the determinationof inorganicP concentration; V2 =volume of extracts.Total P in the first baseextract(TP") or the acid plus baseextract(Tpb) is calculatedusing the following equation.

Tpa or Tpb

50 V2 =P concentration(J..lglmL)·-·. v3

g soli used

[7]

where vl = volume of baseor acid plus baseextractusedin the digestion. The total organic P concentration(Po) is calculatedusing the following equation [8] Extractionwith ConcentratedSulfuric Acid and Dilute Sodium Hydroxide

Reagents 1. Sulfuric acid, concentrated,18 M. 2. Sulfuric acid, 5.5 M: Slowly add 305 mL of concentratedH2S04 to 500 mL of deionizedwater in a l-L volumetric flask. Cool the solution to room temperatureand dilute to 1 L. 3. Hydrochloric acid,SM: Dilute 417 mL of concentratedHCI to 1 L. 4. Sodiumhydroxide, 10 M: Dissolve 400 g of NaOH in deionizedwater and dilute to 1 L. 5. Sodium hydroxide, 2 M: Dilute 200 mL of 10 M NaOH to 1 L with deionizedwater.

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6. Sodiumhydroxide,0.5 M: Dilute SO mL of 10 M NaOH to 1 L. 7. Potassiumpersulfate,K 2S20 g• 8. p-nitrophenol,0.2S%:Dissolve 0.25 g of p-nitrophenolin 100 mL of deionizedwater.

Procedure.Weigh 2.0 g of air-dried soil «0.18 rom) into a SO-mL volumetric flask and add3 mL (or 4 mL if the soil is calcareous)of concentrated H2S04• After mixing, add 4 mL of deionizedwater in I-mL incrementswhile mixing vigorously for S to 10 s after eachaddition. Rinse the interior side of the flask with S to 10 mL of deionizedwater, andmix the suspensionvigorously. Add about20 mL of deionizedwater and, after coolingto room temperature,filter the suspension through Whatmanno. 1 filter paper.Savethe filtrate in a SO-mL volumetricflask for P determination.Quantitativelytransferthe remainingsoil in the flask to the filter paperwith aboutS mL of deionizedwater or more if necessary. Transfer the filtrate to the SO-mL volumetric flask and dilute to volume with deionizedwater. Savethe filter papercontainingthe soil residuefor baseextraction. Placethe soil residueand filter paperin a 2S0-mL Erlenmeyerflask. Add 98 mL of 0.5 M NaOH, shakefor 2 h, and filter through Whatmanno. 1 filter paper. For total P (TP) determination,pipette an aliquot containing2 to 40 Ilg P from the acid or baseextractinto a SO-mL volumetric flask. Add 1 g of K 2S20 g with a calibratedscoopand 2 mL of S.5 M H2S04• Digest the sampleon a hot plateat about IS0aC for 20 to 30 min or until the vigorousboiling subsides.Cool, addfive dropsof p-nitrophenoland adjustthe pH with 10M NaOH until the color just changesto yellow. Determine the P concentrationusing the ascorbicacid methoddescribedin "AscorbicAcid Method." Calculatethe TP in the acid (TP") and base(Tpb) extractsusing Eq. [7]. For inorganicP (Pi) determination,transferan aliquot from the acid or base extractcontaining2 to 40 Ilg P to a 50-mL volumetricflask. Add five dropsof pnitrophenoland adjustthe pH of the acid extractwith 2 M NaOH and of the base extractwith 5 M HCl until the indicatorcolor just changes.Determinethe P concentrationusingthe ascorbicacid methoddescribedin "AscorbicAcid Methods." Calculatethe quantitiesof Pi in the acid (Pf) or base(P~) extractsusing Eq. [6]. The total organicP (Po) fraction in the initial soil sampleis calculatedusing Eq. [8].

AcetylacetoneExtraction

Reagents 1. Hydrochloric acid, S M: Dilute 417 mL of concentratedHCl (12 M) to 1 L with deionizedwater. 2. Hydrochloric acid, 0.1 M: Dilute 20 mL of 5 M HCl to 1 L with deionized water. 3. Acetylacetone(CH3COCH2COCH3), 0.2 M: Add 20 mL of acetylacetone (>99%) to 950 mL of deionized water,adjust pH to 8.0 with 3 M NaOH and dilute to 1 L.

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4. Sodium hydroxide, 3 M: Dissolve 120 g of NaOH in deionizedwater and dilute to 1 L. 5. Sodium hydroxide,0.05 M: Dilute 16.7 mL of 3 M NaOH to 1 L with deionizedwater. 6. Ethyl ether(CH3CH20CH2CH3). 7. Perchloricacid, 70%. 8. Magnesiumchloride, saturated:Suspend600 g of MgCl 2 in 1 L of deionizedwater.

Procedure. Add 2.0 g of soil «0.15 mm) to 40 mL of 0.1 M HCI in a 100mL centrifuge tube. Shake the suspensionmechanicallyfor 30 min, and centrifuge. Discardthe supernatantliquid and repeatthe washingwith another40 mL of 0.1 M HCI. Quantitativelytransferthe soil residueto a 50-mL beakerwith 30 mL of 0.2 M acetylacetone(pH 8.0) and give a 2-h ultrasonictreatment.Transfer the suspensionto a centrifugebottle with 10 mL of acetylacetoneand adjust the pH to 8.0 with 3 M NaOH. Shakethe suspensionfor 16 h and centrifuge.Decant the supernatantliquid in a 500-mL Erlenmeyerflask. Repeatthe extractionfour more times with 40 mL of 0.2 M acetylacetone(pH 8.0) and a 24-h shakingperiod, with an ultrasonic treatment for the final extraction. Add diethyl ether (C2HsOC2HS)and shake well to extract the acetylacetoneand its complexes. Removethe ether using a liquid-liquid separatoryfunnel. Centrifuge the etherextractedsolution at high speedto clarify the solution. Transferan aliquot containing2 to 40 f.lg P to a Folin-Wu digestion tube (Kimble Glass,Vineland, NJ). Add 0.5 mL of saturatedMgCl 2 and 1 mL of 70% HCI04 and digest in an aluminum digestionblock. Slowly raise the temperature to 205°C. When the white fumes appear,cover the tubeswith funnels or beakers, and digest at 205°C for an additional 30 min. Remove the tube, and let cool. Quantitativelytransferthe digestto a 50-mL volumetricflask for P determination by the ascorbicacid methodas outlined in "Ascorbic Acid Method." The digestionalso can be done in a beakeron a hot plate. Cover the beaker with a watch glasswhen white fumes appear. Calculatethe Po as follows. 50 50 Po = P Concentration(f.lglmL) • - • - - - Vt g soil used

[9]

where Vt = samplevolume usedfor the P determination. Comments Condronet al. (1990) showedthat the ignition methodand the acid-alkali extractionmethodspresentedaboveare about equally effective in extractingPo. The concentrated H 2S04 extractionmethodextractsslightly higher Po (Bowman, 1989; Condronet aI., 1990) but less total soil P (Po + Pi). Efficiency in operation and requirementsfor further fractionation of Po are important considerationsin deciding which methodto choose.The ignition method,while simple and suitable for routine soil testingof a large numberof soil samples,destroysthe struc-

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ture of Po and is unsuitableif further fraction of Po is needed.The method of Mehta et al. (1954), for instance,was adoptedby Caldwell and Black (1958a), and Thomasand Lynch (1960) for determiningthe inositol hexaphosphate using column chromatography.Fractionationof soil Po is presentedin more detail in "Fractionationof OrganicPhosphorus." The ignition methodis subjectto severalpotentialerrors.Ignition at 550°C canenhancethe P solubility particularlyfor the strongly weatheredsoils that contain primarily iron- and aluminum-phosphates, leading in some casesto a considerableoverestimateof Po (Williams & Walker, 1967; Williams et ai., 1970). The Pi associatedwith organic matter through bridge bonding of Fe or Ai can contribute to the error in Po estimatebecauseit is taken as Po after the organic matteris oxidized throughignition (Hong & Yamane,1980). In addition, incomplete oxidation of Po (e.g., RNA and phytin) (Dormaar & Webster, 1964) can result in small errorsin estimatesof Po. The strengthof H2S04 and the length of extractioJ)time usedfor the ignition method vary with investigators.The H2S04 concentrationoriginally proposed by Saundersand Williams (1955) is 0.1 M. The concentrationwas increasedto 0.5 M by Walker and Adams(1958) and further to 1 M by Anderson (1960), HanceandAnderson(1962), Bowman(1989),and Condronet ai. (1990). The extractionperiod usedis either2 or 16 h by the aboveauthors.Saundersand Williams (1955) extractedsimilar amountsof Po with 0.1 M H2S04 using 2- or 16-h extractionperiod. Their choice of a 16 h or overnight extractionperiod is simply a matterof convenience. Concentratedmineral acid (e.g., HCI) hydrolyzes native Po (Halstead& Anderson,1970). Anderson(1960) found the similar resultswith inositol hexaphosphate (C6HlS024P6), glycerophosphate(C3H706PNa2), sugar phosphate (C6H l1 0 9PNa), and nucleic acid, and proposeda pretreatmentwith dilute alkali (0.3 M NaOH) to remove the majority of Po prior to acid extraction.The pretreatmentremovedthe majority of Po prior to acid extraction.The pretreatment removedclose to 60% of Po (Condron et aI., 1990), including all of the glycerophosphateand the majority of phytate (Martin, 1964). Ribonucleic acid (RNA), for instance,is not solublein 0.3 M NaOH (Martin, 1964)but hydrolyzed extensivelyin strongacid (Anderson,1960). The hydrolysiscontributeserrorsin Po estimatesproportionateto the quantity of RNA presentin soils. The concernover the hydrolysis of Po in strong acid solution led to some modificationof the Mehtaet ai. (1954)procedure.Kaila and Virtanen(1955)proposedto use 2 M H2S04 insteadof concentratedHCI, whereasAnderson(1960) introduceda pretreatmentwith dilute alkaline solution as a remedial measure. However,the modification by Anderson(1960),while improving the accuracyof Po extraction,increasesthe sequentialextractionsfrom threeto four steps.In contrast, Bowman(1989)observedonly a slight hydrolysisof inositol hexaphosphate among several phosphomonoesters and phosphodiestersadded to concentrated H2S04 and developeda two-sequentialextractionprocedurethat is equally effective as the Anderson-Mehtamethod.The Bowman methodmerits consideration if efficiency is vital. However,Martin (1964) showedthat threeserial extractions with 5 M H2S04 heatedat 100°C for the first extractioncan extract as much as 85% of Po, a level generally achievedby the Bowman method. Several other

PHOSPHORUS

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methodsalso havebeendevelopedfor determiningPo over the years.An important considerationin their developmentis to facilitate the fractionationof Po. Pretreatmentwith 0.1 M HCl to removeCa and Mg is usually required.Thesemethods include three serial extractionswith 0.3 M KOH at room temperature(Martin, 1964)-thecombinationof exchangeresin with alkaline solution (De Serra & Schnitzer,1972)or with acetylacetone (pH 8.3) (Hong & Yamane,1980)-and 0.3 M NaOH extraction sonified to dispersesoil particles (Soltanpour et aI., 1987). Dependingon the method,between78 and 99% of Po can be extracted. IncreasedPo accumulationin the fulvic acid fraction occursin the extraction of Po by the Anderson-Mehtamethod but not by the resin-acetylacetone method(Hong & Yamane,1980). Organic P associatedwith fulvic acid is consideredto be relatively more labile than humic acid associatedP in soils (Bowman & Cole, 1978). When fulvic acid is used to index P transformationand bioavailability in soils, mild and less destructiveextractantssuch as acetylacetonemay be preferredover strong mineral acids. For Pi determination,the "Modified Ascorbic Acid Method" developedby Dick and Tabatabai(1977b) should be employed to avoid the potentialinterferencefrom acid-labile Po, if presentin a sufficient amount.

FRACTIONATION OF SOIL PHOSPHORUS Fractionationof InorganicPhosphorus Principles Inorganic P can reactwith Ca, Fe, or AI to yield discretephosphatessuch as hydroxyapatite[Cas(P04hOH],octacalciumphosphate[Ca4H(P04h2.5 H20] and variscite (AlP04 • 8 H20) (Lindsay, 1979; Lindsay & Vlek, 1977). Their identification in soil, however,is done mostly by solubility equilibria. Scanning transmissionelectron microscopy in conjunction with energy dispersive x-ray analysis of density separatesyields P-rich particles containing various cations including AI, Si, Ca, and Fe (Pierzynskiet aI., 1990). Fractionationschemesutilize the complexingability of r from ammonium fluoride (NH4F) to separateAI-P from Fe-P, followed by removal of Fe-P with NaOH and of the reductant-solubleP with sodium citrate (Na3C6Hs07• 2H20)-sodium dithionite (Na2S204)-sodiumbicarbonate (NaHC03) (CDB) extractions.The calcium-phosphate, which is insoluble in CDB (Williams et aI., 1980), is extractedwith H2S04 or HCI. However, NH4F reactswith CaC03 to form calcium fluoride (CaF2) in calcareoussoils (Smillie & Syers,1972), which in tum forms secondaryprecipitateswith solubilizedP and reducesthe effectivenessof NH4F to extract P. As a result, the NH4F extractionis not recommended for calcareoussoils (Williams et aI., 1971a).

SpecialApparatus 1. pH meterequippedwith a single combinationelectrode. 2. Mechanicalshaker.

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3. Hot waterbath. 4. Centrifuge.

Methods

Fractionation for Noncalcareous Soils Reagents 1. Ammonium chloride(NH4Cl), 1 M: Dissolve 53.3g of NH4CI in deionized water and dilute to 1 L. 2. Ammonium fluoride (NH4F), 0.5 M (pH 8.2): Dissolve 18.5 g of NH4F in deionizedwater and dilute to 1 L. Adjust pH to 8.2 with 4 M ammonium hydroxide (NH40H). 3. Sodiumhydroxide,2 M: Dissolve80 g of NaOH in deionizedwaterand dilute to 1 L. 4. Sodium hydroxide,0.1 M: Dissolve 4.0 g of NaOH in deionizedwater and dilute to 1 L. 5. Sodiumhydroxide,0.1 M, + sodiumchloride (NaCI), 1 M: Dissolve4.0 g of NaOH and 58.5 g of NaCI in deionizedwater and dilute to 1 L. 6. Sulfuric acid, 0.25 M: Dilute 14 mL of concentratedH 2S04 (18 M) to 1 L with deionizedwater. 7. Hydrochloric acid, 2 M: Dilute 168 mL of concentratedHCI (12 M) to 1 L with deionizedwater. 8. Sodiumcitrate, 0.3 M: Dissolve 88.2g of Na3CJls07• 2 H20 in about 900 mL of deionizedwater and dilute to 1 L. 9. Sodiumchloride, 1 M: Dissolve58.5 g of NaCI in deionizedwater and dilute to 1 L. 10. Sodium chloride, saturated:Add 400 g of NaCI to 1 L of deionized water. 11. Sodiumbicarbonate,1 M: Dissolve 84g of NaHC03 in deionizedwater and dilute to 1 L. 12. Sodiumdithionite reagentgrade. 13. Boric acid, 0.8 M: Dissolve50 g of H3B03in deionizedwateranddilute to 1 L. 14. p-nitrophenol,0.25%: Dissolve 0.25 g of p-nitrophenolin 100 mL of deionizedwater.

Procedure. Add 1.0 g «2 mm) and50 mL of 1 M NH4Cl to a 100-mL centrifuge tube and shakefor 30 min to extractthe solubleand loosely boundP. Centrifuge and decantthe supernatantinto a 50-mL volumetricflask and bring to volume with deionizedwater (ExtractA). Add 50 mL of 0.5 M NH4F (pH 8.2) to the residueand shakethe suspensionfor 1 h to extractaluminumphosphate(AlP04). Centrifuge and decantthe supernatantinto a 100-mL volumetric flask (Extract B). Wash the soil sampletwice with 25-mL portionsof saturatedNaCI and centrifuge. Combinethe washingswith Extract B and bring to volume. Add 50 mL of 0.1 M NaOH to the soil residuesand shakefor 17 h to extract iron phosphate (FeP04). Centrifugeand decantthe supernatantsolution into a 100-mL volumetric flask (Extract C). Wash the soil twice with 25-mL portionsof saturatedNaCl

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and centrifuge.Combinethe washingswith extractC and bring to volume. Add 40 mL of 0.3 M Na3C6H507and 5 mL of 1 M NaHC03 to the residueand heat the suspensionin a water bath at 85°C. Add 1.0 g of Na2S204(sodiumdithionate) and stir rapidly to extract reductant-solubleP. Continueto heat for 15 min and centrifuge.Decantthe supernatantsolution into a 100-mL volumetric flask (Extract D). Washthe soil twice with 25-mL portionsof saturatedNaCl and centrifuge. Combinethe washingswith Extract D, and dilute D to volume. Expose Extract D to air to oxidize Na2S204' Add 50 mL of 0.25 M H2S04 to the soil residueand shakefor 1 h. Centrifuge the suspensionfor 10 min and decantthe supernatantinto a 100-mL volumetric flask (Extract E). Wash the soil twice with 25-mL portionsof saturated NaCl, andcentrifuge.Combinethe washingswith the ExtractE and dilute to volume. Transferan aliquot containing2 to 40 Ilg P from eachof ExtractsA, B, C, D, and E to a 50-mL volumetric flask. Add somedeionizedwater and five drops of p-nitrophenolindicatorto the volumetricflask containingExtractsC and E and adjustthe pH with 2 M HCl or 2 M NaOH until the indicator color just changes. Add 15 mL 0.8M H3B03to the volumetricflask containingExtractB. Determine P concentrationusing the ascorbicacid method as outlined in "Ascorbic Acid Method." PrepareP standardsthat containthe samevolumeof extractingsolution as in the extracts. The calculationof variousP fractions can be madeas follows. Pextract =P concentration(llglmL) where VI Vz

0

50

V2

VI

g soil used

-0

[10]

= samplevolume from ExtractA, B, C, D, or E usedfor P determination.

= the volume of Extract A, B, C, D, or E.

Fractionation of Calcareous Soils Reagents. Preparethe reagentsas describedin the previous"Reagents"section.

Procedure. Add 1.0 g «2 mm) of soil and 50 mL of 0.1 M NaOH + 1 M NaCl, shakefor 17 h and centrifuge.Decantthe supernatantinto a 100-mL volumetric flask (Extract A). Wash the soil twice with 25-mL portionsof 1 M NaCl. Combinethe washingswith Extract A and bring to volume. Add 40 mL of 0.3 M Na3C6H507and 5 mL of 1 M NaHC03 to the soil residueand heatthe suspensionin a waterbath at 85°C. Add 1 g of NaZSZ04and stir rapidly. Continueto heatfor 15 min. Centrifugeand decantthe supernatant solution into a 100-mLvolumetricflask (ExtractB). Washtwice with 25-mL portions of saturatedNaCI andcentrifuge.Combinethe washingswith ExtractBand dilute to volume. Exposethe solution to air to oxidize Na2S204'Add 50 mL of 0.5 M HCI to the soil andshakefor 1 h. Centrifugethe suspensionanddecantthe supernatantsolutioninto a lOO-mL volumetricflask (ExtractC). Washtwice with

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25-mL portions of saturatedNaCl, combine the washingswith Extract C, and dilute to volume. Transferan aliquot containing2 to 40 Ilg P from eachextract to a 50-mL volumetricflask. Add somedeionizedwater and five dropsof p-nitrophenolindicator to the volumetric flasks containingExtractsA and C, followed by adjustment of pH with 2 M HCI or 2 M NaOH. The indicator color changesfrom yellow to colorlessfor Extract A and from colorlessto yellow for Extract C. Determine the P concentrationusing the ascorbicacid methodas outlined in "Ascorbic Acid Method." PrepareP standardscontainingthe samevolume of extracting solution as in the extract. The calculationof various P fractions can be madeusing Eq. [10].

Comments Therehavebeenseveralmodificationsof the fractionationschemepresented by ChangandJackson(1957). Petersonand Corey(1966) alteredthe pH of the NH4F extractingsolutionaswell as the sequenceof the extraction.Increasingthe pH from neutral,as originally proposedby Changand Jackson(1957) for NH4F, to 8.2 increasesP extractabilityby NH4F (Fife, 1962). Williams et al. (1967) proposed a second NaOH extraction for calcareoussoil. Williams et al. (1971a) includedsodiumcitrate-sodiumbicarbonateto minimize the resorptionof the dissolvedP by CaC03 in 0.1 M NaOH solution and a secondHCI extractionfor 4 h to increasethe extraction of occluded apatite in the matrix of minerals (e.g., quartz)(Syerset aI., 1967). The inclusion of NaHC03 is consideredto be necessaryto buffer against decreasesin pH during the extractionof reduntant-solubleP to preventthe dissolution of apatite(Williams et aI., 1967). Thus, the recommendedfractionation schemeby Petersonand Corey (1966) was slightly modified to include NaHC03 in the extractionof the reductant-solubleP fraction. Loosely boundP generallyrepresentsa very small fraction of the total P in soils or sediments.While only trace amountsare found in noncalcareoussoils, Ca-boundP can constitutea large proportionof the P in calcareoussoils (Sharpley & Smith, 1985).The majority of Pin noncalcareous soils or sedimentsis present as aluminum phosphateand iron phosphatethat are extractablewith NH4F and NaOH (Williams et aI., 1971b).

Fractionationof OrganicPhosphorus Principles The phosphatemonoesters,which include inositol phosphates,are more resistant to degradationby soil microorganismsthan the phosphatediesters, which include phospholipidsand nucleic acids (Condron et aI., 1990). Of the 50% or lessof soil Po that hasbeenaccountedfor in known compounds,inositol phosphatespredominate(Anderson,1967).The quantitiesof lower phosphatesof inositol are much smallerthan thoseof higher phosphates(penta-and hexaphosphate).The higherphosphatesof inositol (CJI 1206),which canbe synthesizedby

PHOSPHORUS

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soil microorganisms (Caldwell & Black, 1958b),are more inaccessibleto microbial or enzymedegradation(Greaves& Webley, 1969)due to their higher charge densitiesand high binding strengthon the surfacesof soil minerals(Andersonet aI., 1974; Stewart & Tiessen,1987). Stereoisomersof myo-, chiro-, neo- and scyllo-phosphateestershavebeencharacterizedin soils. Inositol phosphatesin soils can be extractedwith alkaline solution. Basically, the organicP is precipitatedas Fe or Ba salts at appropriatepH levels following hypobromitetreatment.The precipitatedFe- or Ba-phytatesare dissolved againin acid solutoin or with H+-resin and fractionatedby anion-exchangechromatography.Separationof Po is basedon the strength of interactionsbetween organicP componentsin the mobile phaseand the exchangesiteson the stationary phase(resin). Phospholipids,a small fraction of total Po «5%) (Stevenson,1982), are definedasphosphateesterssolublein fat solvents such as ether(C2Hs)O, benzene (C6H6), or chloroform (CHCI3). However,soil mineralsinterferewith the extraction of phospholipidsby the solvents,and pretreatmentof soil with a mixture of HCI and HF is necessary toimprove the effectivenessof solventextraction.Phosphatidyl choline (1,2-Diacyl-sn-glycero-3-phospho-choline), phosphatidyl ethanolamine (1,2-Diacyl-sn-glycero-3-phospho-ethanolamine), and phosphatidyl serine(1,2-Diacyl-sn-3-phospho-L-serine) are the predominantforms of phospholipidsin soils. . Like phospholipids,nucleic acids and their derivatives representa small fraction of Po in soil (Anderson,1975; Stevenson,1982). Ribonucleic acid and deoxyribonucleicacid (DNA), which consist of chains of nucleotides,can be adsorbedby soil particles(Goring & Bartholomew,1952). The methodsusedto characterizethis Po fraction havebeenreviewedby Anderson(1967) and are not included in this chapter. Microbial biomass P representsa small fraction of soil P held in soil microorganisms(Tate, 1984). It contains,in addition to Pj, organicP compounds such as RNA, DNA, polyphosphates,and inositol phosphatesand turns over rapidly to supply Pi for plant use.The microbial biomassP is measuredbiologically in fresh soil by CHCl3 fumigation, followed by chemicalextractionto determine the increaseof Pi. The procedurefor its measurementis not includedin this chapter. It is describedin detail by Brookes et al. (1982), Hedley & Stewart (1982), and Walbridge and Vitousek (1987). Special Apparatus

1. 2. 3. 4. 5. 6. 7. 8.

Mechanicalshaker. Centrifuge. Hot water bath. Refrigerator. Fractioncollectors. Aluminum block for tube digestion. Stainlesssteelperchloricacid fume hood. pH meterequippedwith a single combinationelectrode.

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Methods Inositol Phosphates Reagents 1. Hydrochloric acid, concentrated(12 M). 2. Hydrochloric acid, 1.5 M: Dilute 125 mL of concentratedHCI (12 M) to 1 L with deionizedwater. 3. Hydrochloricacid, 0.1 M: Dilute 67 mL of 1.5 M HCI to 1 L with deionized water. 4. Sodiumhydroxide, 10 M: Dissolve400 g of NaOH in deionizedwater and dilute to 1 L. 5. Sodium hydroxide, 2 M: Dilute 200 mL of 10 M NaOH to 1 L with deionizedwater. 6. Sodium hydroxide, 1 M: Dilute 100 mL of 10 M NaOH to 1 L with deionizedwater. 7. Ferric chloride (FeCI3), 60% w/v: Dissolve 1000g of FeCl3 • 6 H20 in deionizedwater and dilute to 1 L. 8. Bromine. 9. Perchloricacid, 70%. 10. Magnesium chloride saturated: Suspend600 g of MgCl2 to 1 L of deionizedwater. 11. Anion exchangeresin, Dowex 1-X8 (0.038--0.075mm) (Dow Chemical, Midland, MI). 12. Cation exchangeresin, Dowex-50W (W). 13. p-nitrophenol,0.25%: dissolve 0.25 g of p-nitrophenolin 100 mL of deionizedwater.

Procedure. Add 5.0 g of soil «0.18mm) and50 mL of 0.1 MHCI to a 100mL centrifugetube and shakethe mixture for 10 min. Centrifugethe mixture to obtain a clear supernatantsolution. Decantthe supernatantliquid and determine the Ca concentration.Repeatthe processuntil the supernatantis free of Ca. Add 50 mL of 1 M NaOH to the soil residueandshakethe mixture for 16 h. Centrifuge and decantthe supernatantliquid to a 250-mL Erlenmeyerflask. Add an additional 50 mL of 1 M NaOH to the residualsoil and heat the mixture in a water bath at60°C for 4 h with occasionalstirring. Centrifugeand combinethe supernatant liquid with the first extract. Wash the residualsoil twice with "50 mL of H20, centrifuge,and add the supernatantto the two alkali extracts. Adjust the pH of the combinedextractsand washingswith concentrated HCI to pH 0.5. Mter 30 min, centrifugethe mixture and decantthe supernatant liquid, which containsfulvic acid, to a 5OO-mL Erlenmeyerflask. Redissolvethe residualhumic acid by twice washingthe residuewith 25 mL of 1 M NaOH. Precipitate the humic acid again with concentratedHCI each time as described above.Centrifugeand combinethe washingswith the fulvic acid fraction in an Erlenmeyerflask. Cool the combinedfulvic acid solution to 5°C and add 20 mL of Br2 in 2-mL incrementswith constantmixing. Keep the mixture at 5°C for 18 h, after which heatthe solution at 60°C for 1 h andremovethe excessBr2 by low temperaturedistillation in vacuo or alternativelyby extractionwith 50-mL por-

PHOSPHORUS

887

tions of ether (C2HS)O four times. Transfer an aliquot to a Folin-Wu digestion tube, add 0.5 mL of saturatedMgCl2 solution, 1 mL of 70% HCI04 and determine the P concentrationas describedin "Ascorbic Acid Method." Adjust the solution pH to 2.0 with 10 M NaOH solution. Add 60% FeCl3 (w/v) to give a FelP (W/W) ratio of about two and heat the solution in a steam bath for 20 min to coagulatethe precipitate.Centrifugethe solution after it has cooled and decantthe liquid. Dissolve the precipitatein 2 M NaOH and reprecipitate at pH 2 using enoughFeCl3 to obtain a FelP ratio of about 1. Centrifuge and decantthe liquid. Dissolve the precipitatein 2 M NaOH and neutralizethe excessalkali by addition of H+ -resin until the pH is about seven.Filter to separate the solution from the resin and reducethe volume of the solution to 5 to 10 mL in vacuo. Ion-Exchange Chromatography. Preparea resin column (10- by 1.2-cm diam.) with the Cl- form of Dowex 1 x 8 anionexchangeresin (0.038-0.075mm) and convert the resin to the formate form by washing with 6 M formic acid (HCOOH) and then with H20. Add an aliquot containing5 to 8 mg of organic P to the column and elute the column with increasingconcentrationof HCI up to 0.7 M by an apparatusthat consistsof two connectedreservoirsof the type describedby Parr (1954): one containing 1550 mL H20 (ReservoirA) and the other 900 mL 1.5 M HCI (Reservoir B). Stir the solution in ReservoirA constantly and keepthe reservoirsat identical levels. Keep the flow rate from Reservoir A at 1 mL per 3 min. Collect successivelO-mL samplesof the eluateup to 50 in an automaticfraction collector. Transferan aliquot containing2 to 40 l!g P to a Folin-Wu digestion tube. Add 0.5 mL of saturatedMgCI2, 1 mL of concentratedH2S04 and 1 mL of 70% HCI04 and digest at 205°C for 30 min as describedin "Procedure"for "AcetylacetoneExtraction."Transferthe digestto a 50-mL volumetricflask with 15 mL of deionizedwater. Add five dropsof 0.25%p-nitrophenolindicator and neutralize the acidity with 10 M NaOH in dropwise additions until the color just turns yellow. Determinethe P concentrationusing the ascorbicacid methoddescribed in "Ascorbic Acid Method." Comments. Treatmentof fulvic acid with NaOBr following the removalof humic substancesby centrifugationis necessaryto destroy extraneousorganic matter that can interfere with the adsorptionand elution of inositol phosphates. The excessBr2 shouldbe removedby low temperaturedistillation as describedin the procedure,by boiling under a fume hood (Wrenshall & Dyer, 1941) or by extraction with ether to facilitate the formation of ferric phytate (Cosgrove, 1963).The brominationof fulvic acid hasminimal effect on the stability of phytin presentin the fulvic acid (Wrenshall& Dyer, 1941). Anderson(1963) observedthat when the Fe concentrationis high and inositol phosphateconcentrationis low, quantitativeyields may not be achieveddue to the formation of solublecomplexesof sodium-phytatewith FeCI3. The desired weight ratio of FelP is about 0.81 (Wrenshall & Dyer, 1941).The ratio of FelP calledfor by the Omotosoand Wild's (1970a)methodis between1:1and 2:1. Ferric chloride applicationbasedon theserations is more than adequateto precipitate the phytin as well as avoid high concentrationsof Fe.

888

KUO

The elution diagramrevealssevenfractions for soil inositol phosphatesas opposedto six for the commercialphytin (Omotoso& Wild, 1970a; Cosgrove, 1963). The first fraction yields Pi> inositol mono-, di-, and triphosphates,plus an unknown inositol phosphate.The lower estersconstituteonly a minor portion of the total Po (Anderson,1956; Martin & Wicken, 1966; Dormaar,1967). However, Cosgrove(1963), using a Cl--form of Dowex 1 x 8 (0.038-0.075mm), failed to identify the presenceof lower estersand the free inositol from the hydrolysis of the Po in this fraction. Fractions 2 and 3 containedtetrainositol phosphate (C6H 1601SP4), and Fractions 4 and 5 contained pentainositol phosphate and scyl(C6H 170 21Ps). Fractions6 and 7 containedthe inositol hexaphosphate loinositol hexaphosphatethat was referred to as isomers of inositol hexaphosphateby Smith and Clark (1951) and later identified by Cosgrove(1963) as scylloinositol hexaphosphatebased on paper chromatographyand comparisonof infrared spectra. A two-stepfractionatioll of Po by chromatographyin alkaline soil extracts without acidification to separatethe humic acid from fulvic acid fractions has been developed(Martin & Wicken, 1966; Omotoso& Wild, 1970b; Steward& Tate, 1971).The Po is separatedfrom Pi by eluting the extractthrougha Sephadex G-25 gel (PharmaciaBiotech, Alameda,CA) column and is further fractionated by eluting through a secondSephadexcolumn following the destructionof extraneousorganics(Omotoso& Wild, 1970b)or by isolation asbariumsaltsand conversion to the free acids (Steward& Tate, 1971). The two-step approachavoids the use of FeCl3 to precipitatephytin, which has a relatively low reactivity with lower esters(Anderson,1975), possibly due to a relatively low chargedensity. Severalother fractionationschemes,differing mainly in the type and form of resinsand elutantsused,also havebeenproposed.Caldwell and Black (1958a) obtainedPo in the fulvic fraction using the procedureof Mehta et al. (1954). The Po was separatedin a stepwiseelution with increasingconcentrationof HCI using weak base polyamine resin. The method, designedprimarily for isolating the polyphosphates (inositol pentaphosphate andhexaphosphate), haspoor resolution for the lower phosphateesters (Dormaar, 1967). McKercher and Anderson (1968a)extractedsoils with 3 M NaOH in boiling water and successfullyeluted the lower and polyinositol phosphatesfrom a column of Dowex-l (format form) of with increasingconcentrationsof NH4-formate. Penta-and hexaphosphates inositol and their isomerscan be separated when the eluatescontainingthem are further eluted with a gradientof HCI from 0 to 1.5 M (McKercher & Anderson, 1968b). Myoinositol hexaphosphateis generally the major componentof the polyphosphateester. Polyphosphateis the major componentof soil Po, although its content varies widely amongsoils. Its concentrationrangesfrom 11 to 30% of total Po (averaging24%) for someEnglandand Nigerian soils (Omotoso& Wild, 1970b), 2 to 30% for some Canadiansoils (Dormaar, 1967; McKercher & Anderson, 1968a,b;Thomas & Lynch, 1960), 8 to 25% for some U.S. soils (Caldwell & Black, 1958c),and 12 to 16% for someAustrian soils(Cosgrove,1963). Other isolates of Po that have been identified include acid-labile sugar phosphate[e.g., glucose-I-phosphate (C6H 130 9P), Omotoso& Wild, 1970b] and of bactepyrophosphate(Anderson& Russell, 1969). In addition, phosphonates

PHOSPHORUS

889

ria origin have been identified by nuclear magnetic resonance(NMR) spectroscopy(Tate & Newman,1982; Hawkeset aI., 1984). Another method of characterizingPo is basedon successiveextra~tions with various extractants(Bowman & Cole, 1978; Tiessenet aI., 1983). These investigatorsdivided the total Po into labile (NaHC03-extractable),moderately labile (dilute NaOH and dilute H2S04-extractable),moderatelyresistant(associatedwith fulvic acid), highly resistant(associatedwith humic acid), and residual fractions. This fractionationschemeaccountsfor 98% of total Po.

Phospholipids Reagents 1. 1.25% HCI + 1.25% HF: Dilute approximate34 mL of concentrated HCI (12 M) and 26 mL of concentratedHF (24 M) to 1 L with deionized water in a l-L polypropylenebottle. 2. Acetone(CH3COCH3). 3. Ether. 4. Chloroform (CHCI3). 5. Light petroleum. 6. Benzene(C6H6)' 7. Methanol (CH30H).

Procedure. Add 1.0 g of soil «0.15 mm) to 10 mL of 1.25%HCI + 1.25% HF solution in a 100-mL polyethylenecentrifuge tube. Shake the suspension overnightand centrifuge.Discardthe supernatantliquid and repeatthe extraction. Wash the soil residuewith 10-mL portions of deionizedH20 until acid free and discardthe washings.Add 50 mL of acetone,cover the tube with a polyethylene sheetheld in placewith a rubberband.Allow the suspensionto standfor 4 h with thoroughshakingevery 30 min. Centrifugeand transferthe supernatantliquid to a 500-mL bottle with a ground-glassstopper.Add 50 mL of light petroleum(boiling point == 40-60°C) and allow it to standfor 4 h with occasionalshaking.Centrifuge and combinethe supernatantwith the acetoneextract. Repeatthe extraction with 50 mL of a 1:4 (v/v) mixture of ethanollbenzeneand then with 50 mL of a 1:1 (v/v) mixture of ethanol(C2HsOH):chloroform. Combinetheseextracts with the previous extracts and evaporatethem to dryness at low temperature (30°C) and pressureunder a fume hood. To purify the P-containinglipid, reextract the residuewith the following sequence:20 mL of a cold 1:1 (v/v) mixture of ether/light petroleumfor 2 min; 20 mL of cold chloroform for 2 min; 20 mL of cold 1:1 (v/v) ether/lightpetroleumbroughtto boiling in a water bath; and finally 20 mL of cold chloroform, heatedto boiling. Combinetheseextractsand evaporateto drynessin a beakerin a water bath and determinethe TP concentration by digestionin HCI04 as outlined in "Procedure"for "AcetylacetoneExtraction." Comments. Soil drying greatly affects the extractability of phospholipids, and for air-dried soils, at least two pretreatmentswith the mixture HCl and HF are necessaryto increasethe efficiency of extraction(Hance& Anderson,1963a). Clay is known to decreasethe extraction of lipid P in bacteriacellular extracts (Goring & Bartholomew,1949).

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There have beenseveralmodificationsto simply the Hanceand Anderson (1963a)method.Theseinclude two successiveextractionswith hexane-acetone followed by (Kowalenko & McKercher, 1970), extractionwith ethanol-benzene extraction with methanol (CH30H)-chloroform (Baker, 1975), and extraction with methanol-chloroform(Chae & Tabatabai,1981). High-performanceliquid chromatographycan be used in combinationwith methanol-chloroformextraction for identifying phospholipidsin soils (Stott & Tabatabai,1985). The concentrationof phospholipidsrangesfrom 0.1 to 13 mg kg-I, representing0 to 14% of the total Po of somesoils (Kowalenko & McKercher, 1970; Hance & Anderson, 1963a; Dormaar, 1970; Chae & Tabatabai,1981; Stott & Tabatabai, 1985). Alkaline hydrolysis of soil lipids yields glycerophosphate, whereas acid hydrolysis produces choline (2-hydroxy-N,N,N-trimethylethanammoniumhydroxide) and ethanolamine(2-aminoethanol)(Hance& Anderson, 1963b). AVAIlABILITY INDICES

General Principles To evaluateP availability in soils, numeroussoil testshavebeendeveloped that extractvarying amountsof P, dependingon the typesof extractantsused.The extractantscan be generallyclassifiedinto severalcategories:

1. Water or unbufferedsalt solutions(e.g., CaCI2). 2. Dilute concentrationsof weakacids(e.g.,lactate,acetate)with or without a complexingagent(P- or EDTA). 3. Dilute concentrationsof strongacids(e.g., HCI, H 2S04) with or without a complexingagent(e.g., P-, lactate,EDTA). 4. Buffered alkaline solutions [e.g., NaHC03, NH4HC03 (ammonium bicarbonate)]with or without a complexingagent(DTPA). 5. Anion exchangeresin or iron oxide-impregnatedfilter paperstrips. 6. isotopic exchangewith 32p. The dilute strongacid solutionssolubilizeCa-P,Al-P, and to a lesserextent, Fe-P(Nelson et aI., 1953). Fluoride is includedwith dilute strong acid solutions (Bray & Kurtz, 1945) to complexAl and preventreadsorptionof P by Fe oxides. Salts and EDTA are further included with P- and dilute strong acid (Mehlich, 1984) to form multielementteststhat simultaneouslyextract macro-and microelements. Like EDTA, DTPA was added to the buffered alkaline solutions (Soltanpour& Schwab, 1977) to facilitate the extractionof microelementsthat can be simultaneouslydeterminedby inductively coupled argon plasma spectroscopy(Soltanpouret aI., 1979). Anion exchangeresin, iron-oxide impregnatedfilter paperstrips, and isotopic exchangemethodsare nondestructiveon soil constituentsin contrastto the chemicalextractants.Iron-oxide impregnatedfilter paperor anionexchangeresin functionsas a sink that simulatesthe actionof plant roots by continuouslyremoving dissolved P from the soil solution. The quantity of P that is isotopically exchangeablewithin a specifiedtime interval gives an estimateof labile surface

P.

PHOSPHORUS

891

Methodsfor a numberof soil testsare presentedalongwith their basicprinciples. For the soil teststo adequatelyreflect soil P availability, the P testsshould respondto soil characteristicsin a similar manneras plants. The relationships amongsoil tests and betweensoil test and plant yield or P uptakeare discussed in the commentssections. Methods Extractionwith Wateror Dilute Salt Solution Principles.Soil solution P representsthe portion of soil P that is in equilibrium with the solid phaseP under the conditionsprevailing in the soil. It is a very small fraction of availablesoil P, but readily accessibleto plant roots. Centrifugation and displacementare the two most commonly used techniques for obtaining soil solution. However, these methodsare impractical for routine soil testing, consideringthe time and the volume of soil solution for P determination.A wider ratio of water to soil is generally recommendedin the interestof efficient operation,recognizingthat the resultingchangesof the chemical characteristicsof the soil solution, such as ionic strength,can influence soil P solubility. Dilute CaCl2 (0.01 M) is usedin placeof waterfor obtaininga clear filtrate (Aslyng, 1964). The amountof P solublein CaCl2 solution is'smallerthan that in water due in part to the enhancementof Ca2+ on P sorption by soils. Reagents 1. Calcium chloride, 0.01 M: Dissolve 1.47 g of CaCl2 • 2H20 in deionized water and dilute to 1 L. Procedure.Add 5 g of air-dried soil «2 mm) and 50 mL of deionized water (or 0.01 M CaCI2) to a flask. Shakethe suspensionfor 1 h and centrifuge to obtain a clear supernatantliquid. If the solution is not free of suspendedsoil particles,filter through a membranepaper(0.45 Ilm) or repeatedlyfilter through the sameWhatmanno. 42 filter paperuntil clear. Pipette an aliquot containing 1 to 20 Ilg P into a 25-mL volumetric flask and determinethe P concentrationby the "Ascorbic Acid Method" if significant amountsof acid-labile Po are expected.PreparestandardP solutionsand a blank that contain the samevolume of extracting solution. Calculatethe amount of P extracted(Pex) as follows

25

Pex (mg kg-I) = P concentration(llglmL)· -

VI

whereVI V2

•

V2

g soil used

[12]

=volume of extractusedfor P determination,

=volume of extract.

Comments.The ratios of soil weight to the volume of H 20 that have been used to measurethe water-solubleP vary widely, including 1:1.25 (Olsen & Watanabe,1970), 1:10 (Olsen & Sommers,1982), 1:60 (van der Paauw, 1971),

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and 1:100(van Diest, 1963). The reactiontimes usedfor the extractioninclude 5 min (Kuo & Jellum, 1987; Olsen & Sommers,1982), 1 h (Olsen & Watanabe, 1970; Mackay et aI., 1984)or 15 h (van Diest, 1963).The water-solubleP should be a function of the quantity of P sorbedand the P sorptioncapacityor, alternatively, the degreeof P surfacesaturation(Murrmann& Peech,1969). It also may be related to the concentrationof physically sorbed P (Ryden & Syers, 1977). However,due to the unbufferedcharacteristicof water, the quantity of water-soluble P, or the P concentrationin solution, is highly dependenton pH (Kuo & Jellum, 1987; Guptaet aI., 1990),ionic strength(Bolan et aI., 1986; Bar-Yosefet aI., 1988) and type and valenceof exchangeablecation (Smillie et aI., 1987; Stoop, 1983; Sharpleyet aI., 1988). For example,Na saturationof soil increasesP solubility in water, whereasCa saturationgenerallydoesthe opposite. Water-solubleP correlatedwell with P uptakeby plants (van Diest, 1963; Thompsonet aI., 1960; Tran et aI., 1988; Keramidas& Polyzopoulos,1983), although results to the contraty also have been shown (Fried & Shapiro, 1956). Underlong-termgrowth conditions,especiallywith plantshaving high P requirements,the water-solubleP may be inadequateto accuratelyreflect the overall soil P availability (Ballard & Pritchett, 1975). The replenishmentof solution P involves the P quantity factor as well,which becomesincreasinglycritical as more P is withdrawn by plants. At comparablesolution P concentrations,soils with higher P buffering capacitiesor clay contentsshouldcontain more labile P to replenishthe solution P removedby root absorption(Hedley et aI., 1982; Jungk, 1987). Thus, the critical solution P concentrationor the critical quantity of water-solubleP required for optimum plant growth is lower for fine-textured soils than for coarse-textured soils (Kamprath& Watson,1980). Unlike water extraction, the dilute CaCl2 (0.01 M) extraction does not require a high-speedcentrifugationto obtain a clear extract (Schofield, 1955). The ratios of the weight of soil to the volume of CaCl2 solution as well as the lengths of time that have been used for the extraction vary considerably.The ratios range from 1:1.25 (Soltanpouret aI., 1974) to 1:10 (White & Beckett, 1964); and the reaction times range from less than 1 h (Baker & Hall, 1967; Aslyng, 1964) to 7 d (Dalal & Hallsworth, 1977). The amount of CaClz-extractableP correlatedwell with the eqilibrium P concentration(Moody et aI., 1983), NaHCOF or NH4HC03-DTPA-extractable P (Labhsetwar& Soltanpour,1985). At the same level of CaClz-extractableP, more P is availablein soils with high clay contents(Olsenet aI., 1983). Thus, the CaCl2-s01ubleP level critical for optimum plant growth is lower for silty clay loam soil than for very fine sandyloam (Fox & Kamprath,1970). The phosphatepotential(1/2 pCa + pH2P04") in 0.01 M CaCl2 extractswas consideredto be a viable parameterfor describingP availability or P solubility in soils (Aslyng, 1964; Schofield, 1955), as the potential is dependenton the P buffering capacityof soil (Barrow, 1967). However, for the phosphatepotential to adequatelydescribeP uptakeby plants,the solution activity of Ca2+ shouldbe maintainedconstantor at least should vary much less than that of phosphate

PHOSPHORUS

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(White & Beckett,1964;Wild, 1964;Olsen& Khasawneh,1980).In this manner, the potentialvaries in proportionto the P activity. Extractionwith Dilute Concentrationof StrongAcids

Reagents

+ 0.0125M H2S04): Add 14 mL of concentratedH2S04 (18 M) and 83.3 mL of concentratedHCI (12 M) to 18 L of deionizedH20 and dilute to 20 L.

1. Extracting solution (0.05 M HCI

Procedure.Add 5.0 g of air-dried soil «2 mm) and about200 mg of charcoal (Darco G60, J.T. Baker, Phillipburg, NJ) to a 50-mL Erlenmeyerflask. Add 20 mL of extractingsolution and shakethe suspensionfor 5 min. Filter the suspensionthrougha membranefilter «0.45mm) or Whatmanno. 42 filter paper(if Whatmanno. 42 filter paper is used and the filter is not clear, pour the filtrate back through the samefilter paper).Transferan aliquot containing1 to 20 I!g of P to a 25-mL volumetric flask and dilute to volume. Determinethe P concentration by the "Ascorbic Acid Method." PreparestandardP solutions and a blank which containthe samevolume of extractingsolution. Calculate the amount of P extracted using Eq. [12] in "Procedure"for "Extraction with Water or Dilute Salt Solution." Comments.The dilute acid extractionprocedureof Nelson et ai. (1953), also known as the North Carolina or Mehlich-l P test (Kamprath & Watson, 1980),extractslarge amountsof nonlabileP in soils that have pHgreaterthan 6.0 (Holford, 1980). It is unreliable for calcareousor alkaline soils (Thomas & Peaslee,1973), soils that have recently received rock phosphateapplications (Yost et aI., 1982),or soils with high cationexchangecapacity(CEC) or high base saturation(Thomas& Peaslee,1973). Thesetypes of soils tend to neutralizethe acid, thereby reducing the capability of the dilute acid to extract P. Clay or hydrousaluminum and iron oxides also can reducethe quantity of P extractable by the acids(Nelsonet aI., 1953; Lins & Cox, 1989; Lins et aI., 1985). The dilute acid, or Mehlich-l, soil test generally correlateswell with the Bray-1 test (Welch et aI., 1957; Shumanet aI., 1988),but not aswell with Olsen's NaHC03 test (Shumanet aI., 1988) amongdiversesoils. However,for the same soil type, they are all highly correlated(Welch et aI., 1957). The amount of P extractedor the recoveryof a known amountof addedP by the Mehlich-1 test is generallygreaterthan that by Olsentest, particularly for soils that have received rock phosphateapplicationsrecently(Menon et aI., 1988). It is slightly less than that by the dilute acid fluoride test (Bray-I) (Ballard & Pritchett, 1975; McLean et aI., 1982; Menon et aI., 1988).The Mehlich-l test has the advantageof simultmeouslyextractingCa and Mg as well (Nelson et aI., 1953). A P level of 20 to 30 mg P kg-1 soil for the Melich-l test is generallyconsideredin the high category(Thomas& Peaslee,1973),althoughadjustmentsfor different soil types are necessary.Kamprathand Watson(1980) indicatedthat a Mehlich-l P level of 20 to 25 mg P kg-1 soil is consideredadequatefor plant

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growth in sandy soils but only 10 ppm P is required for fine-textured soils. Increasingclay contentgenerallydecreasesthe critical P level requiredfor optimum plant growth (Lins & Cox, 1989). Extractionwith Dilute Acid Fluoride Principles. For acid soils, F"" promotesP desorption by decreasingAI activity through the formation of AI and F complexes.Fluoride also is effective in suppressingthe readsorptionof solubilized P by soil colloids. However, the Bray-1 test (Bray & Kurtz, 1945) performsunsatisfactorilyin highly calcareous soils due to the neutralizationof the acid by calcium carbonate(CaC03) and formation of CaF2, that reactswith dissolvedP to form secondaryprecipitates.

Reagents 1. Ammonium fluoride, 1 M: Dissolve 37 g of NH4F in deionizedwater and dilute to 1 L. Storethe solution in a polypropylenebottle. 2. Hydrochloric acid, 0.5 M: Dilute 20.8 mL of concentratedHCI (12 M) to 500 mL with deionizedwater. 3. Extracting solution: Add 15 mL of 1.0 M NH4F and 25 mL of 0.5 M HCI to 460 mL of deionizedwater to obtain a solution containing0.03 M NH4F and 0.025M HCI. Procedure.Add 1.0 g of air-dried soil «2 mm) to a flat-bottomedglass vial or bottle. Add 7 mL of the extractingsolution. Shakethe suspensionvigorously for 1 min and filter through a membranefilter (0.45 flm) or Whatmanno. 42 filter paper(if Whatmanno. 42 filter paperis usedand the filtrate is not clear, pour the filtrate back through the samefilter). Transferan aliquot containing1 to 20 flg of P to a 25-mL volumetric flask and determinethe P concentrationby the "Ascorbic Acid Method" or by the "Modified Ascorbic Acid Method" if sufficient amountsof labile Po are present.PrepareP standardsolutionsand a blank which include the samevolume of extractingsolution. Calculate the amount of P extracted using Eq. [12] in "Procedure"for "Extraction with Water or Dilute Salt Solution." Comments.The soil/extracting solution ratios and shaking times vary amongusers(Jackson,1958; Mackay et aI., 1984; Chien et aI., 1980; Olsen & Sommers,1982). For soil with a high CEC or a CaC03 equivalentthat exceeds 7% base saturation,the Bray-1 test is not suitable (Thomas & Peaslee,1973; Baker & Hall, 1967; Nesseet aI., 1988) due to neutralizationof the acids, unless the ratio of extractantto soil is increasedconsiderably(Randall & Grava, 1971). Maintaininga pH level 0.4 mm and 2 mL of aliquot is taken, place the flask in an oven at 100°C, heat the sampleto dryness,cool the flask, and add2 mL of water), add 4 mL reducingmixture, and proceedin reduction and distillation of the SOi"-S to HzS and estimationas MB. For commentsconcerning theMB determination,see"Comments"under"Total Sulfur." Ion ChromatographicMethod For instruments,reagents,and procedureusedin suppressed-type IC systems,seeTabatabaiand Frankenberger(1996, Chapter8).

Comments The chief advantageof the MB method describedis the rapidity with which the colorimetric determinationmay be completedand the small sample size (1-2 mL) requiredfor determinationof the extractedsulfate.The procedure can detectas little as 2 Ilg of S. If an aliquot of 20 mL is takenfor analysisand reducedin sizebeforeanalysis,the 2 mg of S is equivalentto O.lllg of S per milliliter. Work by Pirela and Tabatabai(1987) showed that sulfate also can be reducedto H2S by reagentcontaining Sn and H3P04. Similar to the reducing mixture used in the 10hnson-Nishita(1952) method, the Sn-H3P04 reagent

942

TABATABAI

reducesa variety of organic S and reducedS compounds(Pirela & Tabatabai, 1987, 1988). Freney(1958) suggestedthat to obtain a measureof the true water-soluble sulfate contentof a soil by using the reduction methodof Johnsonand Nishita (1952), it is necessaryto preparean extract free from colloidal material. Other inorganicS compoundsthat would be estimatedas sulfatemustbe removedfrom the solution. This can be achievedby precipitating the sulfate as BaS04 and removing all soluble compoundsby use of the Pregl filter stick. The reagents described,however,seemnot to extract significant amountsof organic S reducible by the reducing mixture, becausestudiesby Dick and Tabatabai(1979) showedthat the resultsobtainedby the MB methodfor inorganicsulfate in soil extractsobtainedwith severalreagents,including the reagentsdescribedin "Ion ChromatographicMethod", are similar to thoseobtainedby the IC method. The amountsof inorganicsulfate extractedfrom neutral and slightly alkaline soils by the reagentdescribedare almost identical (Tabatabai& Bremner, 1972a; Ajwa & Tabatabai,1993). When the soil pH is 1650°C)can be developedwith the properaddi-

CARBON AND ORGANIC MAITER

973

tions of Sn and Fe, but the temperaturemaximum is held only briefly. The temperaturerises steadily until the susceptorsmelt and fuse with the sample, and thereafter, it falls rapidly. Occasionally this temperaturerise and fall occurs before thermal decompositionof C is complete,perhapsbecauseof inadequate contactbetweenthe sampleand the susceptormaterial. For most soils, this does not appearto be a major problemsinceFe, Sn, and tin-coatedcopperaccelerators (-1 g of each/sample)havebeenfound to yield accuratetotal C valuesin a range of calcareousandnoncalcareous soils and standardcarbonateminerals(Tabatabai & Bremner,1970). If the organic matter contentof the soil is high, the sampleweight should be reducedappropriately.Organic materialscan be analyzedby this technique, but sampleweightsmustbe reducedto 20 or 30 mg if explosionsare to be avoided. Alternatively the organic material in amountsup to 60 mg can be mixed and coveredwith Alundum or Sinderiteas describedin "Procedure"under"MediumTemperatureResistanceFurnaceMethod." The gravimetricdeterminationof CO2 following combustionwith the Leco induction furnace was found by Carr (1973) to yield total C levels comparable with manualwet and dry combustionmethods.In addition to gravimetry,an automatedCO2 analyzerbasedon thermal conductivity measurements of the effluent gaseswas applicableto soil analysis(Tabatabai& Bremner,1970). Alternatively, a titrimetric method was developedto allow estimationof both total C and 14C in soil samplesamendedwith 14C compounds(Cheng& Farrow, 1976). A bypassvalve and a 12S-mL gas washing bottle (e.g., Corning 31760) are used in place of the CO2 absorption bulb of Fig. 34-1. All CO2 releasedby combustionis trappedin SO mL ofO.S M NaOH followed by removal of one aliquot for liquid scintillation counting to quantify 14C2 and a second aliquot for titration with standardHCI to determinetotal C. The total C data obtained were comparableto those obtained by a wet combustionprocedure. Recentdata also indicatesthat titrimetric and thermal condl'ctivity methodsare comparablefor the determinationof CO2 (Winter et aI., 1990).

InstrumentalMethods The following sectiondescribesrepresentativecommercialinstrumentsfor determining total C in soils. They were chosen to illustrate the principles involved in instrumentingtotal C analysis.The inclusion of the following three instrumentsdoes not imply that they are superioror inferior to otherscurrently beingmarketed.As with all instruments,variousevaluationproceduresshouldbe usedto determineand to confirm the validity of data obtainedin comparisonto accepted,standardmethods.Tabatabaiand Bremner(1991) describedautomated instrumentsavailablefor determiningtotal C in soils aswell aswhich instruments are capableof simultaneousdeterminationof N or S.

Carbo-ErbaNA 1500.Carlo-ErbaInstruments(Milan, Italy) developedan automatedinstrumentcapableof simultaneousdeterminationof C, H, and N in geologic materials, soils, and other environmental samples. The principles involved, developmentof the instrumentand a descriptionof modesof operation are presentedby Pella(1990a,b).A sampleis placedin a tin samplecup, crimped

NELSON & SOMMERS

974

Helium (continuous flow) Sample in Sn container

Thermal Conductivity Sample Detector

Reference

Cu NiO

Chro atographic

colum

Co,Oj Ag

J

Fig. 34--2. Schematicdiagramfor Carlo-ErbaModel NA 1500analyzer.

to confineit, andintroducedinto a quartzreactor.For mineralsoils, a typical sample size is 5- to 10 mg, necessitatingthe use of samplesfinely ground in a ball mill or similar apparatus.The quartz reactoris maintainedat 1050°Cwith a constant flow of He. Flashcombustionwill occurif a pulseof O2 is injectedinto the quartz reactorshortly after introduction of the sample.Under thesetemperature and O2 conditions, the tin is oxidized to SnOz resulting in the temperature increasingto 1700 to 1800°Cand the completecombustionof soil organicmatter. The combustionproducts(C02, N oxides,and HzO) are swept by the helium carrier gas through chromium dioxide (Cr02) to catalyzeoxidation of organic fragmentsand C030 4 coatedwith Ag to removehalogensand sulfur oxides.The gasesthen flow through a heatedCu (650°C) column to removeexcessoxygen, Mg(CI04)z to removeH20 and into a chromatographiccolumn for separationof N2 and COz. The different gasesare detectedwith a thermal conductivity detector. A generalizedflow diagramfor this instrumentis shownin Fig. 34-2. Scheperset al. (1989) evaluatedthe NA 1500 coupledwith a massspectrometerfor simultaneousdeterminationof C, N, and 15N in soil and plant materials. The NA 1500 yielded Nand 15N datacomparableto that obtainedwith conventional manual methods(Kjeldahl digestion followed by mass spectroscopy analysis).A detailedcomparisonof C data was not conductedalthoughrealistic values for total C in soils and plant materialswere obtained.An evaluationof samplepreparationmethods(soil grindinglWiley mill vs. ball milling) indicated that homogeneous soil and plant sampleswere essentialto reduceanalyticalvariability, especially in view of the typical sample size of 10 mg. Verardo et al. (1990) have describedproceduresfor using the NA 1500 to determineC and N in marine sediments.Due to the analysisof 5 to 10 mg, careful samplepreparation and grinding are neededon insure that a representativesampleis analyzed. A" with any analyticalmethod,the inclusion of appropriatestandardsand blanks is essentialtinsure valid total C data. The capability of coupling this instrument with a massspectrometeris a potentialbenefit as well.

CARBON AND ORGANIC MATfER

975

Leco Instruments.The LECO Corporation(St. Joseph,MI) has marketed instrumentsfor automatedanalysisof total C in soils and othersolid materialsfor the pastseveraldecades. A descriptionof Leco instrumentsis presentedby Tabatabaiand Bremner (1991). Earlier results with the Leco automatic 70-s C analyzer (Tabatabai& Bremner, 1970; Carr, 1973) indicate that reliable soil total C data are obtained using Fe, Sn, and tin-coatedcopperacceleratorsin an induction furnacefollowed by thermal conductivity to quantitateCO2, The newer LECO IR-12 instrument involves combustionof a soil samplein an induction furnaceusing an O2 atmospherefollowed by passingthe gas mixture over a catalystto convert CO to CO2 and CO2 quantitation with an infrared detector. A related instrument, model DC-12 Duo-Carb,involves combustionof samplesmixed with V 205 (vanadium pentoxide) in an induction furnace heatedto 1000°C under an O2 atmosphere. The COz producedis measuredwith a thermal conductivity detector. In 1993, LECO marketedtwo instrumentsfor determiningtotal C in soils. Both instrumentsutilize resistancefurnacesto combustsamplesat >950°C. In the Model CR-412, a sample containedin a ceramic boat is placed in a specially designedhorizontalresistancefurnacemaintainedat a constanttemperaturein the rangeof 950 to 1400°CunderOz flow. After a delay, Oz is directedonto the sample and carriesthe COz releasedthrough dust and water vapor traps and into an infrared detection system. Merry and Spouncer (1988)evaluated the earlier model CR-12 and found that it gave reasonablesoil organicC valueswhen operated at 1200°C. In an evaluationof combustiontemperatureon C recoveryfrom noncalcareousand calcareoussoils, it was found that both inorganic and organic C were recoveredbetween600°C and 1000°C.Total C determinedby the CR-12 and the Allison method(Allison, 1960; see"Wet CombustionMethod") were in close agreement for20 Iowa soils (Yeomans& Bremner,1991). Chichesterand Chaison(1992) evaluatedthe CR-12for determiningorganic C and inorganicC by combustingsamplesat 575°C and 1000°C,respectively. They recommendedcombustionat 575°C for 250 to 360 s to determineorganic C followed by combustionat 1000°Cfor 250 s to determineinorganicC. In addition, the time requiredfor analysisof inorganicC could be reducedfrom 250 to 60 s by increasingthe combustiontemperaturefrom 1000°Cto 1371°C.In summary, total C could bedeterminedby a single combustionat 1371°C as found by other investigators. A secondLECO instrument is the model CHN 600 which is capableof simultaneousanalysisof C, H, and N. A flow diagramfor the CHN 600 is shown in Fig. 34-3. The successorto the CHN 600 is the model CHN 1000. A soil sample «200mg) is placedin a tin capsuleand combustedin a resistancefurnace at 950°C using Oz as a carriergas.The gasesformed are scrubbedto removeS gases and equilibratedin a ballast chamber.Mter equilibration, the gas mixture flows through two infrared detectorsset to detectCOz and HzO. An aliquot of the gas mixture is analyzedfor N z by thermal conductivity after reduction of N oxides and removal of COz and HzO. The CHN 600 has beenused in severalsoils studies.Total C resultsfrom 20 Iowa soils were essentiallythe sameusing the CHN 600 and a standardwetcombustionmethod(Yeomans& Bremner,1991). A comparisonof dataobtained

NELSON & SOMMERS

976

950C

Dual Heat Zone Furnace

Exhaust

Fig. 34-3. Schematicdiagramfor LECO Model CHN 600 analyzer.

by the CHN 600 with a LECO induction furnaceinstrumentand a wet-oxidation method indicated that the CHN 600 was the most precise total C technique (0.014>.12%C) and enableda technicianto perform 90 to 100 analysesin an 8h d (Sheldrick, 1986). The CHN 600 hasbeenshown to recover 100% of the C in a range of pure organic C compounds[acetanilide(N-acetylaniline),sucrose (C12H 220 1), sulfanilic acid (C6H7N03S),and EDTA (ClOH16N20S)] and, as expected,to yield soil organic C values 16 to 59% greaterthan those obtainedby the Walkley-Black method (McGeehan& Naylor, 1988). In general, the CHN 600 has shown to be a reliable and accurateinstrumentfor the determinationof total C in soils.

Perkin-Elmer CHN2400. The Perkin-Elmer (Perkin-ElmerCorp., Instrument Division, Norwalk, CT) simultaneouslymeasuresC, H, and N using the principles employed in the traditional Pregl and Dumas procedures.A sample containedin a platinum boat is oxidized with O2 at -lOOO°C for 2 min in a combustion tube in the absenceof carrier gas (He) flow. Mter combustion,He flow is initiated and the CO2, H20, and N2 basesproducedby combustionare passed over CuO to convertCO to CO2 and silver mesh(silver vanadateon silver wool) to remove S and halogengases.The gasesthen flow into a tube maintainedat 650°C and packedwith coppergranules between end plugs of silver wool, where quantitative reductionof N oxidesto N2 occurs.The gasesare broughtto constant pressureand volume in a gas mixing chamberand then allowed to expandinto the analyzerportion of the instrument.The analyzerconsistsof threethermalconductivity (TC) detectorsconnectedin series and separatedby two traps. The sequenceof TC detectorsand traps enablingquantificationof H, C, and N is as follows: 1. TC detector1 (output equalstotal gas composition). 2. Magnesiumperchloratetrap to removeH 20. 3. TC detector2 (decreasein output from detector1 is proportionalto H content). 4. Sodaasbestosplus Mg(Cl04)2 trap to removeCO2.

CARBON AND ORGANIC MATTER

977

5. TC detector3 (decreasein output from Detector2 is proportionalto C content). 6. The remaininggasesin the sampleare N2• All operations withinthe instrumentare automatic.Additional work is neededto evaluatethis instrumentfor soil analysis. The above discussionis an overview of instrumentalmethodsfor total C analysisof soils. At present,the LECO instrumentshave beenthe most widely usedfor soil analysis.Severalresearchlaboratorieshavebegunusing Carlo-Erba instrumentsfor total C analysisof soils. Due to rapid changesin technologyand instrumentation,it is essentialthat manufacturersbe contactedfor currently available modelsfollowed by an evaluationof the instrument.

Total Carbonby Wet Combustion Introduction The wet combustionanalysisof soils by chromic acid digestion has long beena standardmethodfor determiningtotal C, giving resultsin good agreement with dry combustion.The main advantagesfor wet combustionare that the cost of apparatusis but a small fraction of the cost for dry combustionequipmentand that the parts neededto assemblethe apparatusare standardequipmentin most laboratories.The chief disadvantageof the earlier wet combustionprocedures (e.g., Heck, 1929) is that they use macro equipment,which is tediousto assemble and disassemble,and which occupiesconsiderablebenchspacemore or less permanently.Wet combustion also is used when the special manometricVan Slyke-Neil apparatus(Van Slyke & Folch, 1940; Bremner,1949) is employedto estimatetotal C in soils. The wet combustionmethodof Allison (1960), describedhere, embodies important refinementsfrom publishedprocedures,such as simple and effective digestionacid mixture (Clark & Ogg, 1942),a simple purification and absorption train assembledon a small panel (McCready& Hassid,1942), and a more rapid procedurethan formerly used(Heck, 1929; Jackson,1958, p. 211). The significant featuresof this apparatus(Fig. 34--4) are as follows: (i) it can be assembled from simple parts and requiresno ground-glassconnections,(ii) the small internal volume precludesthe necessityfor preaerationunder most laboratoryconditions, (iii) it requiresonly a short period of aerationfollowing digestion,and (iv) the entire assembly(F-K) occupiesonly a small area.This methodis satisfactory for salt-affectedsoils high in Cl- and also for the dry residuesof soil extracts rich in organicmatter.A rapid treatmentto removecarbonatesdescribedin "PretreatmentPrior to Wet Combustion"permits determinationof organic C on the residueof a pretreatedcalcareoussoil. The following descriptionof wet combustion methodologywas presentedby Allison et al. (1965).

Principles The soil sampleis digestedin a 60:40 mixture of H2S04 and H3P04 containing K2Cr207' The boiling temperatureof this mixture, 210°C, is high enough matter, yet low enoughto prevent to ensurecompleteoxidation of carbonaceous

NELSON & SOMMERS

978

To Trap

I or"

Trop I

~ Ga,-

MglCIO ) JFiber Qloss .2 Ascarite-

Fibtr Qloss

2t

T'OP

~I

Go.-· .

. "

mon Mods

+

NoOH

•

K Nesbitt bulb

250 ml Side. arm Erlenmeyer

c Fig. 34-4. Diagram of apparatususedto determineC by the wet combustionmethod.Trap I or II is usedfor determinationof CO2 evolved by gravimetricor titrimetric techniques,respectively(diagram is not drawn to scale).

excessivefuming in the condenser.The CO2 evolved is absorbedby a suitable absorbentandweighed,althoughit may be absorbedin a standardbaseand titrated. A combination of fuming H2S04, phosphoric acid (H3P04), iodic acid (HI03) [added to potassiumiodate (KI0 3)], and cr03 has been used for determining C in organic compounds(Van Slyke & Folch, 1940) and in soil (McCready & Hassid, 1942).The reportedadvantagesof this oxidation mixture are that it vigorously attacksand dehydratesresistantforms of C, thereby reducing boiling time for completeoxidation, and that it facilitates conversionof CO to CO2, Carbonmonoxideis often producedwhen readily oxidizablecarbohydrates are presentin the sample.Extensivecomparisonsof the Van Slyke-Folchand the 60:40 H2S04-H 2P04 oxidizing mixtureson many soils indicatethat the two mixturesare equallyeffective in convertingtotal soil C to CO2, The more rapid digestion with the Van Slyke-Folch mixture, resulting in a saving of 3 or 4 min per determination,is not sufficient advantageto offset the difficulties of preparing and maintaininga digestion acid that containsfuming H2S04, Moreover it was found that the needfor HI0 3 in the digestionmixture doesnot exist, which indicatesthat soil organicmattercontainslittle or no active carbohydratecapableof producingCO during digestion(Allison, 1960). Salt-affectedsoils frequently contain sufficient Cl- to give errors by wet combustionanalysiswhetherthe CO2 is determinedtitrimetrically (Clark & Ogg, 1942) or gravimetrically (Allison, 1960). When soil high in Cl- is heatedin a digestion mixture containingCr20-r-, chromyl chloride (CrOzCI2) is formed by the following reactionbeforeboiling begins

CARBON AND ORGANIC MATTER

979

The reddishCr02Cl2 decomposesat about 190°C, releasingfree C12, with a color changeto pale green.Any Cl2 and tracesof undecomposedCr02CI2 that passthe purification train are retainedin the CO2 absorptionbulb to give a positive error. In the methodsdescribed,Cl2 interferenceis preventedby including two traps in the purifying train, one containingKI and one containingsilver sulfate (Ag2S04) (TrapsF and G in Fig. 34-2). The use of Ag2S04 alone gives protection up to about0.2% Cl- (Allison, 1960), but its protectivevalue becomesquestionableat higher Cl- concentrations,SinceKI hasa very high capacityto absorb free Cl 2 by the reaction 2KI + Cl 2 =2KCl + 212,

[4]

the use of a KI trap is recommendedfor soils high in Cl-. With both traps in the system,Cl- up to 5% of the sampleweight does not interfere, provided proper precautionsare observedduring the early stagesof sampledigestion.Use of the Ag2S04 trap in conjunctionwith the KI trap servesto indicatewhen the latter is exhausted.For soils containing trace or low amountsof Cl-, the carrier stream may flow directly into the Ag2S04 trap. Wet CombustionMethod The wet combustionmethodwas describedby Allison (1960).

Special Apparatus The apparatusis shown in Fig. 34-4. Assemblethe apparatusfrom the following parts: (A) Hoke needlevalve: (B) 25-cm high soda-limetower; (C) 100mL Kjeldahl flasks to fit a no. 2 stopper;(D) Allihn four-bulb condenser,fitted with a no. 2 stopperat the delivery end; (E) 60-mL open-topseparatorfunnel; (F-H) 25- by 90-mm shell vials with no. 4 stoppers;(I and1) I5-cm long CaCl2 U-tube; and (K) Nesbitt absorptionbulb. Use neoprenestoppersand gum rubber tubing for all connections.Coat all rubbertube connectionslightly with silicone lubricant. Items C through E can be ground-glassjoint glasswareif desired (Fig. 34-4). All joints are standard-taper24/40. The following parts are needed:(C) 100-mL round-bottomflask (Coming 4320); (C-I) distilling adaptertube (Coming 9421), which containsinlet tube for bubbling C0z-freeair into digestionacid mixture; (D) Allihn condenser,-300-mmjacketlength(Coming2480); (E-1) distilling tube with suction sidearm (Coming 9420) (side arm is connectedto purifying traps); (E) graduatedseparatorfunnel (Coming 6382A). A heatingmantle and rheostatare usedto heat the 100-mL digestionflask. Providea C0z-freecarrier streamby releasingair from an air pressureline through Valve A and passingit through soda-limeTower B. ConnectB in a glass tube 4-mm o.d. that extendsdownwardthroughCondenserD and dips about1 cm below the surfaceof the oxidizing acid in DigestionFlask C. Shortenthe stemof FunnelE to a length of about 9 em, and reducethe tip openingof the stem to a diameterof about 2 mm. Adjust the position of the FunnelE to extendinto D at

NELSON & SOMMERS

980

least 5 cm below the stopperto avoid contactbetweenoxidizing acid and stopper. LubricateStopcockE with the digestionacid mixture or with syrupy H3P04• Regularstopcocklubricant should not be usedon stopcocks. Assemblethe purifying traps,F to J, on a panelto provide stability. Fit the vials of trapsF, G, and H with no. 4 stoppersthey have approximately6 cm of the bottom cut off to provide a tight sealwith the vials. Reducethe tip openings of the inflow tubesin F and G, but do not makethem smallerthan 1 mm in diameter, or sealingmay occur. Fill trapsF and G approximatelytwo-thirds full with 50% KI solution and saturatedAg2S04, respectively.Adjust the inflow tubesso that they extend into the solutionsnot more than 3.8 cm for Trap F and 1.3 cm for Trap G; otherwiseback pressuremay developand causeleaks in the system. Fill Trap H not more than one-thirdfull with concentratedH2S04, Prepare the inflow tube for H from the barrel of a 5-mL pipettewith the tip extendingnot more than 1.3 cm into the acid (note that Trap H connectsdirectly to Trap I). Placea fiberglassdisc in the bottom of the V-tube; and fill the right side, Trap [, with 30-mesh(600 !lm) granularZn for absorbingany acid fumes that escape pastH. Fill the left trap, TrapJ, with anhydrousMg(CI04)2, which absorbswater from the carrier streamcontainingevolvedCO2 beforeit entersK. Fill the Nesbitt absorptionbulb K with any good, self-indicatingabsorbent having a high capacityfor absorbingCO2, Indicarb and Mikhobite are excellent for this purpose.When filled as shown in Fig. 34--4, the bulb containssuccessively a 3-cm layer of 8- to 14-mesh(1.4-2.36mm) absorbent,a 2-cm layer of 14- to 20-mesh(0.85-1.4 mm) absorbent,and a l-cm overlayer of anhydrous Mg(CI04)2, with a wad of glasswool aboveand below the column.

Reagents 1. Digestion acid mixture: Pour 600 mL of concentratedH2S04 into 400 mL of 85% H3P04, cool the mixture, and store it in a glass-stoppered bottle. Keep the bottle well stopperedto prevent absorptionof water vapor. 2. Potassiumdichromate,reagentgrade. 3. Potassiumiodide solution, 50%: Dissolve 100 g of KI in 100 mL of water. 4. Silver sulfatesolution, saturated. 5. Carbondioxide absorbent,self-indicating,7- to 14- (1.4-2.8 mm) and 14- to 20-mesh(0.85-1.4mm) size; Suitablematerialsare Mikhobite (G. FrederickSmith ChemicalCo., Columbus,OH), Caroxiteor Indiearb(FisherScientific, Pittsburgh,PA), or Ascarite(Arthur H. Thomas Co., Philadelphia). 6. Sodalime, 8- to 14-meshsize (0.85-1.4mm). 7. GranularZn, Hg > Cu. Seleniumis not significantly more effective than Hg when catalysisof both clearingand conversion of organicN to NHt-N are considered,but both Seand Hg areconsiderablymore effective than Cu. Considerablecontroversyexists concerningreports that Se causesloss of N when usedas a catalystin Kjeldahl digestion,but a critical evaluation of the literatureon this subjectindicatesthat Se is a safeand effective catalyst when properly usedand that loss of N occursonly if Se is usedin excessive quantity, or with large amountsof K 2S04 or Na2S04,and digestionis fairly prolonged(Bradstreet,1965). Work by Bremner(1960) supportedtheseconclusions and showedthat no loss of N occurredduring Kjeldahl digestionof soils by the selenium-catalyzedmethod recommendedhere ("Regular Kjeldahl Method") evenwhen digestionis continuedfor as long as 12 h. It is difficult to evaluatethe extensiveliteratureon the effectivenessof different catalystsin Kjeldahl digestionof organic N compounds,but if effects on both rate of clearingand conversionof organicN to NHt -N are considered,there is considerableevidencethat Hg is the most effective single catalyst.However, the useof Hg hasbeendiscouragedby the practicaldifficulties encounteredin the determinationof NHt in digestscontainingHg. When a digest containingHg2+ is treatedwith alkali, someof the NHt in the digest reacts withthe HgO precipitated by the alkali to form a Hg-NHt complex(Clark, 1943),and the NHt in this complexis not readily liberatedby distillation with alkali. To avoid low resultsin the determinationof NHt, it is necessaryto add sodiumsulfide or thiosulfateto precipitateHg2+ as HgS, or zinc dust to reducethe HgO to metallic Hg. Numerous difficulties have beenexperiencedwith the techniquesusedto preventinterferenceby HgO in the determinationof NH.t. For example,it hasbeenfound that they can lead to the evolutionof H2S, to the appearance of metallic Hg in the distillate, to seriousbumping when the Hg is precipitatedas the sulfide, and to the formation of a black deposit in the condenserof the distillation apparatus. McKenzie and Wallace (1954) found that when sodium thiosulfatepentahydrate was usedto eliminateinterferenceby HgO, the successof the methodwas dependent on the ratio of thiosulfateto HgO. Somered HgO was precipitatedwhen the Na2S203• 5H20-HgO ratio was three, whereasblack HgS was always precipitatedwhen the ratio was five. Theseobservationshave beenconfirmed by other workers,and it has beenfurther observedthat if a considerableexcessof alkali is usedfor distillation after precipitation of Hg as HgS, the mixture turns yellow, and metallic Hg distills into the flask usedto collect the NH3 (Clark, 1943; Bremner, 1960). The precisereasonsfor someof the difficulties experiencedin distillation of NHt from digestscontainingHg are still obscure.In view of this and of the toxicity of Hg, use of mercury-catalyzedmethodsfor Kjeldahl digestion of soils is not recommended. Kjeldahl methodsin which digestion is performedusing 0.6 to 0.7 g of K 2SOJmL of H2S04 have distinct advantagesover methodsemploying lower concentrationsof K 2S04 in respectto both speedand completenessof digestion

1098

BREMNER

of organic N, and they appearto be free of some of the defects of methods employinghigherconcentrationsof K 2S04 (e.g.,frothing, solidification of digest, risk of loss of N). But experiencewith thesemethodshasbeentoo limited to permit an evaluationof their suitability for soil analysis,and their accuracywhen modified to include N03-N and N02'-N in Kjeldahl analysisof soils hasnot been studied.In contrast,the Kjeldahl methodrecommendedhere("RegularKjeldahl Method") hasbeenfound to give highly reproducibleresultswith a wide variety of soils andto give excellentrecoveriesof N03-N and N02'-N when modified to include theseforms of N as describedin "Permanganate-Reduced Iron Modification of Kjeldahl Methodto IncludeNitrate and Nitrite" and"Salicylic Acid-Thiosulfate Modification of Kjeldahl Method to Include Nitrate and Nitrite." Severalinvestigators(White & Long, 1951; Grunbaumet aI., 1952) have found that the useof sealedtubespermitsdigestionat temperatures above400°C without loss of N and allows completedigestionof heterocyclicN compounds without addition of the substancesusually employedto promote conversionof organicN to NUt -N in digestionwith H 2S04, The use of sealedtubesfor digestion also eliminateslossesof N through bumping and preventscontamination with NH3 from laboratory air. Stevenson(1960) found that these advantages madethe sealedtube techniquevaluablefor Kjeldahl analysisof rocks, silicate minerals, and subsurfacesoils containing small amountsof N. However, this techniqueis too complicatedand hazardousfor routine use in Kjeldahl analysis of soils. Honda(1962) describeda Kjeldahl methodof determiningtotal N in soils involving digestionof the soil samplewith a mixture of H2S04 and H3P04 containing K 2S04 and CUS04 • 5H20. This method appearsattractive for routine total N analysisof soils, becausethe time of clearing(2-7 min) andtime of digestion after clearing (10 min) are much shorter than the correspondingtimes in other methods.Work reportedby Skjemstadand Reeve(1976) indicatedthat a modification of Honda'smethodgavesatisfactoryresultsandcould be adapted to include N03. However,Nelsonand Sommers(1980) found that seriousbumping and spatteringoccurredduring digestionof soil samplesby Honda'smethodand that the clearingtime requiredin this methodwas much longer than 7 min. Early workers attemptedto use H20 2 to accelerateKjeldahl digestion,but found that use of this oxidant can lead to low results.There is evidence,however, that H20 2 can be usedsatisfactorily for Kjeldahl analysis.For example,the Kjel-FossAutomaticAnalyzer developedby FossElectric of Hillerud, Denmark, for rapid Kjeldahl analysisusesH20 2 to reducethe time requiredfor conversion of total N to NHt by the Kjeldahl digestiontechnique,and it has proved satisfactory for determinationof total N in plants,animals,feeds,and fertilizers (Wall et aI., 1975; Noel, 1976; Larson & Peterson,1979; Bjamo, 1980). Another Kjeldahl method involving the use of H20 2 that merits attentionis the method proposedby Hach et ai. (1985, 1987), which involves digestionwith H2S04 and H20 2 in the absenceof K 2S04 and catalyst.Watkins et ai. (1987) found that this methodwas satisfactoryfor routine determinationof total N in variousfeedstuffs andbiological products,andChristiansonandHolt (1989)found that it gavegood recovery(average,98.2%)of total N in six soils analyzed.For useofthis method,

NITROGEN-TOTAL

1099

it is necessaryto purchasethe DigesdahlDigestionApparatusmarketedby Hach Co., Loveland, CO. Sharmaand Sud (1980) describeda rapid method for total N analysisof soils and reportedthat the total N valuesobtainedby this methodwere usually significantly higher than thoseobtainedby conventionalKjeldahl procedures.In their method, the soil sample is digestedwith a mixture of chromium trioxide (Cr03) and concentratedH2S04 at 300 ± 10°C for 15 min, and the NHt produced by this digestionis determinedby estimationof the NH3 liberatedby distillation of the digestwith sodiumhydroxide.Flowersand Bremner(1991a)recentlyevaluated this method by comparing its results with 12 diverse surface soilswith those obtainedby the Kjeldahl methoddescribedin "Regular Kjeldahl Method Using Block Digester."They found that the chromic acid methodrecoveredonly 87.5 to 94.1% (average,90.5%) of the soil N recoveredby the Kjeldahl method and that this was at least partly due to oxidation of ammoniumto nitrate and/or nitrite by chromicacid and subsequent gaseousloss of theseoxidized forms of N. The most significant developmentsince 1965 in regardto Kjeldahl digestion has been the introduction of techniquesin which digestion is performedin Pyrex tubes placed in holes drilled in an aluminum block that is heatedby an electric hot plate or by internal electric heatersconnectedto a temperaturecontrol. Block digestershave significant advantagesover the customarydigestion stands,becausebesidespermitting better temperaturecontrol during digestion, they requirelessattentionand fume hood spaceand allow simultaneousdigestion of 40 to 100 samples.Nelson and Sommers(1972), Schumanet al. (1973), and Gallaher et al. (1976) were the first to describe the use of aluminum block digestersfor Kjeldahl digestion of soils. The digestion techniquesadoptedby theseworkers are outlined in Table 37-2. Aluminum block digesterswith interTable 37-2. Methodsproposedfor Kjeldahl digestionof soils using AI block digesters. Reference Details of method

Nelson & Sommers (1972)

Schumanet al. (1973)

Gallaheret al. (1976)

Pyrex digestion tube Soil sample,g Soil meshsize H2S04, mL K 2S04, g Catalyst

50 ml,t with funnel in neck 0.2 11) can lead to interferencefrom polyvalentcations,which form hydroxideprecipitates(Ananth & Moraghan,1987).

Nitrite. The colorimetricmethodsgenerallyemployedfor determinationof NO are basedon the reactionof NO with primary aromaticamines(diazotizing reagents)under acidic conditionsto producediazoniumsalts, which react (coupIe) with aromatic compoundscontaining suitable amino or hydroxyl groups (coupling reagents)to form coloredazo compoundsthat may be measuredspectrophotometrically(Boltz & Taras, 1978; Fox,1985). Thesemethodshave their origin in work by Griess(1879), who performedthe diazotizationreactionusing sulfanilic acid (C6H7N03S) in H2S04 solution and utilized 1-naphthylamine (ClOH9N) as the coupling reagent.Ilosvay (1889) modified the original method by carrying out the diazotizationand coupling reactionsin acetic acid solution, and this so-called Griess-Ilosvaymethod, or a modification thereof, has been used almost exclusively for determinationof NO in soils and other biological materials.In the versionof this methoddescribedby Bremner(1965) and Keeney and Nelson (1982) for analysisof soil extracts,sulfanilamideis usedinsteadof sulfanilic acid as the diazotizing reagent,andN-(l-naphthyl)-ethylenediamine is used insteadof 1-naphthylamineas the coupling reagent.These modifications have been found to increasethe rateof color development,the stability of the color, and the sensitivity of the method(Shinn, 1941). The Griess-Ilosvaymethodof determiningNOz is more sensitivethan the steam-distillationor microdiffusion methodsfor this determination,and even greatersensitivity has beenachievedif the azo chromophoreis concentratedby solventextraction(e.g., Baveja& Gupta, 1982; Chaubeet aI., 1984), or is measuredby fluorometry (e.g., Nakamura,1980; Rubio et aI., 1984),or is measured by fluorometry (e.g., Nakamura,1980; Rubio et aI., 1984). Thesetechniquesare up to 100 times more sensitivethan the Griess-Ilosvaymethod; however,they also are more complicated,and most have a narrow working rangeof NOz concentrations. Puttannaand PrakasaRao (1986) have describeda two-step modification of the Griess-Ilosvaymethodthat extendsthe working range.In the first step,the sample is treated with sulfanilic acid and N-(l-naphthyl)-ethylenediamineto form the azo chromophore.If the color intensity at 550 nm exceedsthat of the higheststandard(0.12 Ilg of NOz-N mL-l), the solution is treatedwith acetone (C3H60) andsodiumacetate(CzH3Na02),and the absorbanceis measuredat 480 nm. The latter procedurefollows Beer'slaw at NOz concentrationsup to 0.40 Ilg of N mL-l, which is equivalentto 100 Ilg of NOz-N g-l of soil when extractions are performedas describedin "Procedure"under "Extraction of Exchangeable Ammonium and Nitrate and Nitrite," using a 1:10 ratio of soil to extractant.

z

z

z

NITROGEN-INORGANIC FORMS

1151

Automation. Increasingly,automatedanalyzersystemsare being usedfor colorimetricdeterminationsof NHt, NOj", and NO in soil extracts.Most of these systemsutilize flow analysis,in that an aliquot of sampleis mixed with reagents in a flowing carrierstreamto developa color, the intensity of which is measured before dischargeto waste.This techniquewas originatedby Skeggs(1957), and subsequentlyled to the developmentby the TechniconInstrumentsCorporation of the AutoAnalyzer, which was first used for clinical analyses,but was subsequently adaptedfor other applications,including the analysisof inorganic forms of N in water and soil extracts(e.g., Kamphakeet aI., 1967; Henzell et aI., 1968; Henriksen& Selmer-Olsen,1970; Selmer-Olsen,1971; Brown, 1973; Jacksonet aI., 1975). An improved version, the AutoAnalyzer II, remains available, and similar systemsare now offered by manufacturersin severalcountries.All are modular in design,the basiccomponentsbeing a sampler,peristalticpump, mixing manifold, photometer,and recorderor other output device. Additional componentsare often employed,such as a heating bath for color developmentor a dialyzer to eliminate interferencefrom turbidity, and many of the newer models are equippedwith a microprocessoror microcomputer,which is utilized to operate the systemand acquiredata. There are two commonforms of flow analysis.In one form, referredto as continuous-flowor segmented-flowanalysis,the analytical streamis segmented by the regular introductionof air bubblesto limit sampledispersion,promotethe mixing of sampleand reagents,and scrubthe walls of the mixing manifold. In the otherform, referredto as flow-injection analysis,the sampleis introducedinto an unsegmentedcarrierstreamfrom an injection valve, forming a zone that disperses becauseof laminar flow in the mixing manifold. The merits of continuousflow and flow-injection analysishave beenthe subjectof much discussion,and further information can be found in a numberof publications,including thoseof Snyder (1980), Horvai and Pungor (1987), Ruzicka and Hansen (1988), and Smith and Scott (1991). It suffices here to note that either techniquemay be employedfor analysisof NHt, NOj", andNO z in soil extractsand that, with some systems,all three analysescan be carried out simultaneously,at a rate of more than 100 samplesper hour. Severalmethodshavebeenemployedfor automatedanalysisof NH! in soil extracts.Most of thesemethodsare basedon the Berthelotreaction,using either phenol(Henzell et aI., 1968; Selmer-Olsen,1971; Brown, 1973; Rice et aI., 1984; Markus et aI., 1985; Tel & Heseltine, 1990) or salicylate (Rowland, 1983; Adamsenet aI., 1985; Botha & Johnson,1988; Gentry & Willis, 1988), although the Nessler reactionalso hasbeenemployed(Krug et aI., 1979). To preventprecipitation of cadmiumand magnesiumhydroxidesat the high pH requiredfor the Berthelot reaction, color developmentis often carried out in the presenceof a chelatingagentsuch as a tartrateor citrate buffer (Henzell et aI., 1968; SelmerOlsen, 1971; Rowland, 1983; Markus et aI., 1985; Gentry and Willis, 1988) or EDTA (Markus et aI., 1985). Interferencealso can arise from the presenceof amino acids (White & Gosz, 1981; Rowland, 1983; Adamsenet aI., 1985; Burton et aI., 1989), someof which form a colored complex or liberate NHt under the conditions employedto carry out the Berthelot reaction.This type of inter-

z

1152

MULVANEY

ferencehas beenfound to causeseriouserror in NHt analysisof extractsfrom forest soils with a high contentof organicmatter(White & Gosz,1981),andserious error also hasbeenobservedwith soils that were air- or oven dried or fumigated(Burton et aI., 1989). White and Gosz(1981) found that the error observed in their work was practically eliminatedby reducingthe concentrationof NaOH in the alkaline phenol reagentand by decreasingthe temperatureof the heating bath, but subsequentstudiesby Rowland (1983) suggestthesemodificationsto be ineffective. Steamdistillation hasbeenproposedas a meansof avoiding interferenceby amino acidsin automated analyses of soil extractsfor NHt (Burton et aI., 1989). Automatedproceduresinvolving distillation have beendescribedfor NHt analysiswith the TechniconAutoAnalyzer(Keay & Menage,1969; Skjemstad& Reeve,1978). The Griess-Ilosvaymethodhas beenuniversally employedfor automated analysesof NOi" in soil extracts(e.g., Henzell et aI., 1968; Markus et aI., 1985), and the samemethod has been used to determineNO)" plus NOi", from which NO)" is obtainedby difference,following reductionof NO)" to NOi" by hydrazine and CUS04• 5HzO (Henzellet aI., 1968; Best, 1976; Markus et aI., 1985),nitrate reductasefrom a bacterialculture (Lowe & Gillespie, 1975; Rice et aI., 1984), copperizedcadmium (Henriksen& Selmer-Olsen,1970; Adamsenet aI., 1985; Botha & Johnson,1988; Tel & Heseltine,1990), or cadmium-silverwire (Willis & Gentry, 1987).The useof cadmiumis generallypreferred,as hydrazinereduction requires a high pH, which effects precipitation of Ca and Mg (Ananth & Moraghan,1987), and also is subject to interferenceby soluble organic matter (Rowlandet aI., 1984),whereascell-culturetechniquesmustbe employedto produce nitrate reductase,and the reduction is incompletewith 2 M KCl extracts (Rice et aI., 1984). Methods Ammonium Principles. In the proceduredescribed,NHt extractedfrom soil with 2 M KCI is determinedby measuringthe intensity of the emeraldgreen color that forms upon treatmentof an aliquot of the extractwith salicylateand hypochlorite at high pH. A catalyst(sodium nitroprusside)increasesthe rate and intensity of color development,and a chelatingagent(EDTA) preventsthe precipitationof divalent and trivalent cationsas hydroxides. Figure 38--3 shows a three-stepmechanism forcolor developmentby the methoddescribed.In the first step,NH3 reactswith hypochloriteto form monochloramine (NH 2CI). The monochloraminethen reactswith salicylate to form benzoquinonemonoimine, which coupleswith salicylate to give the colored indophenoldye. For maximum sensitivity and accuracyin NHt analysesby the method described,absorbancemeasurements are madeat 667 nm. When measurements are performedat this wavelengthusing a light path of 1 cm, the methodobeys Beer'slaw up to at least20 flg of NHt-N in 25 mL of solution (the volume recommendedfor color development).Under theseconditions,the limit of detection is approximately0.2 flg of NHt-N. With a 5-cm light path, the detectionlimit is about0.04 flg of NHt-N.

NITROGEN-INORGANIC FORMS

1153

(A)

NH,CI + OW (I)

(8)

00.

+

NH,CI

COO· (II)

(C)

00. HNQO - ·ONQO +

COO·

(II)

COO·

·OOC

COO·

(III)

Fig. 38-3. Reactionsin methodfor colorimetricdeterminationof NHt: (A) monochloramine(I) for· mation; (B) oxidative imination of salicylate(II); (C) coupling reactionto form indophenoldye (III).

Method 3 SPECIAL APPARATUS

1. Spectrophotometer, equippedto provide a l-cm light path and capable of absorbancemeasurements at 667 nm. REGENTS

1. Sodium salicylate-sodiumnitroprussidereagent.Dissolve 7.813 g of sodium salicylate (NaC7Hs03) and 0.125 g of sodium nitroprusside [disodium pentacyanonitrosylferrate, Na2Fe(CN)sNO• 5H20] in approximately 80 mL of deionizedwater in a 100-mL volumetric flask, and bring the solution to volume with deionizedwater. Mix thoroughly, and then transferthe solution to an amberbottle for protectionfrom light. Store in a refrigerator. 2. Buffered hypochlorite reagent.Dissolve 2.96 g of sodium hydroxide (NaOH), and then 9.96 g of sodium monohydrogenphosphateheptahydrate(Na2HP04• 7H20), in approximately60 mL of deionizedwater in a lOO-mL volumetric flask. Add 10 mL of sodium hypochlorite (NaOCl) solution(Clorox bleachor equivalent).Adjust the pH to 13.0 with NaOH, and dilute to 100 mL with deionizedwater. 3. Ethylenediaminetetraacetic acid reagent.Dissolve 6 g of the disodium acid (Na2EDTA) in 100 mL of deionsalt of ethylenediaminetetraacetic ized water in a volumetric flask. 4. Standardammoniumsolution. Dissolve0.4717g of ammoniumsulfate [(NH4hS04] in deionizedwater (a primary-standardreagentis available from FisherScientific), and dilute the solution to a volume of 1 L in a volumetric flask. If pure, dry (N~hS04 is used,the solution contains 100 Ilg of NJIt-N mL- 1. Store the solution in a refrigerator. To preparea working standardthat contains2 Ilg of NHt -N mL-1, dilute 4 mL of the concentratedsolution to 200 mL in a volumetric flask with deionizedwater.

3 After

Nelson(1983).

1154

MULVANEY

PROCEDURE.Pipettean aliquot (nonnallyS;3 mL, not to exceed5 mL) ofthe soil extractinto a 25-mL volumetricflask. Add 1 mL of EDTA reagent,and swirl the flask to mix the contents.Then add 4 mL of salicylate-nitroprusside reagent, swirl the flask, and bring the volume to approximately20 mL with deionized water. Add 2 mL of bufferedhypochloritereagent,immediatelybring the volume to 25 mL with deionizedwater, and thoroughly mix the contentsof the flask. Placethe flask in a waterbath at 37°C to developthe color. After 30 min, remove the flask from the bath, allow the solution to cool to room temperature(approx. 10 min), and measureits absorbanceat 667 nm againsta reagentblank solution. Calibrateabsorbancemeasurements by analysisof standardscontaining0, 2, 4, 6, 10, and 20 Ilg of NHt -N. To preparethesestandards,pipette into six 25mL volumetricflasks the samevolume of 2M KCI (the extractingsolution)asthe aliquot of soil extract taken for analysis,and add 0, 1, 2, 3, 5, or 10 mL of the working standard(NH4)2S04 solution (2 Ilg of NHS-N mL- 1). Then carry out color development,and measurethe absorbanceby the proceduredescribedfor analysisof the extract. CALCULATIONS. Detenninethe NHS concentrationof the sampleusing the equationobtainedby linear regressionof the concentrationsof the standardson the correspondingabsorbancemeasurements. Alternatively, make this detennination by referenceto a calibrationcurve preparedfrom analysesof standards. COMMENTS. The method describedis essentially a modification of the indophenolblue methoddescribedby Keeneyand Nelson (1982), the major difference being that sodium salicylate is used to carry out color development insteadof phenol. Sodium salicylateis neither toxic nor caustic,and the reagent is suppliedin the fonn of a powderthat is easily weighedand transferred. The addition of EDTA is necessaryto preventinterferencefrom precipitation of polyvalent cations, particularly Ca2+ and Mg2+. According to Nelson (1983), the EDTA treatmentemployedin the methoddescribed(1 mL of a solution containing60 mg of Na2EDTA) will complexat least 8 mg of Ca2+ or 5 mg of Mg2+, which greatly exceedsthe nonnal content of theseions in 5 mL (the maximum volume recommendedfor analysis)of a 2 M KCI soil extract(10 mL of KCI g-l of soil). If a precipitatedevelopsupon addition of the bufferedhypochlorite reagent,the analysisshould be perfonnedusing a smalleraliquot of soil extract, or 2 mL of the EDTA reagentmay be addedto double the capacityfor complexation. A pH of 13 is required for maximal color developmentin the method described(Nelson,1983).A lower pH will lead to a loss of sensitivity. For example, Nelson(1983) found that absorbancewas reducedby up to 41% when color developmentwas carried out at pH 12 insteadof pH 13. The methoddescribed may be employedfor analysisof soil extractsobtainedusing acidic extractants, but the extractshouldfirst be neutralized(by addition of NaOH) to ensureproper color development. The buffered hypochlorite reagent is unstable and should be prepared immediatelybeforeuse.The salicylate-nitroprusside reagentis stablein the dark, but should be preparedweekly to ensuremaximal sensitivity. For proper color

NITROGEN-INORGANIC FORMS

1155

development,the salicylate-nitroprussidereagent must be added before the bufferedhypochloritereagent. As described,color developmentis carried out at 37°C for 30 min, which maximizes molar absorptivity (Nelson, 1983). Alternatively, the color can be developedat room temperature(23°C); however, maximal color development requires2 h, and molar absorptivity is somewhatlower than when the color is developedat 37°C (Nelson, 1983). The color is stablefor at least 4 h (Nelson, 1983). Like othermethodsbasedon the Berthelotreaction,analysesby the method describedare subjectto interferenceby organic forms of N, particularly amino acids. Caution is advisedin the use of this methodfor analysisof extractsthat may contain appreciableamountsof organic N, such as those from fumigated soils or forest floor litter samples.In suchcases,analysesmay be performedby the steam-distillationmethodsdescribedin "Methods" under"Steam-Distillation Methods," or by the microdiffusion methods described in "Methods" under "Microdiffusion Methods."

Nitrate Principles. In the proceduredescribed,N03" extractedfrom soil with 2 M KCI is reducedto NO by passage through a columnof copperizedcadmium,and the NO formed is determinedby a modified Griess-Ilosvaymethod.Quantitative reductionof N03" to NO is accomplishedby carryingout the reactionwith copperizedcadmiumin an NH4Cl matrix, at a pH of 5 to 10. Nitrite is estimatedcolorimetrically after treatmentof the column leachatewith a diazotizing reagent [sulfanilamide(CJISN20 2S)] and a couplingreagent[N-(1-naphthyl)-ethylenediamine] in HCl solution to form an azo chromophore(see"Nitrite"). The intensity of the reddishpurplecolor that develops isproportionalto the concentrationof N03" in the soil extract,or to the concentrationof N03" plus NO if NO is present. For maximum sensitivity and accuracy, absorbancemeasurementsare madeat a wavelengthof 540 nm, although,if necessary,any wavelengthmay be employedbetween510 and 550 nm. When the measurements are madeusing a light path of 1 em, the methodobeysBeer'slaw up to at least20 ~g of N03"-N in 100 mL of solution(the volume recommendedfor analysis).The excellentsensitivity of the methodallows low levels of N03" to be measured,and therebypermits dilution of soil extractsto minimize any interferencethat might ariseduring color development.

z

z

z

z

z

Method' SPECIAL APPARATUS

1. Nitrate reductiontube. A Pyrex tube (1.2-cm o.d., 30 cm long) is fitted with a fritted glassplate and stopcockat one end to serveas an outlet, and with a 5-cm length of 5.1-cm o.d. tubing at the other end to serve as a reservoirfor 75 mL of solution. The outlet end of the stopcockis 4 After

Keeneyand Nelson(1982), and Dorich and Nelson(1984).

MULVANEY

1156

connectedvia vinyl tubing to a Pyrex tube (4-mm o.d.) insertedthrough a two-hole size 00 rubberstopper.A secondtube (4-mm o.d.) through the stopperconnectsto a flow regulatorand a vacuumsource. 2. Spectrophotometer, equippedto provide a 1-cm light path and capable of absorbancemeasurements at 540 nm. REAGENTS

1. Copperizedcadmiumreagent.PlaceSO g of cadmiummetal (coarsely powderedor granular,0.2S-to 1-mm diam., :::;2 mm long; a satisfactory material is availableas cat. no. C-3 from FisherScientific) in a SOOmL beaker,and treat it with 2S0 mL of 6 M hydrochloric acid (HCI). After 1 min, decantthe HCI, and thoroughly wash the cadmiummetal with deionizedwater. Then treat the metal with 2S0 mL of a cupric sulfate pentahydrate(CUS04 • SH20) solution (20 g of CUS04 • SH20 L- 1). Allow the mixture to stand for at least 1 min; then decantthe CUS04 • SH20 solution, and wash the metal with deionizedwater. Retreat with 2S0 mL of CUS04 • SH20, decant the solution, and thoroughly wash the copperizedcadmium with deionized water until all traces of blue and light gray color have disappearedfrom the wash water. Transferthe metal to a NO)" reductioncolumn as describedin the previous"SpecialApparatus"section. 2. Ammoniumchloride (NH4CI) solution,concentrated.Dissolve100 g of reagent-gradeNH4CI in SOO mL of deionizedwater. Storein a glassor plastic bottle. 3. Ammonium chloride solution, diluted. Dilute 50 mL of concentrated NH4CI solution to 2 L in a volumetric flask with deionizedwater. Mix thoroughly,and transferto a glassor plastic bottle for storage. 4. Diazotizing reagent.Dissolve 0.5 g sulfanilamidein 100 mL of 2.4 M HCI in a volumetric flask. Storethe solution in a refrigerator. 5. Coupling reagent. Dissolve 0.3 g of N-(l-naphthyl)-ethylenediamine dihydrochloridein 100 mL of 0.12 M HCI in a volumetricflask. Transfer the solution to an amberbottle for storagein a refrigerator. 6. Standard nitrate solution. Dissolve 0.3609 g of potassium nitrate (KN03) in deionized water (a primary-standardreagent is available from FisherScientific), and dilute to 1 L in a volumetric flask. If pure, dry KN03 is used,this solution contains50 j..lg of NO)"-N mL-l. Store the solution in a refrigerator.To preparea working standardthat contains 2 j..lg of NO)"-N mL-l, dilute 20 mL of the concentratedsolution to 500 mL in a volumetric flask with 2 M KCI. PROCEDURE PREPARATION OF CADMIUM COLUMN. With the outlet stopcockclosed,fill the NO)" reduction tube with dilute NH4CI solution (Reagent3) to the baseof the upper reservoir.Pour in sufficient copperizedcadmiumparticlesto give a depth of 20 cm, tapping the tube as the cadmiumparticlesare addedto ensurethat all air bubblesare removed.Drain the excessNH4CI solutionthroughthe outlet stopcock, and then wash the cadmium column thoroughly (10 pore-volumes)with

NITROGEN-INORGANIC FORMS

1157

dilute NI4CI solution,usinga flow rateof approximately8 mL min-to Leavesufficient solution in the tube that the cadmiumcolumn is coveredto a depth of at least 1 cm. Immediatelybeforecarrying out NOj" analyses,add 1 mL of concentrated NI4CI solution (Reagent2) to the cadmium column,and usethe outlet stopcock to lower the level of liquid until it is evenwith the top of the column. Then add 75 mL of dilute NI4CI solution (Reagent3) to the upper reservoirof the NOj" reductiontube, attach a 100-mL volumetric flask to the outlet stopcock-...ia the rubberstopper,and by regulatingthe supplyof vacuumand/orthe position of the stopcock,allow the NH4CI to flow throughthe column at a rateof 110 mL min-to ANALYSIS OF EXTRACT. Drain excessNI4CI solution from the cadmiumcolumn until the liquid level is evenwith the top of the column.Pipette1 mL of concentratedNI4CI solution (Reagent2) onto the top of the column, and then an aliquot of soil extract(normally 2-5 mL) containingnot more than 20 ~g of NOj" -N. Attach a lOO-mL volumetric flask to collect the outlet flow from the column, and open the stopcock to drain the treated extract into the cadmium column. Immediatelywash the inside of the reduction tube with 2 mL of dilute NH4CI solution, and add75 mL of dilute NH4CI solution to the upper reservoiron the reductiontube. Use the stopcockand/or the supply of vacuumto adjust the flow rate to 110 mL min-to When the level of liquid reachesthe top of the cadmium column, close the outlet stopcock,and disconnectthe flask used to collect the flow from the column.Add 2 mL of the diazotizingreagentto the contentsof the flask, and mix thoroughly. Allow to stand5 min; then add 2 mL of the coupling reagent,bring to volumewith deionizedwater,and mix thoroughly.After 20 min, measurethe absorbanceof the solution at 540 nm againsta reagentblank solution. Calibrateabsorbancemeasurements by analysisof standardscontaining0, 2,4,6,10,and 20 ~g of N03"-N. To perform theseanalyses,add 0,1,2,3,5,and lO mL of the working standardN03" solution (2 ~g of N03"-N mL- 1) to the cadmium column, and carry out reduction and color developmentas describedfor the soil extract. The method describeddeterminesany NOz initially present in the soil extract,plus that formed by reductionof N03". If necessary,the amountof N03" in the extractcan be determinedby treating the aliquot under analysiswith sulfamie acid to decomposeNO as describedin "Methods" under"Steam-Distillation Methods"or "Methods"under"Mierodiffusion Methods,"or by carryingout a separatedeterminationfor NO by the Griess-Ilosvayproceduredescribedin the "Nitrite" sectionunder"Colorimetric Methods."

z

z

CALCUlATIONS. Determinethe N03" concentrationof the sampleusing the equationobtainedby linear regressionof the concentrationsof the standardson the correspondingabsorbancemeasurements.Alternatively, make this determination by referenceto a calibrationcurve prepared fromanalysesof standards. COMMENTS. The reductionof N03" to NOzis the most likely sourceof difficulty in the methoddescribed,and care is requiredin the preparationand operation of the copperizedcadmiumcolumn employedfor this reduction.The cad-

MULVANEY

1158

mium metal usedshouldbe of the size specified,as the efficiency of reduction dependsupon particle size, and a different flow rate may be requiredif the cadmium particlesare largeror smallerthan specified.To ensurequantitativereduction of NO)", copperizationof the cadmiummetal mustbe complete.The copperized metal must be thoroughlywashed,as excessCu causesNO)" to be reduced to an oxidation state lower than NOz (Nydahl, 1976). The cadmium column shouldbe packedto a depthof 20 cm; Dorich and Nelson(1984)obtainedincomplete reductionof NO)" with a 10- or IS-em column,and incompleterecoveryof NOzwith a 30-cm column. Air bubblesin the column seriously interfere with NO)" reduction,so careshouldbe takento ensurethat all air bubblesareremoved during packing of the column, and that a layer of liquid is always maintained abovethe cadmiummetal to excludeair. If thereis any failure to maintainsucha layer, the columnshouldbe repacked.The flow ratemustbe regulatedat 110 mL rnin-1 during operationof the column.Dorich andNelson(1984)obtainedincomplete recoveryof NO)" and NO when elution was carriedout at a rate of 7 to 10 mL min-I, asrecommendedin severalothermanualmethodsfor NO)" analysisby cadmium reduction (e.g., Huffman & Barbarick, 1981; Am. Public Health Assoc., 1992), which was attributedto reductionof NO Excessiveflow rates will lead to incompletereductionof NO)". A cadmiumcolumn has a finite capacityfor reductionof NO)", and with use,a declinewill eventuallyoccur in the efficiency of reduction(normally after severalhundredsamples),as indicatedby a decreasein the absorbancevalues measuredduring calibration.In suchcases,columnefficiency may be checkedby comparing analysesfor NO)" and NO using samplesthat contain the same amountof N (e.g., 10 llg). A difference in absorbancevalues indicatesincomplete reductionof NO)", and althoughanalyseswill still be possibleif the amount of NOzformed is proportionalto the amountof NO)" in the sample,the cadmium metal should be recopperized(as describedin the previous"Reagents"section) to restoresensitivity. Soil extractsobtainedwith 2 M KCI are sometimescolored,but no difficulty has been encounteredin analysis of colored extracts by the method described.Becauseof the excellentsensitivityof the method,soil extractsmay be diluted with deionizedwater beforeanalysis,and by this technique,evenhighly coloredextractsmay be analyzedwithout difficulty. Although pH is not critical to the reduction of NO)" by the method described,extractsobtainedusing acidic or alkaline extractantsshould be neutralized (by addition of NaOH or HCI) before analysis,to ensureproper color developmentby the Griess-Ilosvaymethod.The solutionpH after additionof the diazotizingreagentshouldbe about1.6, andthe pH after additionof the coupling reagentshouldbe about1.5 (Bremner,1965). The methoddescribedis free from interferenceby any commonconstituent of soil extracts,but the efficiency of N03" reductionis reducedby high concentrationsof sulfide (Wood et aI., 1967)or phosphate(Olson, 1980).Sulfide reacts with Cd to form CdS,whereasphosphateis adsorbedby the cadmiumcolumn. The standardNO)" solution is stablefor at least6 mo if storedin a refrigerator. Owing to the excellentsensitivity of the method for NO)" and NO the

z

z.

z,

z,

NITROGEN-INORGANIC FORMS

(A)

RONH Z + NO; + 2H+ - - - - - - R O N. N + 2H zO (I)

(8)

1159

RON.N+

8

R'

(II)

Fig. 38-4. Diazotizationandcouplingreactionsin methodfor colorimetricdeterminationof N0Z-: (A) diazotizationreaction-(I)sulfanilamide(R =-S02 • NH2); (B) coupling reaction-(II)N-(l-naphthyI)-ethylenediamine(R' =-NH • CH2CH2 • NH2).

deionizedwater usedto preparethis solution shouldbe as free as possiblefrom theseforms of N. Controlsmust be included in eachseriesof analysesto allow for the color producedby the reagentsemployed(including the reagentsusedin extractionof the soil sample).

Nitrite Principles. In the proceduredescribed,NO z in a 2 M KCI soil extract is determinedby a modification of the Griess-Ilosvaymethod. An aliquot of the extract is treatedwith a diazotizing reagent(sulfanilamidein HCI solution) to convert the NO to a diazonium salt, and then with a coupling reagent[N-(1naphthyl)-ethylenediamine]to convert the diazoniumsalt to an azo compound. The concentrationof NO is proportional to the intensity of the reddish purple color that developsas a resultof thesetreatments.The diazotizationand coupling reactionsin this methodof analysisare illustratedby Fig. 38-4. The sensitivity and accuracyof the methodusedfor colorimetric determination of NO are greatest whenabsorbancemeasurements are madeat 540 nm (the wavelengthof maximal molar absorptivity), and the method obeys Beer's law up to at least61lg of NOz-N in 50 mL of solution (the volume recommended for analysis)when measurementsare performedat this wavelengthusing a light pathof 1 cm. A spectrophotometer is employedin the methoddescribed,but a colorimetercan be usedif equippedwith a greenfilter having maximal transmittance in the range of 520 to 550 nm. A 5-cm light path provides a fivefold increasein sensitivity, with a correspondingdecreasein dynamicrange.

z

z

z

MULVANEY

1160

Method5 SPECIAL APPARATUS

1. Spectrophotometer, equippedto provide a 1-cm light path and capable of absorbancemeasurements at 540 nm. REAGENTS

1. Diazotizing reagent.Dissolve 0.5 g of sulfanilamidein 100 mL of 2.4 M HCI. Storethe solution in a refrigerator. 2. Coupling reagent. Dissolve 0.3 g of N-(l-naphthyl)-ethylenediamine dihydrochloride in 100 mL of 0.12 M HCI. Store the solution in an amberbottle in a refrigerator. 3. Standardnitrite solution. Dissolve 0.2463g of sodiumnitrite (NaN02) in deionizedwater, and dilute the solution to 1 L in a volumetric flask. If pure, dry NaN02 is used, this solution contains 50 J..lg of NOz-N mL-1. Storethe solutionin a refrigerator.To preparea working standard that contains1 J..lg of NOz-N mL-l, dilute 20 mL of the concentrated solution to 1 L in a volumetric flask with deionizedwater. PROCEDURE.Pipette an aliquot (usually 2 mL) of the extract into a 50-mL volumetricflask, and adddeionizedwaterto makethe total volume about45 mL. Add 1 mL of the diazotizing reagent(Reagent1), and mix the solution. After 5 min, add 1 mL of the coupling reagent(Reagent2). Mix the solution, and allow it to standfor 20 min. Then make the solution to volume, mix it thoroughly,and measureits color intensity at 540 nm againsta reagentblank solution. Calibrateabsorbancemeasurements by analysisof standardscontaining0, 1,2,3,4,5,and 6 Ilg of NOz-N. Pipette0-, 1-,2-,3-,4-,5-,and 6-mL aliquots of the working standardNO solution into 50-mL volumetricflasks, and measure the absorbances obtainedby the proceduredescribedfor analysisof the extract.

z

CALCUlATIONS. Determinethe NOz concentrationof the sampleusing the equationobtainedby linear regressionof the concentrationsof the standardson the correspondingabsorbancemeasurements. Alternatively, make this determination by referenceto a calibrationcurve preparedfrom analysesof standards.

z

COMMENTS. Although otherextractantshavebeenemployedfor analysisof

NO by the Griess-Ilosvaymethod(Bremner,1965), no difficulty is encountered

in the analysisof extractsobtainedusing2 M KCI, and this reagenthasthe advantageof extractingexchangeableNHt in addition to NO}" and NO which allows all threedeterminationsto be madeby a single extraction.Extractsobtainedwith 2 M KCl are sometimescolored,but this doesnot normally interferewith analyses by the method describedfor determinationof NOz, becausethe extract is diluted extensivelywith water before analysis.If necessary,a correctioncan be made for color interferenceusing the techniqueof Montgomery and Dymock (1961). With this technique, an additional control analysis is performed that involves treatmentof the samplewith the diazotizing reagentas in the normal

z,

5

After Barnesand Folkard (1951), Bremner(1965), and Keeneyand Nelson (1982).

NITROGEN-INORGANIC FORMS

1161

procedure,but the treatment with N-(l-naphthyl)-ethylenediamineis omitted. The optical density of this sampleis then measured against distilled (or deionized) water, and the value obtainedis subtractedfrom the absorbancedetermined for the sampleby the normal procedure. Little information is available concerningthe stability of NO in extracts obtainedby the proceduredescribedin "Extraction of ExchangeableAmmonium and Nitrate and Nitrite" or by other methodsthat have beenproposedfor extraction of soils for colorimetric determinationof NO Therefore,extractsshouldbe analyzedimmediatelyafter their preparationwheneverpossible;otherwise,they should be storedin a refrigeratoror freezer. The accuracyof the resultsobtainedby modified Griess-Ilosvaymethods of determiningNO dependson proper adjustmentof pH for the diazotization and coupling reactions(Rider & Mellon, 1946; Barnes& Folkard, 1951). Therefore, the sampletaken for analysisshould not be strongly buffered, and samples that are acidic or alkaline should be neutralized(by addition of NaOH or HCI) beforeanalysis.In the methoddescribed,the pH after addition of the diazotizing reagentshould be about 1.6, and the pH after addition of the coupling reagent should be 1.5 (Barnes& Folkard, 1951). The rate of color developmentin the methoddescribedis very rapid, full color developmentbeing achievedin 10 min at 25°C. The color formedis stable for severalhours. Barnesand Folkard (1951) observedno fading of the colors obtainedby this methodor by other modificationsof the Griess-Ilosvayprocedure using lightproof vessels,and concludedthat the fading sometimes observed using thesemethodsis probably due to exposureof the azo compoundto light. Therefore, samples prepared for colorimetric measurementby the method describedshouldbe storedin the dark if they cannotbe analyzedsoonafter developmentof the color. The Griess-Ilosvaymethodof determiningNO is highly sensitiveand specific, and it is not subjectto interferenceby high concentrationsof variouscations that havebeenfound to interand anions(Rider & Mellon, 1946).The substances fere with the method(Boltz & Taras,1978) are unlikely to occur in soil extracts and will not be discussedhere. However,sincecoppersaltshave beenemployed to preparesoil extractsfor colorimetricdeterminationof NO.3 and NO and since copperand mercurysaltsare sometimesusedto preventmicrobial activity in soil extracts,attentionmay be drawn to the fact that both Cu2+ and Hg2+ have been found to interfere with the determinationof NO by Griess-Ilosvaymethods. Mercury causeshigh results,and Cu2+ catalyzesthe decompositionof the diazonium salt and causeslow results.Interferenceby high concentrationsof NHt also hasbeenreported. The coupling reagentemployedin the methoddescribedhas the disadvantagethat it becomesdiscoloredand tendsto precipitateevenwhen it is kept in the dark. This increasesthe reagentblank and decreasesthe sensitivity and precision of the method.However,the reagentis usablefor at least2 mo if it is storedin a refrigerator. Experiencehas shown that the standardNO solution is stablefor at least 6 mo if storedin a refrigerator,and that there is no needto add CHCl3 or to take other precautionsgenerallyrecommendedfor preparationand storageof standard

z

z.

z

z

z,

z

z

MULVANEY

1162

N02" solutions.The N02" standardcanbe calibratedby titration with KMn04 (Jeffery et al., 1989),or by the MgO-Devarda'salloy steam-distillationor microdiffusion methodsdescribedin "Methods"under"Steam-DistillationMethods"and "Methods" under"Microdiffusion Methods." Controlsmust be includedin eachseriesof analysesto allow for the color producedby the reagentsemployed(including the reagentsusedfor extractionof the soil sample). Ion-SelectiveElectrodes Introduction Upon first consideration,the useof electrodesto determineNH4", NO)", and N02" in soil extractswould appearto offer many advantagesover other methods for thesedeterminations.The proceduresinvolved are extremelysimpleandconvenient,requiring only an electrodeand a suitablepH meter.However,the electrodesare expensive,and the membraneor sensormodule has a limited operationallifetime (a few daysto severalmonths).Moreover,the electrodesfor measuring NO)" lack sensitivity, requirecontinualrestandardization,and are subject to numerousinterferences.Information regardingthe theory and designof ionselectiveelectrodescan be found in publicationsby Carlsonand Keeney(1971), Covington(1974), Yu (1985), andTalibudeen(1991). Ammonium. An NH4 electrodeutilizing a cation-sensitiveliquid membraneis commerciallyavailable(Unicam Ltd., Cambridge,UK), but it is subject to seriousinterferenceby K+, which precludesanalyseswhen KCI or K 2S04 are

usedto extract exchangeable NH4" from soil. A much more practical alternative is the NH3 gas-sensingelectrodewith an internal referenceelectrode,which is availablefrom severalcommercialsources(Table 38-5). With the latter type of electrode,the solution underanalysisis madealkaline (PH > 11) to convertNH4 to NH3, and the activity of NH3 is determinedby measuringthe pH of an interTable 38-5. Commonlyavailableelectrodesfor measuringNH4, NOj", and NOi. Type of electrode

Concentration range Interferences

Manufacturert

Model no.

Coming Orion Unicam

476130 95-12 80003

mgNL-l Ammonium Gas-sensing(NH3)

0.Q1-14oo Volatile amines,Hg2+

Nitrate Liquid membrane:/: 0.14-14()()(} C10", 1-, CIOj", CN-, NOi, HS-, Coming Br, HCOj", CI-, COr, pot Orion Unicam Nitrite CO2, HCOj", col-, volatile acids Orion Gas-sensing(NO..) 0.07-140

476134 93-07 80113 95-46

t Addresses:Coming,Inc., Coming,NY 14831;Orion Research,Inc., The SchrafftCenter,529 Main

Street,Boston,MA 02129; Unicam Ltd., York Street,Cambridge,UK CBl 2PX. :/: Requiresexternalreferenceelectrode.

NITROGEN-INORGANIC FORMS

1163

nal filling solution into which NH3 diffusesthrough a semipermeablemembrane. The pH measurementmay be madeusing any meterwith a resolutionof 0.1 mY. Several studies have indicated that the NH3 electrodeis satisfactoryfor determinationof NH.t in soil extracts(Banwartet aI., 1972; Hofman et aI., 1980; du Preezet aI., 1989). The most comprehensiveevaluationhasbeenthat of Banwart et al. (1972). No difficulty was encounteredin analysisof extractsprepared using 2 M KCI, and the resultsfor samplescontaining0.05 to 3 mg of NH.t-N per liter agreed closely with those obtained by the steam-distillation method describedin "Ammonium-Nitrogen"under "Proceduresin Absenceof Nitrite." Interferencetestsshowedthat analysesby the NH3 electrodewere unaffectedby any commonconstituentof soil extracts,including inorganiccationsand anions, amino acids, amides,hexosamines,purines, and pyrimidines. However, serious interferenceoccurredwith volatile amines(methylamineand ethylamine),which the electrodedetectsas NH3, and with Hg2+, which forms a complex with NH3 under alkaline conditions.According to the manufacturer(Orion Research,Inc., 1989), the latter interferencemay be eliminated by treating the sample with iodide to complex Hg2+. A commercially available automatedinorganic N analyzer (Model 380; Alltech Associates,Inc., Deerfield,IL) utilizes many of the sameprinciplesasthe NH3 electrode.In this instrument,the sampleis treatedwith alkali to convertNH.t to NH3, which diffuses through a semipermeablemembraneinto a stream of water and is determinedfrom the resulting increasein electrical conductivity (Carlson, 1978). The diffusion-conductivity method has been successfully employedfor determinationof NH.t in Kjeldahl digests(Carlson,1978) and natural waters(Scott et aI., 1989),and for determinationof NH.t and N03" in soil and plant tissueextracts(Carlsonet aI., 1990). Nitrate_ Electrodeswith selectivity for N03" are available from manufacturersin the USA and UK (Table 38-5). Theseelectrodesutilize a NO) -selective

ion exchanger,either a long-chainalkyl NH.t salt (e.g., Coming, Inc.) or a metal salt of orthophenanthroline (e.g.,Orion Research,Inc.). The ion exchangeris dissolved in an organic solvent and impregnatedin a thermo-settingplastic membrane, which separatesthe solution under analysis from an internal reference solution. Whenthe electrodeis immersedin a solutioncontainingN03" , the N03" binds to the ion exchangerand is transportedacrossthe membrane,such that an electricalpotentialdevelopsbetweenthe two sidesof the membrane.The magnitude of this potential dependson the differencein N03" contentbetweensample and referencesolutions,and is measuredrelative to a constantpotentialgenerated with an externalreferenceelectrode. The N03" electrodemeasuresthe level of free N03" in solution, or N03" ion activity (aN03"), which is relatedto molal concentration(mN03") by

aN03" = (yN03")(mN03"),

[1]

whereyN03" is the activity coefficient.The measuredpotential,E, is relatedto the activity by a modification of the Nernstequation,

E = E' - Slog (aN03" + 'LKjaj),

[2]

MULVANEY

1164

whereE' is a constant(determinedby the particularNO)" and referenceelectrodes employed);S is the electrodeslope (at 25°C, 59.16 mV per decadechangein ionic activity); K j is the selectivity coefficientfor interfering anion, i, with activity aj; and L denotessummationof Kjaj for all interfering ions. According to Orion Research,Inc. (1990a), a Nemstianresponseis obtainedwith NO)" concentrationsas low as 1.4 mg of N L-l. Measurementsmay be madeat somewhat lower concentrations(the detectionlimit is specifiedas 0.14 mg of NO)"-N L-l) (fable 38--6), but calibrationwill be nonlinear. In the absenceof interference,aNO)" closely approximatesmNO)" at concentrationsup to about 150 mg L -1 (~34 mg of NO)" -N L -1) (Table 38--6). At higher concentrations,the ionic strengthincreasessufficiently that aNO)" underestimatesmNO)", and with concentratedsalt solutions(e.g., 2 M KCI), the effect is so severethat the electrodebecomesunresponsiveto NO)" (Myers & Paul, 1968). Therefore,such solutionscannotbe employedas extractantsfor analyses with the NO)" electrode,althougha dilute salt solution may sometimesbe desirable, especiallywith soils having significant anion-exchangecapacity (Black & Waring, 1978), or to ensurethat all analysesare carried out at the same ionic strength.A variety of solutions have been employed for the latter purpose, including 0.025 M Al z(S04h (Baker & Smith, 1969); 0.01 M CUS04 (0ien & Selmer-Olsen,1969; Bartuzi et aI., 1976; Hadjidemetriou,1982); a mixture of Al Z(S04)3(0.01 M), AgzS04 (0.01 M), H3B03 (0.02 M), and sulfamic acid (0.02 M) (Milham et aI., 1970); 0.1 M sodiumcitrate(C6HsNa307)(Raveh,1973); 0.01 M or saturatedCaS04(Bound, 1977; Pedrazziniet aI., 1979; Li & Smith, 1984); 0.04 M KAl(S04)Z (Hofman et aI., 1980); and 0.12 or 0.04 M (NH 4)zS04 (Li & Smith, 1984; Orion Research,Inc., 1990a). Severalanionshavesufficiently high selectivity coefficients(Kj in Eq. [2]) that they interfere at very low concentrations.The most serious interference occurswith CI04", 1-, CIO)", and CN-, but theseions are seldompresentin soil extractsor otherbiological samples.The major interferingions in soil extractsare N02", HCO)", Cl-, and C01-, although PO]- (and also HPOi- and HzPO,n can interfere if the concentrationis exceptionally high (Table 38-7). Organicanions also can interfere, and such interferencehas been observedin analysisof plant extracts(Paul & Carlson,1968) and naturalwaters(Csiky et aI., 1985). A correctioncannotnormally bemadefor anion interference,sinceevenif anionconcentrationsare known, ionic strengthmay vary amongsamples,and this Table 38--6. Concentrationand activity of NO) in KN0 3 solutionsat 25°C (Langmuir & Jacobson, 1970). Concentration

Activity

Concentration

Activity

200 300 400 500 750 1000

188 278 367 454 668 876

mgL-l 1 5 10 25 50 100 150

1.00 4.95 9.86 24.4 48.4 95.5 142

NITROGEN·INORGANIC FORMS

1165

will affect ion activities. Therefore,interfering anions must be removed before analysis.ExtractantscontainingAg2S04 may be used to reduceCl- interference (Milham et aI., 1970; Hulanicki et aI., 1974; Orion Research,Inc., 1990a).Paul and Carlson (1968) suggestedthe use of a silver-treated ion-exchangeresin (Dowex 50-X8), but this techniquewas found to be unsatisfactoryin subsequent work by Bakerand Smith (1969). Dilute H2S04 may be usedto removeCOj- and HCOj" (Orion Research,Inc., 1990a). Nitrite interferencecan be eliminated by decomposingNOi with sulfamicacid (Mahendrappa,1969; Milham et aI., 1970; Orion Research, Inc., 1990a), by complexing NOi with sulfanilamide (C6HsN302S)(Francis& Malone, 1975), or by oxidizing NOi to NOj with acidified KMn04 (Morie et aI., 1972). The latter approachis more complicated,but permitsdeterminationof both NOj and NOi. Interferenceby organicanionsmay be reducedthrough acidification to suppressionization (Baker & Smith, 1969; Orion Research,Inc., 1990a).Csiky et ai. (1985) have describeda simple procedure using disposableadsorptioncolumnsto removehumic substancesfrom natural watersbefore analysiswith the NOj electrode. The nitrate electrodeis relatively unaffectedby pH. The operationalrange is from pH 2 to 12 (Davieset aI., 1972; Hulanicki et aI., 1974); however,a much narrower range is required for analysesat low NOj concentrations«2 mg of NOj-N L- 1) (Potterton& Shults, 1970; Davies et aI., 1972), and, in somecases, measurementshave been carried out with pH buffered at 2.2 or 3.0 (Baker & Smith, 1969; Milham et aI., 1970; Hulanicki et aI., 1974). This practicedoesnot appear necessaryfor analysis of soils or soil extracts (Bremner et aI., 1968; Raveh, 1973), but may be adviseablewith plant tissue extractsto reduceinterferenceby organicacids (Baker & Smith, 1969). The referenceelectrodemust be chosenwith care, as it can be a major source of error and variability (Potterton & Shults, 1967; Carlson & Keeney, 1971; Dahnke, 1971). The referenceelectrodeshould give a stable and reproducible output that is unaffectedby stirring, and there should be no contamination of the solution under analysisby interfering anionsfrom the internal filling solution. Theserequirementscannotbe met with most of the referenceelectrodes usedto measurepH. A variety of referenceelectrodeshave been employedwith nitrate electrodes, including silver/silver chloride electrodeswith a single (Bremneret aI., Table 38-7. Concentrationsof anionscommonly presentin soil extractsrequiredto causea +10% error in measurement of NO:) with the Orion Model 93-07electrode(Orion Research,Inc., 1990a). NO:) concentration(mg L-I)t 10

Anion

100

mgL-I NO

z

RCO:) Cl-

COlpot

S01-

t One milligram of NO:) L-I

2 44 76 86 339 6857

=0.22 mg of NO:)-N L-I.

23 440 760 860 3390 68570

230 4400 7600 8600 33700 685700

1166

MULVANEY

1968;Keeneyet al., 1970;Sommerfeldtet aI., 1971;Simeonovet al., 1977; Black & Waring, 1978; du Preezet aI., 1989)or double (Goodman,1976; Pedrazziniet aI., 1979; Hadjidemetriou,1982;Li & Smith, 1984)sleevejunction, and calomel electrodeswith a fiber (Baker & Smith, 1969; Mahendrappa,1969; Onken & Sunderman,1970; Francis& Malone, 1975)or ceramic(Morie et aI., 1972)junction. Double-junctionelectrodesarenow generallypreferred,as the useof singlejunction electrodescan lead to significantcontaminationby Cl- from the internal filling solution (Potterton& Shults, 1967; Keeney et aI., 1970; Dahnke, 1971), whereascalomel electrodesare affected by the rate of stirring and may cause readingsto drift (Sommerfeldtet aI., 1971).To avoid contaminationby Cl- from the double-junctionelectrode,the outerchamberis fIlled with a dilutesolutionof (NH4)zS04 instead of KCl (Hadjidemetriou, 1982; Li & Smith, 1984; Orion Research,Inc., 1990a).Bound and Fleet (1977) havedescribeda solid-statereferenceelectrodethat usesno filling solution, but the potential is affectedby pH and ionic strength. Numerousprocedureshavebeendescribedto determineNO)" in soils with the NO)" electrode.Most of theseproceduresinvolve an extraction withwater or a dilute salt solution (Bremneret al., 1968; Mahendrappa,1969; 0ien & SelmerOlsen, 1969; Milham et aI., 1970; Onken & Sunderman,1970; Raveh, 1973; Black & Waring, 1978; Knittell & Fischbeck,1979; Pedrazziniet aI., 1979; Hofman et aI., 1980; Li & Smith, 1984), but analyseshave often been performed without extraction, using a well-mixed soil suspension(Bremner et aI., 1968; Simeonovet aI., 1977;Hadjidemetriou,1982;du Preezet aI., 1989)or a soil paste (Bound, 1977). Bremneret a1. (1968) obtainedexcellent agreementwhen NO)" analyseswere performed on stirred soil suspensionsand on the filtrates from thesesuspensions,whereasMack and Sanderson(1971) found that the presence of dispersedcolloidal material in water extractsof soils led to erroneouslyhigh measurements with a nitrate electrode.The latter finding can likely be attributed to the fact that a saturatedcalomelelectrodewas employedas the referenceelectrode. Bound (1977) reportedsimilar difficulties when a calomel electrodewas used with soil pastes,but the problem was eliminated by using a silver/silver chloride referenceelectrodewith a nonporousliquid junction (Bound & Fleet, 1977). The responsetime for the nitrate electroderangesfrom a few secondsat high concentrationsof NO)" to severalminutesat low concentrations(Potterton& Shults, 1967; Davieset aI., 1972). A period of 1 to 2 min is generallyemployed (Dahnke,1971),althougha longerperiod may be requiredwhen analysesareperformed on samplesof widely differing NO)" content(Potterton& Shults,1967). The nitrate electrodeis subject to drift (Knittel & Fischbeck,1979) and shouldbe standardizedat regularintervals(normally after every 10-20samples). Standardizationcan be accomplishedby constructinga plot of the measured potential(E) againstthe logarithmof NO)" concentrationfor a seriesof standards, and if the meteremployedis equippedwith a logarithmic scalefor direct readout of concentration,calibrationmay be carriedout usingonly two standardsolutions that differ by tenfold in concentrationand encompassthe rangeof concentrations to be encounteredwith samples(Potterton& Shults, 1967;Dahnke,1971). Static electricity cancauseseriousinstability in readingswhen relative humidity is low,

NITROGEN·INORGANIC FORMS

1167

in which casethe electrodesshouldbe coveredwith a shieldof aluminumfoil and groundedto the meter(Onken & Sunderman,1970; Sommerfeldtet aI., 1971). Severaltechniqueshave beenproposedto increasethe speedand conve· nienceof analyseswith the nitrate electrode.Milham (1970) hasdescribeda sim· pIe flow-through cell for manualanalyses.Flow-injection techniquesutilizing the nitrate electrode have been employed for automatedanalysis of soil extracts (Hansenet aI., 1977; Ruzicka et aI., 1977; Schalschaet aI., 1981). Goodman (1976) has describedan apparatusthat automaticallyextractsup to 60 soil samples, and then carries out an analysisof each extract with the nitrate electrode. Besidesbeing rapid and convenient,thesetechniquestend to give better accura· cy and precisionthan can otherwisebe attainedwith the nitrate electrode,since operationalvariablesare standardized. A compactmeterfor measuringNO)" hasbeendevelopedby Horiba Instru· mentsCo., Kyoto, Japan.This unit, known as the Cardy nitrate meter, utilizes a miniature version of the nitrate electrodein a replaceablesensormodule. The entire assembly,consistingof the sensormodule and a digital meterwith readout in partsper million (mg of NO)" -N L- 1), is so compactthat it easily fits in a shirt pocket. Analysesare readily performedon soil extracts,and the extractionsare simplified considerablyby using a speciallydesigned plastic filter bag developed by SpectrumTechnologies,Inc., Plainfield, IL. The Cardy meter has been recommendedfor on-farm monitoring of soil and crop N status(Hartz et aI., 1993), but no evaluationof analytical performancehas yet beenpublished. A simple type of ion-selectiveelectrodecan be madeby immersingcopper wire in an electroactive reagentcontaininga liquid ion exchanger.The result is commonly referred to as a coated-wireelectrode(Cattrall & Hamilton, 1984). Lee et ai. (1986) have describeda coated-wireelectrodefor NO)" that utilizes a graphite-epoxy paste, and their design has subsequentlybeen evaluated by Goodroadand Shuman(1990) for analysis of soil extracts. The results of this evaluationindicate that performanceis adequatefor applicationsnot requiring a high degreeof accuracy,and that the analysesare subject to the same interferencesas a conventionalnitrate electrode.The low cost of coated-wireelectrodes makesthem disposable,and this can be a major advantage,particularly when the membranehasbeencontaminatedby exposureto a high concentrationof an interfering ion (Goodroad& Shuman,1990). Siegel (1980) has describeda methodfor measuringNO)" in soil extracts that utilizes the ammoniaelectrode.In this method,NO)" is determinedas the differencein the concentrationof NH3-N obtainedfor duplicate aliquots of extract made alkaline with NaOH, one of which also is treatedwith Devarda'salloy to convert NO)" to NH 3. Sensitivity and precision are about the same as can be obtainedwith the nitrate electrode,but most interferencesare eliminated, and analysescan be performedon KCl extracts.The major limitation appearsto be the needfor careful control of temperature,which affectsthe rate of reductionof NO)", the rate of NH3 loss from solution, and the measuredNH3 activity. Nitrite. Nitrite in aqueoussolution can be determinedwith a gas-sensing electrodefor nitrogen oxides(NO x ) availablefrom Orion Research,Inc., Boston, MA (Table38-5). With this electrode,the solution underanalysisis acidified (pH 1.1-1.7)to convertNO to NOx species(NO, N02, N20 3, N20 4), which diffuse

z

MULVANEY

1168

througha gas-permeable membraneinto an internalfilling solutionand are determined by measuringthe pH of this solution againstan internal referenceelectrode. According to the manufacturer(Orion Research,Inc., 1990b), the NOx electrodeexhibitsa linear responsein the range,5 x 10-6 M to 10-2M, which correspondsto a N02" concentrationof 0.07 to 140 mg of N L-1. The only studyin which the NOx electrodehasbeenusedto determineN02" in soil extractshasbeenthat of Tabatabai(1974),who found analysesby the electrode to be in good agreementwith those by the Griess-Ilosvaymethod (see "Nitrite" under"Colorimetric Methods")when extractionswere performedusing water, 1 M KCI, 0.1 M LiCI, 0.01 M CaCI2, 0.008M Ca(H2P04)2 • H20, or saturatedCaS04'Seriousoverestimationoccurredwith 2 M KCI, but sucherror can be eliminated by increasingthe concentrationof the electrodefilling solution (Orion Research,Inc., 1990b).No interferencewas observedby cationsor anions commonly presentin soil extracts,or by other substancesthat may be present under some conditions, such as Cu2+ or Hg2+, both of which interfere in the Griess-Ilosvaycolorimetric method of analysis for N02" (see "Comments" in "Nitrite" section under "Colorimetric Methods"). Interferencedid occur when acidified samplesof extracts were treated with calcareoussoil, which can be attributedto the formation of CO2 bubbles.Accordingto the manufacturer(Orion Research,Inc., 1990b), any compoundthat reactswith water to form an acidic solution will interfere, including CO2, S02' and volatile weak acids [acetic acid (~H402)' formic acid (H2C02), HF, lactic acid (C3H 60 3), pyruvic acid (C3H40 3)]· Methods

Ammonium

Principles. The soil extract is made alkaline by addition of NaOH (pH 11-12) to convertNHt to NH3, and the electromotiveforce (emf, in mY) is measuredwith an NH3 electrode.The concentrationof NHt -N is estimatedby comparison to the values obtained in analysis of NHt -N standardsby the same method.

MethotP SPECIAL APPARATUS

1. Ammonia electrode. 2. pH-millivolt meterwith resolutionof 0.1 mY. REAGENTS

1. Sodium hydroxide, 0.25 M. Dissolve 10 g of NaOH in 800 mL of deionizedwater, and dilute to 1 L in a volumetric flask. 2. Standardammoniumsolution. Dissolve 0.3818 g of ammoniumchloride (NH4CI) in 1 L of 2 M KCI in a volumetric flask. If pure, dry NH4CI is used,the solutioncontains100 Ilg of NHt-N mL-l. Storethe solution in a refrigerator.To preparea seriesof working standardsthat 6 After

Banwartet al. (1972),and Keeneyand Nelson (1982).

NITROGEN-INORGANIC FORMS

1169

contain0.1, 0.5,1,5,and 10 Ilg ofNH.t-N mL-l, pipette1-,5-,10-,50-, and 100-mL aliquots of the concentrated solution into 1-L volumetric flasks, and dilute to volume with 2 M KCI. If an extractantotherthan 2 M KCI is used,preparethe standardsin this solution. PROCEDURE.Pipettea 20-mL aliquot of a 2M KCI soil extract into a 30-or 50-mL beakercontaininga Teflon-coatedstirring bar. Placethe beakeron a magnetic stirrer, add 2 mL of 0.25 M NaOH, and immersein the solution the ammonia electrodeconnectedto a pH-millivolt meter. Stir the solution for 1 min, and then recordthe meterreading(in mV). To calibratethe meter,carry out the same procedureusing 20-mL aliquotsof the working standardNH.t solutions. CALCULATIONS. If the meteris designedfor direct readoutof concentration, determinethe NH.t concentrationof the samplefrom the meter reading. Otherwise, calculatethe concentrationusing the equationobtainedby regressionof the concentrationsof the standardson the correspondingmillivolt readings,or determine the value by referenceto a calibrationcurve preparedfrom analysesof standards. COMMENTS. The exactprocedurefor measuringelectrodepotentialdepends on the particularmeteremployed,and referenceshouldbe madeto the operating instructionsprovidedby the manufacturer.The metershouldbe calibratedimmediately beforeeachseriesof analyses.If excessivedrift is observed,or if the electrode slope deviatesmarkedly from 57 to 59 mV per decadechangein NH.t-N concentration,the electrodemembraneshouldbe replaced.To avoid lossof NH 3, measurements must be madewithin 1 to 2 min after the addition of NaOH. The electrodeshouldbe held at a 20° anglewith respectto the vertical, so that air bubbles cannot be entrappedunder the electrode.The rate of stirring during the analysisshould not be so rapid as to form a vortex. The samplesand standards analyzedin eachseriesof analysesshouldbe at the sametemperature. Nitrate

Principles. The concentrationof NO)"-N is estimatedby comparisonof the electromotiveforce (emf, in mV) for the sampleto the valuesobtainedin analysis of NO)"-N standardsby the samemethod. Method' SPECIAL APPARATUS

1. Nitrate electrode. 2. Double-junction referenceelectrode(Orion Model 90-02 or equivalent). Fill outer chamberwith 0.02 M (NH4)zS04. 3. pH-millivolt meterwith resolutionof 0.1 mY. REAGENTS

1. Ammonium sulfate, 2 M (for ionic strengthadjustment).Dissolve 264

g of reagent-grade(NH4)zS04in 1 L of deionizedwater. 7

After Bremneret al. (1968), and Orion Research,Inc. (1990a).

MULVANEY

1170

2. Standard nitrate solution. Dissolve 0.7218 g of potassium nitrate (KN03) in 1 L of deionizedwater in a volumetric flask. If pure, dry KN0 3 is used,the solution contains100 Jlg of N03"-N mL-l. Store the solution in a refrigerator.To prepareworking standardsthat contain 1 and 10 Jlg of N03"-N mL-l, pipette10- and 100-mL aliquotsof the concentratedsolution into 1-L volumetric flasks, and dilute to volume with deionizedwater. PROCEDURE.Pipettea 20-mL aliquot of an aqueoussoil extractinto a 30- or 50-mL beakercontaininga Teflon-coatedstirring bar. Placethe beakeron a magnetic stirrer, add 2 mL of 2 M (NH4)zS04,and immersein the solution the N03" and referenceelectrodesconnectedto a pH-millivolt meter. Stir the solution for 1 min, and recordthe meterreading(in mY). To calibratethe meter,carry out the sameprocedureusing 20-mLaliquots of the working standardN03" solutions. CALCULATIONS. If the meteris designedfor direct readoutof concentration, determinethe N03" concentrationof the samplefrom the meter reading. Otherwise, calculatethe concentrationusing the equationobtainedby regressionof the concentrationsof the standardson the correspondingmillivolt readings,or determine the value by referenceto a calibrationcurve preparedfrom analysesof standards. COMMENTS. To ensureproper performanceand extend electrodelife, care should be taken to follow manufacturer'sguidelinesconcerningthe preparation, storage,and standardizationof the N03" and referenceelectrodes.Calibration shouldbe carriedout immediatelybeforeeachseriesof analyses,and after every 10 to 20 samples.If excessivedrift is observed,or if the electrodeslopedeviates markedlyfrom 57 to 59 mY per decadechangein NHt-N concentration,the sensor moduleshouldbe replaced.Chemicaltreatmentof the extractmay be required to eliminateinterferenceby solublesalts,Cl-, or NOz. The extractmustbe stirred for 1 min to ensurecompletemixing. Stirring shouldnot be so rapid that the electrode tip is exposedto air bubbles.Use of a thermal-insulatingbarrier between the beakerand the stirrer is adviseableto avoid temperature fluctuations (Keeney et aI., 1970).If desired,analysescan be performedon soil suspensions,using the proceduresdescribedby Bremneret a1. (1968) or Keeneyand Nelson(1982).

Nitrite Principles. The soil extract is acidified by addition of a Na2S04-H2S04 buffer (pH 1.2) to convertNOz to gaseousNOx , and the emf (in mY) is measured with an NOx electrode.The concentrationof NOz-N is estimatedby comparison to the valuesobtainedin analysisof NOz-N standardsby the samemethod.

Method 8 SPECIAL APPARATUS

1. NOx electrode(Orion Model 95-46). 2. pH-millivolt meterwith resolutionof 0.1 mY. 8

After Tabatabai(1974).

NITROGEN-INORGANIC FORMS

1171

REAGENTS

1. Acid buffer reagent. Dissolve 190 g of anhydroussodium sulfate (Na2S04)in approximately800 mL of deionizedwater in a 1-L volumetricflask, slowly add53 mL of 18 M (concentrated)H2S04, andcool and dilute the solution to volume with deionizedwater. 2. Standardnitrite solution. Dissolve0.4926g of sodiumnitrite (NaN02) in 1 L of deionizedwater in a volumetric flask. If pure, dry NaN02 is used,the solution contains100 Ilg of N02"-N mL-l. Store the solution in a refrigerator.To prepareworking standardsthat contain0.1, 1, and 10 Ilg of N02"-N mL-l, pipette1-, 10-, and 100-mLaliquotsof the concentratedsolutioninto l-L volumetricflasks, anddilute to volumewith 1 M KCl. If an extractantother than 1 M KCI is used,preparethe standardsin this solution.

PROCEDURE.Pipettea 20-mL aliquot of aIM KCI soil extractinto a 30- or 50-mL beakercontaininga Teflon-coatedstirring bar. Placethe beakeron a magnetic stirrer, add 2 mL of acid buffer, and immersein the solution the NOx electrode connectedto a pH-millivolt meter. Stir the solution for 1 min, and record the meterreading(in mV). To calibratethe meter,carry out the sameprocedure using 20-mL aliquotsof the working standardN02" solutions. CALCULATIONS. If the meteris designedfor direct readoutof concentration, determinethe N02" concentrationof the samplefrom the meter reading.Otherwise, calculatethe concentrationusingthe equationobtainedby regressionof the concentrationsof the standardson the correspondingmillivolt readings,or determine the valueby referenceto a calibrationcurvepreparedfrom analysesof standards. COMMENTS. Referenceshould be made to the instruction manual for the

NOx electrodefor details regardingpreparation,storage,and standardizationof

the electrode.Calibrationshouldbe carriedout immediatelybeforeeachseriesof analyses.If excessivedrift is observed,or if the electrodeslopedeviatesmarkedly from 57 to 59 mV per decadechangein N02"-N concentration,the electrode membraneshould be replaced.To avoid loss of NOv measurementsmust be madewithin 1 to 2 min after the additionof the acid buffer. The electrodeshould be held at a 20° angle with respectto the vertical, so that air bubblescannotbe entrappedunderthe electrode.The rate of stirring during the analysisshouldnot be so rapid asto form a vortex. The samplesandstandardsanalyzedin eachseries of analysesshouldbe at the sametemperature. DETERMINATION OF NONEXCHANGEABLE AMMONIUM Introduction

Threetypesof methodshavebeenusedto determine nonexchangeable NH4 in soils. In one,the soil sampleis distilled with NaOH, and a duplicatesampleis distilled with KOH, nonexchangeable NHS being estimatedas the difference betweenthe amountsof NH4 releasedby the two distillations. In another,the soil

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sampleis heatedto remove exchangeable NUt and organicN, and non-exchangeable NHt is determinedfrom analysisof the residueby the Kjeldahl procedure. In the other, the soil sampleis treatedwith HF to decomposethe clay minerals containingthe nonexchangeable NHt, and the NHt releasedby this treatmentis determined.ExchangeableNHt is removedbefore the treatmentwith HF or is estimatedseparatelyand subtractedfrom the NHt releasedby HE The duplicate distillation procedurefor estimatingnonexchangeable NH,t in soils was originally proposedby Barshad(1951). This procedurehas not been widely used because:(i) distillation with NaOH does not effect quantitative releaseof nonexchangeable NHt from soils and minerals, (ii) trace amountsof K+ effectively block the releaseof nonexchangeable NHt during distillation with NaOH, and (iii) the amountof NHt releasedfrom organic-Ncompoundsis likely to differ for distillations with NaOH and KOH. Mogilevkina (1964) suggesteda methodof determiningnonexchangeable NH,t in soils, in which the sampleis heatedat 400°C for 24 to 72 h dependingon the organic matter content, and nonexchangeable NHt is estimatedby total-N analysisof the residuefrom this heat treatmentusing a Kjeldahl procedure.This methodinvolves the assumptionthat the heat treatmentusedeffects quantitative removalof organicN and exchangeableNH,t but doesnot releasenonexchangeable NHt, in which caseN in the residuewill occur exclusivelyas nonexchangeable NHt andwill representall of the nonexchangeable NHt originally presentin the sample.However,Nelson and Bremner(1966) showedthis assumptionto be invalid, as the heattreatmentusedled to loss of nonexchangeable NHt, and their work further showed that the Kjeldahl procedureused failed to quantitatively recover N in the residue from this treatment.Mogilevkina's method has been found to give much lower values than other methodsof determining nonexchangeableNHt in soils (Nelson& Bremner,1966; Bremneret aI., 1967; Mogilevkina, 1969; Moyano & Gallardo, 1988) and is unsatisfactory. A variety of methodshavebeendescribedthat involve the useof HF to estimatenonexchangeable NHt in soils. In the first study to provide evidencefor the presenceof substantialamountsof nonexchangeableNHt in soils, Rodrigues (1954) showedthat a significant amountof NHt was releasedfrom tropical soils by treatment withHF and concludedthat this NHt was derived from NHt held by clay minerals.In his work, nonexchangeable NHt was extractedby treatment of the soil samplewith a mixture of four volumesof 20 M (40%) HF and onevolume of 9 M (50%) H2S04, NHt in the extract being determinedby distillation with NaOH after removalof HF with H2S04, This is a drasticprocedure,and subsequentwork by Bremner(1959) showedthat it causesextensivedecomposition of organic-N compoundsto NHt. Milder HF procedureshave been adoptedin most investigationsconcerningthe occurrenceand distribution of nonexchangeable NH,t in soils. The most commonextractantshavebeen5 M HF-0.75M Hel0.3 M H2S04 solution (Dhariwal & Stevenson,1958), 1 M HF-1 M HCI solution (Bremner,1959), and 5 M HF-1 M HCI solution (Silva & Bremner,1966). Various pretreatmentshavebeenemployedto removeorganicmatter inHF NHt in soils and therebyreducethe risk methodsof estimatingnonexchangeable of interferenceby decompositionof organic-Ncompoundsto NHt. In the method describedby Dhariwal and Stevenson(1958) for estimationof nonexchangeable

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NHt, the soil sampleis heatedwith 1 M KOH in an autoclavefor 8 h beforetreatment with HF-HCI-H2S04 solution. Schachtschabel (1960, 1961) suggestedthe NHt useof H20 2 to oxidize organicmatterbeforeextractionof nonexchangeable with HF-H2S04' and further recommendedthat the H20 2 treatmentbe carriedout in the presenceof KCl to preventfixation of the NHt producedby this treatment and that correctionbe madefor any releaseof NHt from organiccompoundsduring extractionwith HF-H2S04. SIlva and Bremner(1966) describeda methodfor estimationof nonexchangeable NHt, in which the soil sampleis treatedwith a mixture of KOBr and KOH before extraction with HF-HCI. The KOBr-KOH mixture is more effective for removal of organic matter than hot KOH or H20 2 (Bremneret aI., 1967), and the high concentrationof K+ eliminatesany possibility of NHt fixation by soil mineralsduring treatmentwith this reagent.Moreover, the KOBr readily convertsNHt to N2, thereby ensuringrapid removal of exchangeableNHt and NHt derivedfrom organic-Ncompounds. The method of Silva and Bremner(1966) is describedbecauseit has no apparentdefectsand has beenusedwidely to estimatenonexchangeable NHt in soils. However, there is no way of verifying the accuracyof this method, and attentionshould be drawn to the possibility that soil mineralsmay contain highly labile organic-Ncompoundsthat are not removedor decomposedby treatment with KOBr-KOH, but are releasedby treatmentwith HF-HCI and decompose extensively to NHt during this treatmentor upon subsequentdistillation with KOH. Freney(1964) observeda closecorrelationbetweenorganic-Cand total-N analysesof residuesobtainedby the KOH-HF methodof Dhariwal and Stevenson (1958) and suggestedthat nonexchangeable NHt estimatedby this methodis an artifact arising from the decompositionof labile organic-Ncompounds.However, there is little supportfor this view amongsoil scientists,and all indications are that the greatmajority of NHt liberatedby KOH-HF or KOBr-HF methodsis nonexchangeable NHt from silicate minerals. A secondpossible defect of the procedure described by Silva and Bremner (1966) for estimation of nonexchangeableNHt is that clay mineralsmay be partially dissolvedby the KOBrKOH pretreatmentusedto decomposeorganic-N compoundsbefore addition of HF-HCl, in which casesomeof the nonexchangeable NHt might be lost. Loss of NHt also could occur from the presenceof taranakitesor other complex phosphatesthat are subjectto quantitativeremovalby treatmentwith KOH or KOBrKOH (Bremneret aI., 1967).However,thereis no evidencethat soils containsignificant amountsof NHt in the form of suchcompounds.

Principles In the methoddescribed,the soil sampleis treatedwith alkaline potassium hypobromite(KOBr-KOH) solution to removeexchangeable NHt and organic-N compoundsthat may yield NHt under the conditionsemployedfor releaseand estimation of nonexchangeableNHt, and the residue from this treatment is washedwith 0.5 M KCl and shakenwith 5 M HF-1 M HCl solution for 24 h to decomposemineralscontainingnonexchangeable NHt. The NHt releasedby the HF-HCI treatmentis determinedfrom the amountof NH3 liberatedby stearndistillation of the soil-acid mixture with KOH.

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The alkaline KOBr solution employedto eliminate interferenceby organic-N compoundsoxidizessoil organicmatteralmostcompletelyunderconditions NHt and that that are unlikely to causesignificant releaseof nonexchangeable involve no risk of fixation of NHt by soil minerals.The NHt formed on oxidation of soil organicmatterby this reagentis immediatelyconvertedto N2 by the following reaction 2NH3 + 3KOBr ~ N2 + 3KBr + 3H20. Method9

SpecialApparatus 1. Steam-distillationapparatusdescribedin "Special Apparatus" under "Steam-DistillationMethods"(Fig. 38-1). 2. Distillation flasks. The flasks employedare 250-mL Pyrex round bottom boiling flasks with standard-taper joint (T24/40) that havebeenfitted with a socketjoint (~ 28/15) for attachmentto the distillation apparatus. Their dimensionsshould be such that when the flasks are connectedto the steam-distillationapparatus,the distancebetweenthe tip of the steam-inlettube and the bottom of the flask does not exceed4 mm. 3. Microburette(5-mL, graduatedat O.Ol-mL intervals)or automatictitrator.

Reagents 1. Potassiumhypobromitesolution. Dissolve 22 g of KOH in 200 mL of deionizedwater in a 250-mL Erlenmeyerflask containinga Teflon- or glass-coatedmagneticstirring bar. Then immerse the flask in a container of crushedice, and allow the solution to cool to 5°C. With constant stirring, add 6 mL of bromine (Br2) dropwisefrom a burette at a rate of approximately 0.5mL min-I. Storethe solution in a refrigerator. 2. Potassiumchloride solution, approximately0.5 M. Dissolve 186 g of reagent-gradeKCI in 5 L of deionizedwater. 3. Hydrofluoric acid-hydrochloricacid solution (approximately5 MHF-1 M HCI). To 1.5 L of deionized water in a 2.5-L polyethylene or polypropylenebottle markedto indicate a volume of 2 L, add 167 mL of 12 M (concentrated)HCI (sp. gr. 1.19) and 325 mL of 31 M (concentrated,~52%) HE Then dilute the solution to the 2-L mark, and mix it thoroughly. 4. Potassiumhydroxidesolution, approximately10 M. Dissolve2.8 kg of KOH in approximately4 L of deionizedwater, and dilute the cooled solution to a volume of 5 L. Storethe solutionin a tightly stopperedbottle.

9

After Bremner(1965), and Silva and Bremner(1966).

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5. Boric acid-indicator solution. Prepare this reagent as described in "Reagents"under"Steam-DistillationMethods." 6. Sulfuric acid, 0.0025M standard.

Procedure Placea 1-g sampleof finely ground «150!lm) soil in a 200-mL tall-form beaker,and add20 mL of KOBr solution. Swirl the beakerto mix the soil and KOBr, cover the mouth of the beakerwith a watch glass,and allow the covered beakerto standat room temperaturefor 2 h. Then add60 mL of deionizedwater, and after replacingthe watch glass, heat the beakeron a hotplate until the soilKOBr mixture hasboiled vigorously for 5 min. Allow the mixture to cool and settle in the covered beaker (preferably overnight); then decant and discard the supernatantliquid, and transferthe residuewith 0.5 M KCI to a 100-mL polyethylene or polypropylenecentrifugetube that is markedto indicatea volume of 80 mL. Use a wash bottle containing0.5 M KCI to perform this transfer,and bring the contentsof the tube to the 80-mL mark with the washingsobtainedby rinsing the beakerseveraltimes with 0.5 M KCI. Fit the neck of the tube with a polyethylenecap or rubberstopper,and after shakingthe tube manuallyfor a few seconds,centrifugeit (1100 x g) for 10 min. Decantthe clearsupernatant liquid,add 0.5 M KCI to the 80-mL mark, andshakeandcentrifugethe tube as describedpreviously. Decantthe clear supernatantliquid, and add 20 mL of 5 M HF-1 M HCl solution from a 25-mL polyethylenegraduatedcylinder. Then stopperthe tube, and shakeit for 24 h on a reciprocatingshaker. After completionof the treatmentwith HF-HCl solution, transfer15 mL of 10 M KOH to a 250-mL distillation flask, and place a long-stemmedpolyethylene filter funnel in the neck of the flask so that the stem extendsbelow the surface of the KOH solution. Transferthe contentsof the centrifugetube tothe distillation flask through this funnel, and completethe transferby rinsing the centrifuge tube and funnel with approximately25 mL of deionized water from a wash bottle. Then stopperthe flask, swirl it to mix the contents,and allow it to standfor a few minutes.During this period of standing,add 10 mL of boric acidindicator solution to a 250-mL beakerthat is marked toindicatea volume of 100 mL, and place the beakerunderthe condenserof the steam-distillationapparatus so that the tip of the condenseris in contactwith the side of the beakerand about 1 cm below the top. Attach the distillation flask to the steam-distillationapparatus as shownin Fig. 38-1, and immediatelycommencesteamdistillation by closing the lower stopcockon the steam-bypass assembly.When the distillate reaches the 100-mL mark on the receiverflask (~12 min is required),rinse the tip of the condenser,and stop the distillation by opening the lower stopcock on the steam-bypassassembly.Titrate the distillate with 0.0025M H2S04, At the endpoint, the color changes fromgreento a permanent,faint pink.

Calculations Calculatenonexchangeable NHt-N in the samplefrom the expression(SC) x T, whereS is the volume of H2S04 usedin titration of the sample,C is the volume usedin titration of a control (obtainedby steamdistillation of 20 mL of

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5 M HF-l M HCI solution with 15 mL of 10 M KOH solution), and T is the titer of the titrant (for 0.0025M H2S04, T = 70 J..lg N mL- 1). Comments Potassiumhypobromitetendsto decomposeto form KBr and O2 (2KOBr 2KBr + O2), so this reagentshouldbe preparedimmediatelybeforeuse.However, the rateof decompositionis markedly reducedat low temperaturesin the absenceof light, and the reagentmay safely be usedfor up to 1 mo if stored ina refrigerator. Studiesby Silva and Bremner(1966) have shown that the KOBr pretreatment employedin the proceduredescribed removes 96 to 99% of the organicN in soils containingup to 150 g of organicmatterkg-1 and that the residuesfrom this pretreatmentrelease only trace amounts of NHS under the conditions employedfor distillation of the NHS releasedby the treatmentwith HF-HCI solution. The 5-min boil after treatmentwith KOBr-KOH solution at room temperature for 2 h increasesthe amountof soil organicN convertedto Nz, and removes material that releasesNHS under the conditionsemployedfor distillation of the NHS releasedby treatmentwith HF-HCI solution. Boiling also causesflocculation of the soil solids and permits the KOBr-KOH solution to be removedby decantationwithout preliminary centrifugation.To ensurecompleteremoval of labile organic-Ncompounds,boiling shouldbe vigorous. Testsusingseveralsoils haveshownthat the resultsobtainedby the method describedare not significantly affected if the period of treatmentwith KOBrKOH solution is increasedfrom 2 to 12 h, or if the period of shakingwith 5 M HF-l M HCI solution is increasedfrom 24 to 48 h (Silva & Bremner,1966). Careis requiredto avoid lossof suspendedmaterialduring the decantations in the proceduredescribed.The useof a rubberpolicemanis recommendedto aid removal of adheringsoil particles during washing in transferof the potassium hypobromite-treated soil to the centrifugetube and of the soil-HF-HCI mixture to the distillation flask. Hydrofluoric acid is a volatile reagentand interfereswith the determination of NHS by distillation with alkali (Bremner & Harada, 1959). In the method described,this interferenceis eliminatedby the techniqueusedfor addition of the HF-treatedsampleto the distillation flask, and by swirling the flask and allowing it to standfor a few minutesafter this addition. No loss of NH3 has beendetected using this procedure,but the risk of loss is reducedif the 10 M KOH solution employedfor distillation is kept cold by storing it in a refrigerator. The apparatusused for separationof NHS by steamdistillation, and the reagentsusedfor estimationof NHS in the distillate are describedand discussed in "Methods"under"Steam-DistillationMethods,"and referenceshouldbe made to this sectionfor additionalinformation concerningthe procedurefor separation and estimationof NHS. ~

REFERENCES Adamsen,FJ., D.S. Bigelow, and G.R. Scott. 1985.Automatedmethodsfor ammonium,nitrate, and nitrite in 2 M KCI-phenylmercuricacetateextractsof soil. Commun. Soil Sci. Plant Anal. 16:883-898.

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Allen, S.E., and H.M. Grimshaw. 1962. Effect of low-temperaturestorageon the extractablenutrient ions in soil. J. Sci. Food Agric. 13:525-529. Al- Wehaid,A, and A. Townshend.1986. Spectrophotometric flow-injection determinationof nitrate basedon reductionwith titanium (III) chloride. Anal. Chim. Acta 186:289-294. American Public Health Association. 1992. Standardmethods for the examination of water and wastewater.18th ed. Am. Public Health Assoc.,Washington,DC. Ananth, S., and J.T. Moraghan.1987. The effect of calcium and magnesiumon soil nitrate determination by automated segmented-flow methods.Soil Sci. Soc. Am. 1. 51:664-667. Axelrod, H.D., 1.E. Bonelli, and J.P. Lodge, Jr. 1970. Fluorimetric determinationof trace nitrates. Anal. Chim. Acta 51:21-24. Baker, AS. 1967. Colorimetric determinationof nitrate in soil and plant extractswith brucine. J. Agric. Food Chern. 15:802-806. Baker, AS., and R. Smith. 1969. Extracting solution for potentiometricdeterminationof nitrate in plant tissue.1. Agric. Food Chern. 17:1284-1287. Balks, R., and I. Reekers.1955. Bestimmungdes Nitrat- und Ammoniastickstoffsim Boden. Landwirtsch. Forsch.8:7-13. Banwart, w.L., M.A Tabatabai,and 1.M. Bremner. 1972. Determination of ammonium in soil extracts and water samples by an ammonia electrode. Commun. Soil Sci. Plant Anal. 3:449-458. Barak, P., and Y. Chen. 1987.Three-minuteanalysisof chloride, nitrate, and sulfateby single column anion chromatography.Soil Sci. Soc. Am. J. 51:257-258. Barker, AV. 1974. Nitrate determinationsin soil, water and plants. Massachusetts Agric. Exp. Stn. Res. Bull. 611. Univ. Massachusetts, Amherst. Barnes,H., and AR. Folkard. 1951. The determinationof nitrites. Analyst (London) 76:599-603. Barshad,I. 1951. Cation exchangein soils: I. Ammonium fixation and its relation to potassiumfixation and to determinationof ammoniumexchangecapacity.Soil Sci. 72:361-371. Bartuzi, J., I. Dechnik, and Z. Stepniewska.1976. Zastosowanieelektrod selektywnychdo pomiaru aktywnoscichlorkow i azotanoww glebie. Rocz. G1ebozn.27:15-26. Baveja, AK., and v.K. Gupta. 1982. Extractive spectrophotometricdeterminationof trace amounts of nitrite and nitrate in irrigation water and soil. Fert. Technol. 19:80-84. Berthelot,M.P.E. 1859. Violetd'aniline. Rep. Chim. Appl. 1:284. Best, E.K. 1976. An automatedmethodfor determiningnitrate-nitrogenin soil extracts.Queensland 1. Agric. Anim. Sci. 33:161-166. Best, E.K., and E.T. Craswell. 1985. Interferenceby magnesiumin the determinationof nitrate in 2 M KCl extractsof soil by steamdistillation. Commun.Soil Sci. Plant Anal. 16:1189-1198. Biggar, J.W. 1978. Spatialvariability of nitrogen in soils. p. 201-211.In D.R. Nielsen and J.G. MacDonald (ed.) Nitrogen in the environment.Vol. 1. Acad. Press,New York. Black, AS., and S.A Waring. 1978. Nitrate determinationin an oxisol using K 2S04 extraction and the nitrate-specificion electrode.Plant Soil 49:207-211. Boltz, D.F., and M.J. Taras.1978. Nitrogen. p. 197-251.In D.F. Boltz and J.A. Howell (ed.) Colorimetric determinationof nonmetals.2nd ed. Wiley-Intersci., New York. Botha,AD.P., and J.C. Johnson.1988.A vacuumfiltration and leachingmethodfor the colorimetric determination of ammonium and nitrate nitrogen in soils. S. Afr. Tydskr. Plant Grond 5:196-198. Bound, G.P. 1977. Determinationof nitrate in soil pastesby ion selective electrodes.J. Sci. Food Agric. 28:501-505. Bound, G.P., and B. Fleet. 1977. The developmentof a solid statereferenceelectrodefor use in soil measurements. J. Sci. Food Agric. 28:431-435. Bradfield, E.G., and D.T. Cooke. 1985. Determinationof inorganicanionsin water extractsof plants and soils by ion chromatography.Analyst (London) 110:1409-1410. Breimer, T., and J.H.G. Slangen.1981. Pretreatmentof soil samplesbefore NOrN analysis.Neth. 1. Agric. Sci. 29:15-22. Bremner,J.M. 1959. Determinationof fixed ammoniumin soil. 1. Agric. Sci. 52:147-160. Bremner,1.M. 1965. Inorganic forms of nitrogen. p. 1179-1237.In C.A Black et al. (ed.) Methods of soil analysis.Part 2. Agron. Monogr. 9. ASA, Madison,WI. Bremner,J.M., and L.G. Bundy. 1973. Use of ferrous hydroxide for determinationof nitrate in soil extracts.Commun.Soil Sci. Plant Anal. 4:285-291. Bremner,1.M., L.G. Bundy, andAS. Agarwal. 1968. Use of a selectiveion electrodefor determination of nitrate in soils. Anal. Leu. 1:837-844.

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Bremner,J.M., and A.P. Edwards.1965. Determinationand isotope-ratioanalysisof different forms of nitrogenin soils: I. Apparatusand procedurefor distillation and determinationof ammonium. Soil Sci. Soc. Am. Proc. 29:504-507. Bremner,J.M., and T. Harada.1959. Releaseof ammoniumand organic matter from soil by hydrofluoric acid and effect of hydrofluoric acid treatmenton extractionof soil organic matter by neutraland alkaline reagents.J. Agric. Sci. 52:137-146. Bremner,J.M., and D.R. Keeney.1965. Steamdistillation methodsfor determinationof ammonium, nitrate and nitrite. Anal. Chim. Acta 32:485-495. Bremner, J.M.,and D.R. Keeney. 1966.Determinationand isotope-ratioanalysisof different forms of nitrogen in soils: 3. Exchangeableammonium, nitrate, and nitrite by extraction-distillation methods.Soil Sci. Soc. Am. Proc. 30:577-582. Bremner,J.M., D.W. Nelson, and J.A Silva. 1967. Comparisonand evaluationof methodsof determining fixed ammoniumin soils. Soil Sci. Soc. Am. Proc. 31:466-472. Bremner, J.M., and K Shaw. 1955. Determinationof ammonia and nitrate in soil. J. Agric. Sci. 46:320-328. Brown, M.W. 1973. A highly sensitive automatedtechniquefor the determinationof ammonium nitrogen.J. Sci. Food Agric. 24:1119-1123. Buckett,J., W.D. Duffield, and R.E Milton. 1955.The determinationof nitrate and nitrite in soil. Analyst (London) 80:141-145. Burton, D.L., DA Gower, P.M. Rutherford,and W.B. McGill. 1989. Amino acid interferencewith ammoniumdeterminationin soil extractsusing the automatedindophenolmethod.Commun. Soil Sci. Plant Anal. 20:555-565. Carlson,R.M. 1978. Automatedseparationand conductimetricdeterminationof ammoniaand dissolvedcarbondioxide. Anal. Chern. 50:1528-1531. Carlson,R.M., R.I. Cabrera,J.L. Paul.,J. Quick, and R.Y. Evans.1990. Rapid direct determinationof ammonium and nitrate in soil and plant tissue extracts. Commun. Soil Sci. Plant Anal. 21:1519-1529. Carlson,R.M., and D.R. Keeney.1971.Specificion electrodes: Techniques and usesin soil, plant, and water analysis.p. 39-65. In L.M. Walsh (ed.) Instrumentalmethodsfor analysisof soils and plant tissue.SSSA,Madison,WI. Cataldo,D.A, M. Haroon,L.E. Schrader,and V.L. Youngs. 1975. Rapid colorimetric determination of nitrate in plant tissueby nitration of salicylic acid. Commun.Soil Sci. Plant Anal. 6:71-80. Cattrall, R.W., and I.e. Hamilton. 1984. Coated-wire ion-selective electrodes.Ion-Select.Elec. Rev. 6:125-172. Cawse,P.A 1967. The determinationof nitrate in soil solutions by ultraviolet spectrophotometry. Analyst (London) 92:311-315. Cbaube,A, AK Baveja,and V.K Gupta.1984.Determinationof ultra traceconcentrationsof nitrite in polluted watersand soil. Talanta31:391-393. Clarke, AL., and A.C. Jennings.1965. Spectrophotometricestimationof nitrate in soil using chromotropic acid. J. Agric. Food Chern. 13:174-176. Clausen,e., B.R. Bock, G.A Peterson,and R.A Olson. 1980. The magnesiumproblem in nitrate determinationby steamdistillation. Soil Sci. Soc. Am. J. 44:1326-1327. Clausen,e.R., M.P. Russelle,A.D. F1owerday,and R.A Olson. 1981. Problemsin direct steamdistillation of soil for mineral nitrogen determinationdue to carbonates.Soil Sci. Soc. Am. J. 45:1238-1240. Conway,E.J. 1947. Microdiffusion analysisandvolumetricerror. 2nd ed. CrosbyLockwood, London. Covington,AK 1974. Ion-selectiveelectrodes.CRC Cril. Rev. Anal. Chern.4:355-406. Csiky, I., G. Marko-Varga,and J. A. Jonsson.1985. Use of disposableclean-upcolumnsfor selective removal of humic substancesprior to measurementswith a nitrate ion-selectiveelectrode. Anal. Chim. Acta 178:307-312. Dahnke,w.e. 1971. Use of the nitrate specific ion electrodein soil testing. Commun.Soil Sci. Plant Anal. 2:73-84. Davies, J.E.w., G.J. Moody, and J.D.R. Thomas. 1972. Nitrate ion selective electrodesbasedon poly(vinyl chloride) matrix membranes.Analyst (London) 97:87-94. Dhariwal, AP.S., and EJ. Stevenson.1958. Determinationof fixed ammonium in soils. Soil Sci. 86:343-349. Dick, W.A., and M.A Tabatabai.1979. Ion chromatographicdeterminationof sulfate and nitrate in soils. Soil Sci. Soc. Am. J. 43:899-904. Dorich, R.A, and D.W. Nelson. 1983. Direct colorimetric measurementof ammoniumin potassium chloride extractsof soils. Soil Sci. Soc. Am. J. 47:833-836. Dorich, R.A, and D.W. Nelson. 1984. Evaluationof manualcadmium reductionmethodsfor determination of nitrate in potassiumchloride extractsof soils. Soil Sci. Soc. Am. J. 48:72-75.

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Published 1996

Chapter39 Nitrogen-Organic Forms F. J. STEVENSON,University of Illinois, Urbana, Illinois

INTRODUCTION Chapter32 (Stevenson,1982a)of the secondedition of ASA publicationMethods ofSoil Analysis (Pageet aI., 1982) hasbeenmodified and updated.The reader is referredto this publicationand Chapters85 (Bremner,1965),96(Stevenson, 1965a),and 97 (Stevenson,1965b)of the first edition (Blacket aI., 1965)for detailed background information,including a historicalaccountof the development of methodsfor determining organic forms of N in soils. Detailed reviews of organicN compoundsin soil havebeenprovidedby Stevenson(1982b). Most studieson the forms of organicN in soils are basedon the use of hot mineral acids(or bases)to liberatenitrogenousconstituentsfrom organiccolloids and clay minerals.In a typical procedure,the soil is heatedwith 3 or 6 M hydrochloric acid (HCI), after which the N is separatedinto severaldiscretefractions (Table 39-1). Identifiable organic N compoundsare the amino acids and amino sugars. The N remainingin the soil residueis usually referredto as acid-insoluble N; that recoveredby distillation with magnesiumoxide (MgO) is ammonia-N (NH3-N). The nitrogen not accountedfor in the aboveforms is referredto as the

HUN fraction (hydrolyzable unknown N).

In addition to amino acidsand amino sugars,soils containtracequantities of nucleic acids and other known nitrogenousbiochemicals.However, specialized techniquesare required for their separationand identification. Only onethird to one-halfof the organic N in soils can be accountedfor in known compounds. The methodfor hydrolyzing the soil has not beenstandardized,and many variations in hydrolytic conditionshave been employed.The variablesinclude: (i) type and concentrationof acid, (ii) time and temperatureof hydrolysis, (iii) ratio of acid to soil, and (iv) pretreatment.In general,hydrolysis is done under reflux with 6 M HCI for 12 to 24 h. A large amountof soil N, usually about 25 to 35%, is recoveredas acidinsoluble N (i.e., N recoveredin soil residue).At one time it was thOUght that this fraction was an artifact resulting from the condensationof amino acids with reducing sugarsduring hydrolysis, but it is now believed that some of this N Copyright © 1996 Soil ScienceSociety of America and American Society of Agronomy, 677 S. SegoeRd., Madison,WI 53711,USA. Methods of Soil Analysis. Part 3. Chemical Methods-SSSA Book Seriesno. 5. 1185

STEVENSON

1186

Table 39-1. Steamdistillation methodsfor determiningthe variousforms of N in a soil hydrolysate. Form of N

Methodt

Steamdistillation with NaOH after Kjeldahl digestionwith H2S04 and a Total hydrolyzableN K2S04-catalystmixture SteamdistiJIationwith phosphate-borate buffer after treatmentwith Amino acid-N NaOH at 100°Cto removeaminosugarsplus NH4 and with ninhydrin (PH 2.5, 100°C)to converta-amino-Nto NHi SteamdistiJIationwith phosphate-borate buffer at pH 11.2; correction Amino sugar-N for NH3-N SteamdistiJIationwith MgO Ammonia-N Acid-insolubleN Obtainedby difference(total soil N-hydrolyzableN) HydrolyzableunknownN Obtainedfrom the differencebetweentotal hydrolyzableN and the N (HUN fraction) accountedfor as (NH3 + amin,! acid + amino sugar)-N t In eachmethod,the NH3 liberatedby steamdistiJIationis collectedin a H3B03-indicatorsolution and determinedby titration with standard(0.0025M) H2S04•

occursas a structuralcomponentof humic substances.Chenget at. (1975) found that the percentageof the soil N recoveredin acid-insolubleforms could be reducedby pretreatingthe soil with hydrofiouric acid (HF) beforeacid hydrolysis. Freney(1968)and Griffith et at. (1976)showedthat part of the insolubleN could be dissolvedwith dilute baseand subsequentlysolubilized by acid hydrolysis, therebyreducingthe acid-insolublefraction to about 10% of the total N. Another uniquefeatureof soil N fractionationschemesis that a large proportion of the soil N, about 20 to 25% for surfacesoils, is recoveredas NH3 by distillation with MgO. In the older literature, this form of N was referredto as the "amide-N" of proteins, but it is now known that very little of the NH3 is derived from the amino acid amides, asparagine(C4HgN20 3) and glutamine (CSHlON203). Some of the NH3 is derived from indigenousfIxed NHt, part comesfrom partial destructionof amino sugars.It also is known that NH3 can arise from the breakdownof certain amino acidsduring hydrolysis.Tryptophan (Cll H 12N20 2) is lost completely;others,suchas serine(C3H7N03) and threonine (C4H9N03) are partially destroyed(Stevenson,1982a). Methods commonly used for determiningamino acid-N in soil hydrolysatesare thosebasedon the ninhydrin (CgHJI4) reaction(Fig. 39-1). They include measurements for the CO2 and NH3 formed throughReactionA and for the blue-coloredproductformed through ReactionB. The ninhydrin-C02 methodfor amino acid-N is highly specific in that it requiresthe presenceof both a COOH and an adjacentNH2 or NH-CH2 group. This procedureis difficult and time-consumingand requiresmanometricequipment that demandsboth skill and experiencefor successfuloperation. On the otherhand,the ninhydrin-NH3methoddescribedherein(see"Amino Acid-Nitrogen") is relatively simple and doesnot require highly specializedequipment.Of equalimportanceis the fact that the N is recoveredin a form suitablefor isotopicratio analysis.A colorimetric procedurefor amino acid-N also is describedherein (see"Colorimetric Method for Amino Acid-N"). Two methodsare given for amino sugar-N,an alkaline decompositionprocedurein which the N is recoveredand measured as NH3 (see"Amino SugarNitrogen") and a colorimetric methodbasedon the Elson-Morganreaction(see "Colorimetric Method for Amino Sugars").

NITROGEN-ORGANICFORMS

1187

A pH

o

((I

R-CH-COOH --";'---l~~ I 5 cmol+ kg-1 of both CEC and AEC. None of the soils they studiedshowedany of these characteristicsevenremotely. If this methodis to be usedto estimateexchangeablecations,care should be taken to preparestandardsin the same matrixas the unknownsolutions;Ca2+ in particular suffers some interference from sOi- with air-acetyleneatomic absorptiondetermination.When soils contain soluble salts, pretreatmentshould be usedto removethem, but this can causeerrorsin the estimationof exchangeable cations.Howeverthe presenceof soluble saltswill havelittle or no effect on the measurementof CECCE• The reasonfor the use of an equilibrating solution of divalent cationsof ionic strengthequalto 0.006M is becauseit approximatesthat of the "soil solu-

1218

SUMNER & MILLER

tion" of many highlyweatheredsoils (Gillman & Bell, 1978). In soils where the ionic strengthof the soil solution differs substantiallyfrom 0.006M, an appropriate value shouldbe used. Although BaCl2 is not usually used as an electrolyte to measuresoil pH, pHBaCI2 shouldbe a reliable estimateexceptfor salinesoils wherethe removalof solublesaltsgenerallyresultsin an increasein pH. For greaterconvenience,a conductivity meter which has the capability of being operatedin a ratio mode is preferred,althougha meterwith a single electrode will suffice. With experience,it becomespossiblewith highly bufferedsoils to overadjust pHBaCI2 with 0.1 M H2S04, knowing that an upward drift will occur. Similarly, adjustmentof CR to 1.0 by the addition of deionizedwater is soil-dependent. Shouldthe capacityof the centrifugetube be exceeded,simply transferthe contentsto a weighed beaker and continue. A modification to the procedure which makesit lesstime-consuminghasbeenproposedby Sumneret aI. (1994), in which insteadof dilution with water to bring the CR to 1.0 in the final step,the sampleis centrifugedprior to water addition and the EC of the supernatantis measured.From a calibrationcurve relatingEC of the supernatantto wateradded, the amountof water that would have beenaddedcan be estimated. Cation exchangecapacity values obtained by the compulsive exchange methodare similar to thoseobtainedby the silver thioureamethod(Searle,1986; Gillman et aI., 1983),sumof exchangeable cations(ECEC) (Gillman et aI., 1983; Grove et aI., 1982), and 0.2 M NH4CI (Grove et aI., 1982) methods.

UnbufferedSalt ExtractionMethod Introduction This method is basedon the original proposalof Schofield (1949) which enabledthe measurementof the CEC of a soil at its "field pH" value. An unbufferedsalt solution is usedin placeof the bufferedsolutionssuch as NH40Ac and BaCl2-TEA which were in vogue at that time for saturatingthe exchange complex.The methodpresentedhereis a modificationof the proceduredescribed by Grove et aI. (1982). It involves the saturationof the exchangesiteswith NHt using an unbufferedNH4CI solution, reducingthe ionic strengthto an appropriate value (or removingthe entrainedsalt with water),assessingthe volume of the solution which is entrainedand then displacingNH4+ with a solution of KN0 3. The quantitiesof NHt and CI- in the final extract are correctedfor the amounts in the entrainedsolution. If the volume of entrainedsolution is measured,this methodalso permitsthe estimationof the anion exchangecapacityfrom the quantity of Cl- adsorbed.

Apparatus 1. 2. 3. 4.

Bench-topcentrifuge. 50-mL centrifugetubeswith caps. Vortex stirrer. End-over-endor reciprocatingshaker.

CATION EXCHANGE CAPACITY & EXCHANGE COEFFICIENTS

1219

5. Top-loadingbalancereadingto 0.01 g. 6. Dispensers. 7. 250-mL volumetric flasks. Reagents 1. Saturatingsolution, 0.2 M NH4CI. Dissolve 10.7 g NH4Cl and make up to 1L with deionizedwater. 2. Equilibrating solution, 0.04 M NH 4CI. Dissolve 2.1 g NH4Cl and make up to 1 L with deionizedwater. 3. Extracting solution, 0.2 M KN0 3. Dissolve 20.2 g KN0 3 and make up to 1 L with deionizedwater. Procedure Weigh 5 g of soil into a preweighed50-mL centrifugetube. Add 30 mL of 0.2 M NH4CI and shakefor 5 min, centrifugeand decantsupernatantinto a 250mL volumetric flask, being careful to avoid loss of soil. Add 30 mL of 0.2 M

NH 4CI, resuspendthe soil using the Vortex stirrer (Scientific Industries,Bohemia, NY), shake for 5 min, centrifuge and decant supernatantinto volumetric flask. Repeatthis processthreemore times, combiningsupernatants prior to making up to volume with 0.2 M NH4CI. Savethis solution for the determinationof exchangeableNa, K, Ca, Mg and AI [Chapter 18 (Bertsch & Bloom, 1996), 19 (Helmke & Sparks,1996), and 20 (Suarez,1996)]. Two options are possibleat this point: EITHER washthreetimes with deionizedwater and discardthe supernatant,OR add 3 x 30-mL portionsof 0.04M NH4CI, resuspend,shakefor 5 min, centrifuge and discard the supernatanteach time, and then weigh tube to determine volume of entrainedsolution. Add 30 mL 0.2 M KN0 3, resuspend,shake for 5 min, centrifuge and collect supernatantin a 250-mL volumetric flask. Repeatthis processa further four times, combiningthe supernatants. Analyze this solution for NHt [Chapter38 (Mulvaney, 1996)] and, if entrainedsolution was measuredand AEC is desired,for Cl- [Chapter31 (Frankenbergeret aI., 1996)]. Calculations With water wash, CEC = (NHt x 5)/18 where NHt = NHt in KN0 3 extract in milligrams per liter. CEC = cation exchangecapacityin centimolesof cation chargeper kilogram. With correctionfor entrainedsolution, CEC = 0.2775 x NHt - 0.80 X

VEn

where NHt = NHt in KN03 extractin milligrams per liter. VEn = volume of entrainedsolution in milliliters

SUMNER & MILLER

1220

AEC =0.14 x Cl- - 0.8 X YEn

where CI- =CI- in KN03 extractin milligrams per liter AEC = anionexchangecapacityin centimoles ofanionchargeperkilogram Comments

Whenwaterwashesare usedinsteadof estimatingthe volume and concentration of the entrainedsolution, the soil may begin-to disperse,and a higher speedcentrifugemay be necessaryto separatethe phases.The concentrationof 0.04 M NH4CI wasselectedas that which would preventthe deflocculationof the clay in mostsoils. It is in the middle of the rangeusedby Matsue& Wada(1985). The valuesof CEC obtainedat this concentrationare similar to thoseobtainedin the compulsiveexchangemethodat a concentrationof 0.001M BaCI2• Grove et al. (1982) suggestedthat the net chargeon the soil was given by the value for CEC obtainedusingthe water wash,which wasessentiallyequalto the valueobtainedwhenthe AEC wassubtractedfrom the CEC. This value alsowas equalto the sum of exchangeable cations(ECEC). AmmoniumAcetate(pH 7) Method Introduction

Although this methodhas beenusedfor many years,it overestimatesthe "field" CEC of soils with a pH !cation exchangecapacity in soils which do not containsaltsand carbonates.

Apparatus As listed under"Preparations"for "Cation ExchangeCapacityof All Other Soils," or "Apparatus"for "UnbufferedSalt Extraction Method."

Reagents As listed for "Reagents"for "Cation Exchange Capacity of All Other Soils," or "Reagents"for "Unbuffered Salt Extraction Method."

Procedure DetermineCa, Mg, K, Na andAl by atomic absorptionspectrometryasoutlined in Chapters 19 (Helmke & Sparks, 1996), 20 (Suarez, 1996), and 18 (Bertsch& Bloom, 1996).

1222

SUMNER & MILLER

Calculations Calculateexchangeableions (W+) in centimolesof cation chargeper kilogram as M"+

=(M"+ x Vx n)/(WxA)

whereM"+ = concentrationof cation in extract in milligrams per liter V =volume of extract(mL) n = valenceof cation W =weight of soil (g) A = atomic weight of cation ECEC = Ca + Mg + K + Na + Al

where Ca =exchangeableCa in centimolesof cation chargeper kilogram Mg =exchangeableMg in centimolesof cation chargeper kilogram K =exchangeableK in centimolesof cation chargeper kilogram Na = exchangeableNa in centimolesof cation chargeper kilogram AI = exchangeableAI in centimolesof cation chargeper kilogram

Comments For soils containingsalts and carbonates,this procedureresults in highly inflated valuesfor cation exchangecapacitybecauseof the appreciablesolubility of thesematerials in the extracting solutions. For all other soils, the values obtainedare very similar to those measuredby the methodsdesignedto determine the CEC at "field pH" as describedin "UnbufferedSalt ExtractionMethod" and "CompulsiveExchangeMethod" above.This agreementis to be expectedon theoreticalgrounds. Over a wide variety of soils, valuesfor the sum of basic cationsextracted by a varietyof extractantssuchas 1 M NH 40Ac, 0.2 M NH4CI, 0.2 M BaCI2> 0.2 M CaClz, 1 M BaClz-TEA and 0.01 M SrClz (Bache, 1976; Grove et aI., 1982; Gillman & Hallman, 1988; Hendershot & Duquette, 1986; Matsue & Wada, 1985) plus AI extractedwith 1 M KCI or 0.2 M NH 4CI [Skeen & Sumner, 1967a,b;Chapter18 (Bertsch & Bloom, 1996)] were essentiallythe same,indicating that almost any extractantis suitablefor estimationof ECEC.

MEASUREMENT OF SELECTIVITY COEFFICIENTS Introduction A wide rangeof methodsand computationalapproacheshavebeenusedin measurementsof selectivity coefficients,despite a common objective: to bring solutions containing varying ratios of two competing cations into equilibrium with an exchangerphaseat fixed pH and ionic strength,and to measureboth solution and exchangercompositions.In arriving at a method to suit a particular objective,severalchoicesmust be made:

CATION EXCHANGE CAPACITY & EXCHANGE COEFFICIENTS

1223

ExchangerPhase

In using referenceclays or standardexchangermaterials,size-fractionated suspensionsare typically preparedfor use in the exchangeexperiments.With soils, whole soil materialmay be used,or the colloid fraction separatedandhandled as a suspension. SaturatingTechnique

Homoionicclaysor soils may be useddirectly in the exchangeexperiment by addingmixed salt solutionsand allowing exchangeto occur. Or, the exchanger may be preadjustedto a fractionalcoverageof onecationby washingwith concentratedsolutionscontainingactivity ratios (AR) equivalentto desiredcation equivalentratios on the clay. Using a homoionicexchangeris more convenient, but requirescarefulselectionof solution ionic strengthand solution/soilratio, in orderto providesufficient solublecationsto exchangewith the homoionicclay. Rangeof SurfaceCoverage Most studieswill attemptto spanthe entire rangeof 0 to 100% coverageof the exchangerwith eachcation; however,choiceof a more limited rangeof interest is acceptable,althoughwill not allow computationof Keq (see"Formulation of ExchangeReaction"). Determinationof ExchangeableCations After equilibrium is attained(2-12 h), cationsheld on the exchangermay be measuredby displacementwith an addedsalt, with or without a washingstep to removeentrainedsolution.Or, total dissolutionof the clay may be performed, after accountingfor entrainedsolution and the presenceof cations in the clay matrix (Sposito et at, 1981). Alternately, at low surfacecoverages,the disappearanceof one cation from solution may be usedas a measureof adsorptionif a homoionicexchangersaturatedwith the competingion is used(Milberg et aI., 1978). ComputationalMethod Any or all of the exchangecoefficientsmentionedearlier(see"Formulation of ExchangeReactions")may be computedwith the soluble and exchangeable cationdata.Activity correctionsshouldbe madeif the competingions are of dissimilar valance,or appreciableS04 or other complexinganions are presentin solution. The following methodis an exampleof a combinationof the abovechoices used in published coefficient determinations.Possible modifications are discussedat the end of the methoddescription. Use of Homoionic Clay Fractionsto Determinethe Vanselow Selectivity Coefficient

This method, modeledafter Spositoet ai. (1981) and Sposito& Fletcher (1985), uses20 g L- 1) homoionic suspension containingat least0.1 M salt to preventhydrolysisand clay dissolution.

Procedure Ten or moremixed solutionsof the cationsspanningthe rangeof 1 to 100% exchangercoverageare suggested,with at least three replicatesof each. In formulating solution concentrations,assumenonpreferentadsorptionand use Fig. 4~1 to choosesolution equivalentratios (E) that yield roughly evenly spaced exchangerequivalentratios(E). The formulas in Table 4~2 may then be usedto calculateconcentrationsfor the individual cations at eacht and a given ionic strength(1). Sufficient solution cationsmust be addedto the homoionicclays to ensure that exchangedoesnot overly depletethe solution of one cation. Choiceof mass of addedclay and concentrationand volume of salt addedare the importantvariables. For example,in equilibrating a calcium-saturatedclay with a 100% Na solution, the total equivalentsof Na addedshould be 6 to 10 times the equivalents of exchangeableCa. Thus, 0.25 g of clay with a CEC of 50 cmole kg-1 contains 0.125 cmole Ca2+, 50 mL of 0.025 M NaCI contains1.25 cmole, providing a 10:1 ratio of addedsolution cationsover thoseinitially exchangeable. Before use, the clay suspensionshould be washedto a lower solution I «0.01 M), and the clay concentrationcarefully measuredby drying an aliquot at 105°C; the salt contentalso should be measuredin the solution phase.Pipetting from a vigorously stirred suspensionmay be used to dispensethe clay into weighedcentrifugetubes(50 or 100 mL) after computingthe volume neededto deliver the neededmassof clay. Water and concentrated(i.e., 0.5 M) salt solution of the individual cations are then addedto achievethe required concentrations computedfrom Table 4~1. Rememberto accountfor the cation transferredwith the clay suspensionin the calculations. After equilibration (typically with shaking,8-24 h), centrifuge the tubes and fully decantthe solution phaseand savefor cation analysis.Weigh the tubes to determinethe volume of entrainedsolution, then displacethe exchangeable cationsby addinga volume of concentrated(0.2-1.0M) salt containinga different cation than the two of interest (see "Problems in Measurementof Cation Exchange Capacity"). Corrections should be made for the contribution of entrainedsolution to the cationlevelsin the displacementsolution(see"Problems in Measurementof Cation ExchangeCapacity"). Table 40-2. Solutionequivalenceconditions,ionic strength,and concentrationformulas forexchange reactionsof Cationsi at a given solution equivalentratio (E) and ionic strength(l).:f: Type of exchange Equation

mono)-monoZ

Equiv. condition Ionic strength Solve for Cj, Cm

E =c;I(c) + cz)t j

I

=c) + Cz

cj=EI

t Cj is expressedin moleslliter.

Ej =2cd(2c) + 2cz) =Cj /(c) + cz) 1= 3c) + 3cz Cj

=E;I/3

Em =cm/(cm + 2cd)

1= cm + 3Cd

cm = zEml/(3 - Em)

:f: c =concentration,d =divalent cation, m =monovalentcation; anionsare assumedmonovalent.

CATION EXCHANGE CAPACITY & EXCHANGE COEFFICIENTS

1225

Determinecationsin the equilibrium and displacementsolutionsby atomic absorption spectroscopy or othersuitablemethod[seeChapters19 (Helmke & Sparks,1996) and20 (Suarez,1996)]. Extremecareshouldbe exercisedin these analyses,including the use of standardsmade in matricesto match samples,in order to obtain the most accurateresultspossible. Calculations Solution concentrationsmay be expressedin moles per liter or millimoles per liter (mmol L- 1); activity ratios and coefficientsfor mono-divalentexchange will differ by a factor of 31.6 (i.e., -V1000) when computedusing thesetwo sets of units. Exchangeableions may be computedas mole fractions,using the sum of the moles of exchangeableions as denominatorsfor M;, or as equivalentfractions. For mono-monovalentor di-divalent exchange,Ky may be calculated directly from [17] using free concentrations(i.e., correctedfor complexationusing the approachof Sposito& Mattigod, 1977) of cationsi andj ratherthan activities,sincethe activity coefficients of the two cations are equal. For mono-divalentexchange(i.e., Na-Ca), [18] individual'Y valuescan be calculateddirectly using eqs.4-6, or solutionsmay be speciatedusing geochemicalspeciationprogramssuch as GEOCHEM (Sposito & Mattigod, 1977) or MINTEQ (Allison et aI., 1991). The Ky valuesobtainedover a range of M; may be plotted as In Ky vs. M; to examine the dependenceof Ky on exchangercomposition. Integrals for the curve will ~ield Keq. Plots similar to Fig. 40-1 also may be made after computing E; and E; and comparedwith the nonpreferenceisothermsshown. Comments Rieu et al. (1991) have pointedout the large analyticalerrors in determination of selectivity coefficients,stemminglargely from uncertaintyin exchangeable cation measurementsat low M; or E;. They recommend an empirical approachto relating E; and E; ratherthan a thermodynamicone due to their estimatesof 15 to 25% error in computedK values;adequatereplication (3-5), conscientious measurements,and careful choice of initial solution concentrations will minimize theseproblems.Overall error should be computedfor eachset of replicatesas a standarddeviation in K, and reportedor usedin statisticaltesting. Many variations onthe proposedprocedureare possible,dependingon personal preference,experimental objectives, etc. Whole soils may readily be employed,with soil masschosenbasedon the expectedCEC of the clay fraction.

SUMNER & MILLER

1226

Samplingfrom soil containersshould be done carefully to avoid samplingerror, and salt releaseshould be checkedby analysesof selected equilibriumsolutions for releaseof cations(K, Mg, AI) from the solid phase. It is possibleto preadjustM j to a value close to that expectedat equilibrium for eachsolution compositionby presaturatingthe solid phasewith concentrated salt solutions. This eliminatessubstantialchangein composition of the solution phaseafter equilibration. The concentratedsolutions(0.5 M), chosento span therangeof M j of interest,are subsequentlyreplacedwith more dilute solutions of the sameactivity ratio [i.e., (Na)/(Ca)l/2 for Na-Ca exchangeusing the Gaponequation] until the final desiredionic strength(typically 0.05-0.01M) is reached.This approach has advantagesin that a wider and morepredictablerange of E j is obtained,comparedto beginningthe exchangeprocesswith a homoionic clay. However, the processis considerablymore time-consumingin that each tube must go through numerous washingstepswith the progressivelymore dilute solutions.Details of this method are describedby Levy et al. (1988) and Miller et al. (1990). Any of the otherexchangecoefficientsdiscussed(Gapon,Gaines-Thomas) may be computedusing the dataobtained,in order to compareresultswith other suchvaluesin the literature. However, it should be noted that much early work did 'not correctsolution concentrationsto activities, and may in somecasesthereby be in error. Anion exclusionduring selectivity coefficient measurements hasthe potential to reduceCEC, particularly with high-chargeclays (>50 cmole kg-I) at equilibrium ionic strengths

-200

,g c '0'"

-100

E

c

CL

0 10'

::::;

10°

c

10-'

~

0

~ C

'" V

C 0

U

10-2 10-3

d

10-4 10-5 0

20

10

30

40

Distance from surface (nm)

Fig. 41-1. A negativeconstantchargesurfacewith a diffuse double layer (ion concentration=0.001 M, ion valence=1, surfacecharge=-1.11 mole kg-I): a) distribution of positive (counterion)and negative(coion) ions at the particle-solutioninterface, b)variation of chargewith distancefrom the surface,c) variation of electric potentialwith distancefrom the surface,d) variation of cation and anion concentrationwith distancefrom the surface.

called the double layerthickness.Double layer thicknessis an indication of ion distribution from the surfaceand is important in determiningcolloidal stability. Thus the concentrationof counterionsdecreases withincreasingdistancefrom the surfacewhile the concentrationof coions increases.The diffuse layer charge density ( K., IL, C" C2, Inner- and outer- Through GCSGi and electrolyte Kcat, KAm Ksi sphere

(gWl)

(:5:0.01) Applicable to variable[, suffersconstraintproblems

t Nt = site density; K_ = equilibrium constantfor

SOH = So- + W; K. = equilibrium constantfor SOH + W =SOH·;Ksi =equilibrium constant(s)for cationor anion adsorbatebinding (e.g., Reactions 1-4); C" C2 capacitances; andKcat> KAn TLM equilibrium constantsfor outersphereelectrolyte binding. :j: GCSG = Gouy-Chapman-Stern-Grahame charge potential relationship: -00 = ad = -0.1174 ~[

=

=

sinh(zFljld/2RT). § Hayeset al. (1991).

The SCM have found wide application in the descriptionof the acid-base propertiesand adsorptionbehaviorof inorganic and organic cations and anions on a variety of single phasesorbentsrelevant to soils including iron and aluminum oxides and 1:1 and even2:1 phyllosilicates(see comprehensiverecent reviews by Davis & Kent, 1990; Goldberg, 1991a). The successof SCM in describingconditionsresultsfrom the validity of the assumedsurfacecomplexation process(i.e., conformationto massaction and massbalancelaws) and wide flexibility in terms of adjustableparameters,rather than from the mechanistic accuracyof the interfacial structureassumedby any of the SCM. Severalauthors (Westall & Hohl, 1980;Hayeset al. , 1991) havedemonstratedthat variousSCM are equally capable of fitting acid-basetitration data, although different but nonuniquevaluesfor analogousparameterswere requiredby the variousmodels (Dzombak& Morel, 1987).Indeed,this ambiguity hasled someto caution(Johnston & Sposito, 1987; Goldberg, 1991) that surface complexation modeling, alone, cannotbe usedto supportmechanisticconclusionsregardingsurfacespeciation or interfacial structureand that their strengthlies more as a heuristictool to gain chemical insight (Charlet & Sposito, 1989) and visualize the effects of complexsimultaneousequilibria. Certain stepsare normally taken in the application of a SCM to a single phase sorbent/sorbatesystems(Fig. 44-1). Parameterestimation is the most involved part of the procedure.It is critical to recognizethat the adjustablemodel parameters(Table 44-1) are poorly constrainedand show great interdependence (Dzombak& Morel, 1987). Site density(i.e., numberof reactivehydroxyl sites/ m2 of surface,Nt) has beendeterminedby crystallographiccalculations,tritium exchange,and adsorptionmeasurements (seeJames& Parks,1982,for methods; and Davis & Kent, 1990for a summaryof valuesrelevantto soil minerals).Common valuesreportedfor soil mineralsare summarizedin Table 44-2. Equilibrium constantsfor surface reactionsare determinedsuccessively from material balancedatawherethe reactivity of a single componentcan be isolated (i.e., H+, M"'+, or LI-), beginningfirst with ionization reactionsand subse-

1317

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

Select SCM DLM CCM TLM OIher

Select or Measure N, Crystallography Trttium Exchange Adsorption

Acld-8_ Tltntion Data Detannine 1 K s2' Ks3 optimizedon one soil over differentpH ranges

Ksi

From aluminum-goethite From Al-goethite

Aluminum, iron oxides§

Aluminum, iron oxides§ EGME surfacearea Optimizedwith Ksi Packingarea(empirical Aluminum, iron oxides§ partitioning into two sites N2 gassurfaceareaof Optimized with Ksi soil

Packingarea:j:

S

t qmax = sorbateadsorptionmaxima. :j: S = qrnax N Aa whereN A is Avagadro'snumberand a is the sorbatepackingdensity. § Averageof averagevaluesreportedfor iron and aluminum oxides-K+ (int) =107.35; K_ (int) = 10-8/ 95. ~ ~ = differencein Nz surfaceareabeforeand after extractionwith citrate-dithionite-bicarbonate.

Zacharaet aI., 1989

Goldberg& Glaubig, 1988b AsOl-

Goldberg& Sposito,1984

15 soils Surfacesoil

Model

Goldberg& Glaubig, 1986 B(OH)4' SeOiSpositoet aI., 1988

Solid

CCM

PO]-

Sorbate

44 soils

Anions

Citation

Table 44-3. Applicationsof SCM to anion adsorptionon soils and heterogeneous natural sorbents.

~

I,C

.... ....

~

~ ~

==

~

~

rJ'J

Z

c;1

-

~

~ ::

~

c::

g

Cd

Mollisol-smectite clays

Extraction#

N2 surface area

Fit to experimental data From kaolinite, aluminumgoethite From aluminum and From aluminum and silicon oxide silicon oxides

From amorphous Fe203° nH 20 From kaolinite, aluminumgoethite

Extrapolationfrom experimentaldata

Extrapolationand regression:j: Extrapolation

Ksi

Extrapolation ion retention From arnorphous§ From amorphous§ Fe203° nH20 Fe203° nH20

K±

Acid-base Extrapolatedfrom titration experimentaldata

600 m2/g Fe203° nH 20

NR## titration,

t

S

700 m2/g Fe203° nH20 a) surfaceareatt t b) extraction

Acid-base titration

Multisite NEM [kaolinite aluminumgoethite Surfacearea~~ TLM for smectiteedges

TLM

CCM

Extraction~

Acid-base titration Acid-base titration

Nt

t Surfaceareanot requiredin the NEM. :j: Authors noted the following relationshipthat was usedto estimatebinding constantsfor Pb, Ni, Ca, and Mg-log X Brurface =0.945 log x BfydrOlysis + 5.6. § constantsfrom Dzombakand Morel (1990). ~ Nt = Fe(ex)[g/kg)°moles sites/g-Fe203° nH20(am) where Fe(ex) is from NH20H ° HCl extraction. # Sameas footnote ~ but ammoniumoxalateusedas extractant. tt Nt.kaolinite =Ns.kaolinite ° N2 surfacearea,where Ns is the reportedhydroxyl site density on kaolinite. :j::j: sameas Footnote~ exceptCDB extractionused. §§ TLM linked with ion-exchangehalf-reactionapproachfor multisite/multireactioncalculations. ~~ Sameas Footnote:j::j: but the site density of smectiteedgesNs.smectitewas used. ## Not requiredin this application.

Zacharaet ai., 1992

Cowanet ai., 1992

Payne& Waite, 1991

Osaki et ai., 1990

DLM

Pb, Zn, Ni, Cd, SandyWisconsin Cu aquifer sediment Fe(III), Co, Zn Freshwaterand estuarine sediments U Weatheredschist "ultisol like" Cd Ultisol clay fractions

NEM

Model

Loux et ai., 1989

Solid

TLM

Sorbate(s)

Mouvet & Bourg, 1983 Cu, Zn, Cd, Pb, River sediment Ni, Ca, Mg Charlet& Sposito,1987 Na oxisol

Citation

Table 44-4. Applicationsof SCM to cation adsorptionon soils and heterogeneous natural sorbents.

8

~

= Ro

~

N

=

.... c.>

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1321

(Anderson& Benjamin, 1985, 1990; Lovgren et aI., 1990). Consequently,SCM have beenapplied to soils with assumptionsregardingthe natureof the sorbing surfaceand its properties.It is often assumedthat (i) iron/aluminum-oxidesor layer silicatesare the primary sorbentsin soil, and (ii) that the acid basebehavior of thesesoil sorbents(K+, K_), or functional groups,can be approximatedby their single-phaseconstants(Tables44-3 and 44--4). Total reactivesite concentrations(Nt) havebeenestimatedfor anionsfrom sorptionmaxima(qmax) at lower pH (Table 44-3). A different procedurehas been usedfor cations(Table 44--4) becauseprecipitation of hydroxide or carbonatesolids may confound sorption at the higher pH values that are required.Total reactive maxima measurements site concentrationsfor cationshave beendeterminedby estimatingthe mass/surface areaof the sorbing phasein the soil by extractionor surfaceareameasurement and equatingthis directly to the site densitiesreportedfor well-studiedreferencephases(e.g., Fe203• nH20, or KGa-1). THERMOCHEMICAL DATA Data Sources All geochemicalequilibrium models that use the equilibrium constant approachto predict the equilibrium composition rely on thermochemicaldata basesconsistingof equilibrium constantsfor aqueouscomplex species,solubility productsfor solid phases,and the redox potentialsfor variousoxidation/reduction reactions. These data are generally tabulatedfor zero ionic strength and ambient temperatureconditions (25°C). Typically these are experimentaldata obtained from the standardcompilations of equilibrium constants(Martell & Smith, 1976) or computedfrom listings of standardchemicalpotentialsfor various species(Wagmanet aI., 1982). Some of the equilibrium models also may include estimatedthermochemicaldata for known aqueousand solid speciesfor which reliable experimental data is unavailable (Mattigod & Sposito, 1977; Langmuir, 1979).Typically, the geochemicalequilibrium models contain data basesthat containequilibrium constantsfor hundredsof reactions.In many cases thesedata may not be internally consistentor reliable. Therefore,it is important for the model user to verify that the subsetof thermochemicaldata for reactions of interestare both reliable and consistent. Verificationof thermochemicaldata setscan be conductedby following the generalstrategiesdescribedin detail by Nordstromand Munoz (1985).

Nonideality Corrections The thermochemicaldata basesfor equilibrium models contain data that are relevant to zero ionic strength. Becausereal soil systemshave finite ionic strengths,the constantsin the databaseshave to be correctedto reflect this nonideality. The nonideality correctionfor eachthermodynamicequilibrium constant is conductedby using activity coefficientsfor dissolvedspeciesinvolved in that specific reaction.For instance, the thermodynamicequilibrium constantfor a reaction

1322

MATIIGOD & ZAeHARA

Ca2+ + HCOj"

=CaHCOj

[14]

is representedby K

=(CaHCOj)/(Ca2+)(HC03)

[15]

and the quantities within parenthesesrepresentsthe activities of dissolved species.The activities of eachof thesespecies(a;) is related to their respective concentrations(m;) and activity coefficients(Yi) by the relationship

[16] Thereforefor Reaction[15] K = (CaHCOj)/(Ca2+)([HCOj"D =

[CaHCOj]YcaHCOi[Ca2+]'Yca[HCOj"]HCOJ

[17]

where the quantitieswithin the bracketsrepresentconcentrations.The thermodynamic equilibrium constantis relatedto the activity coefficientsby the relationship,

K =Kc YCaHCO/YCaYHC03

[18]

Where the conditional constantKc = [CaHCOj]/[Ca2+][HCOj"]. Therefore the nonideality correction for K for this reaction involves computing the activity coefficientsof eachof the relevantaqueousspecies.Typically theseactivity coefficients for ionic species(Yi) are computedby the Daviesequation log Yi

=-AZ7[ {/1I2j(1 + /112)} -

0.31]

[19]

WhereA is a constant,Zi is the valenceof the species,and I is the ionic strength of solution (~0.5 mollL) calculatedby the relationship,

/ =1/2 I. m;Zr

[20]

Where mi is the molar concentrationof each ionic species.The activity coefficients of neutral aqueousspecies are computed by the expression(Sposito, 1981b) log 'Yi

=0.11

[21]

For solutions of ionic strengthsexceeding0.5 mol/L, extendedforms of the Debye-Huckelexpressionsuch as the Pitzer equation are used to compute the activity coefficients(Pitzer, 1973).

Temperature Corrections Both temperatureand pressureaffect the eqUilibrium. Small pressurevariationsthat occur within soil systemsdo not cause significant changesin equilib-

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1323

rium. Changesin temperaturehowever,causemeasurablechangesin equilibrium. Generally,if the temperatureof a systemvarieswithin ± lOoC of the ambienttemperature(25°C), the temperaturedependence of the equilibriumconstantmay not exceedthe uncertaintiesof the value listed for the ambienttemperature.Predicting equilibrium at temperaturesbeyondthis rangerequirescorrections tobe made for equilibrium constantslisted for 25°C. The temperaturedependenceof an equilibrium constantis relatedto otherthermochemicalparametersby the expression

olnK/OT = (-l/R1)(oMr/01) + (Mr/R'f2) + (l/R)(M.s%1)

[22]

where l!J{0, llSo, R, and T are the standardenthalpy,standardentropy, gas constant,and the temperatureof reaction,respectively.For narrow rangesof temperaturesthe first and the third termson the right-handside of Eq. [22] becomenegligible quantitiesand the resultingexpressionis the well-known Van't Hoff equation. Use of this simplified expressionfor larger temperatureranges leads to appreciableerrors becauseit ignoresthe temperaturederivativesof the standard enthalpyand standardentropy of reactions. ThereforeintegratingEq. [22] resultsin an expression(Eq. [23]) (Mattigod & Kittrick, 1980) that includes the temperaturedependenceof these thermochemicalparameters.

InKr

=(Tr/T)lnK298 + [(llSO 29s1RT)(T -

Tr)] -

[(l/R1) (LlCOp)OT] + (l/R) LlCJ1)OT

[23]

whereLlCop is the heatcapacityof reaction.The temperaturedependenceof reactions can be more accuratelyevaluatedif the standardentropy and heat capacity for datafor all reactantsand productsare available.Mattigod and Kittrick (1980) provided a detaileddescriptionand examplesfor evaluatingtemperaturedependent equilibria for soil systems.

GEOCHEMICAL EQUILffiRIUM MODELS USED IN SOIL CHEMISTRY Thereare a plethoraof geochemicalequilibrium computercodescapableof predictingequilibrium speciationfrom inputs of measuredtotal concentrationsof various components.A detailed history, description, and pedigree of various codesbeing usedin various geochemicaldisciplineshave beenreviewedby several authors(Nordstrom& Ball, 1984; Waite, 1989; Bassett& Melchior, 1990, p. 2-14). However,only a limited numberof suchcodeshavefound widespreaduse amongsoil chemists.The principal reasonfor such selecteduse of certain codes appearsto be the modificationsand improvementsthese codes contain that make them particularly suitable for equilibrium calculations involving soil systems. Following are brief descriptionsof someof thesecodes.

MATIIGOD & ZACHARA

1324

GEOCHEM This equilibrium code was developedas a modified version of another equilibrium code REDEQU (Mattigod & Sposito,1979). The GEOCHEM differed from REDEQL2 in including: (i) additional thermochemicaldata that are relevantto soil systems,(ii) a methodfor describingcation exchangereactions, and (iii) a mixture model and a quasiparticlemodel to predict metal-DOM interactions. Other important types of interactionssuch as dissolution/precipitation, and oxidation/reductionalso were includedin this code. Additional characteristicsof this code included ways to account for metastability of solid species including mixed solids, corrections for ionic strength, and the capability to predict changesin speciationas a function of changingpartial pressuresof CO2, N2, and O2 (via redox reactions)in the soil gaseousphase. The GEOCHEM usesthe eqUilibrium constantapproachto solvespeciation problems.The set of nonlinearalgebraicequationsresulting from massbalance for all the componentsprovideseqUilibrium speciationin dissolved,solid, and adsorbedphases,respectively.The codeoffers sufficient flexibility for the userto define a specificproblemby appropriatemodification of a numberof input parameters.Since its inception GEOCHEM has beena useful tool in understanding and interpretingexperimentalobservationsrelatedto solid and solution speciation and their effects on adsorption/ionexchange,nutrient uptakeby plants and to estimate the effects of changing important parameterssuch as pH, ionic strength,redox potential, and concentrationof selectedcomponentson speciation. Currently thereare two versionsof GEOCHEM that can be run on personal computers.One of theseversionsretainsthe data input format of the original versionthat can be editedor changedusing an ASCII editor. The secondpersonal computerversion of GEOCHEM (GEOCHEM-PC V 2.0) includes both an interactiveand data file mode of data input, and was developedspecifically for use in soil chemistryproblemsrelatedto plant nutrients(Parkeret aI., 1995). In addition to the mode of data input, other maJor modifications in this version include a thermodynamicdatafile that hasbeencustomizedby including a number of dissolvedspeciesrelevantto plant nutrition, and the eliminationof adsorption/ion exchangesubroutines(Parkeret aI., 1987). A numberof minor modificationsalso are included to enhancethe utility of this personalcomputer(PC) version. SOILCHEM The SOILCHEM was developedby Sposito and Coves (1995) with the same data base structure and computationalbasis as GEOCHEM. However, SOILCHEM differs from GEOCHEMprincipally by: (i) building the input file in an interactivemode; (ii) calculating theactivity coefficientsof dissolvedspecies in soil solutionsof ionic strengthapproaching2 mol dm-3; (iii) computingmetalDOM interactions through the improved Scatchardquasiparticlemodel; (iv) using activities (PH) insteadof concentrationsof protonsto improve the compu-

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1325

tational precision;and (v) incorporatingthe CCM (one of the SCM's) to predict adsorptionreactions.A compiledMacintoshPC versionof this codealso is available from the authors(Sposito& Coves,1990).

MINTEQ This codewas formulatedby Felmy et ai. (1984) combiningthe computational format of MINEQL (Westall et aI., 1976), and a data baseupdatedfrom WATEQ3 (Ball et aI., 1981). The MINTEQ also usesthe equilibrium constant approachto solve the speciation problems. The MINTEQ differs from other codesdescribedhere by allowing equilibrium computationsto be madeat temperaturesother than 298 K. This task is accomplishedby using the Van't Hoff equationor analyticalexpressionsrelatingthe equilibrium constantsas a function of temperature.However,as discussedpreviously,useof the Vant'Hoff equation for computing equilibria over large temperatureranges should be avoided. Adsorptionand ion exchangereactionscan be computedby a numberof options that have beenincorporatedin this code.Theseinclude the use of a distribution coefficient (Kd), a Langmuirian adsorptionmodel, a model basedon the Freundlich isotherm,a simple ion exchangemodel basedon selectivity coefficients for major cations,and SCM'ssuchas the ConstantCapacitance approach and the TCM for adsorption.A PC (IBM compatible)versionof MINTEQ also hasbeen compiled(Petersonet aI., 1987). Another PC version of MINTEQ is the MINTEQA2/PRODEFA2 code (Allison et aI., 1991).In this code,an interactivepreprocessor, PRODEFA2eases input file preparationwith extensiveediting capabilities.The MINTEQA2 has three significant features that distinguishesit from MINTEQ. These include, implementationof the model of Dobbs et ai. (1989) to simulate humate-metal interactions,the ability to perform calculationsat multiple pH values(pH scans), and improvedcapabilitiesto computeion exchange.

MINEQL+ This PC software(version3.00) wasdevelopedat EnvironmentalResearch Softwareas a userfriendly version of the chemicalequilibrium code, MINEQL (Westall et aI., 1976). This programincludesan user interface,an extendedthermodynamicdata basefrom MINTEQA1, and an on-screenmultiple run managing system.Adsorption phenomenacan be addressedwith anyoneof the three surface complexationmodels (CC, generalizedDLM, and TLM). One of the uniquefeaturesof this softwareis that the output canbe regulatedand displayed graphically. The MINEQL+ has beenset up so that the usercan easily manipulate and customizethe attachedthermodynamicdatabase. C-SALT This recently developedchemical equilibrium model was designedby Smith et ai. (1995) to predict speciationin solutionsof ionic strengthexceeding 0.5 moVL. The nonideality correctionsare computedby Pitzer equations.This

1326

MATIlGOD & ZACHARA Basic Sorption da..

t

Ilaction sorbed 01

TOTM _ 10·'

adsorpllon denslly

pH __ FITEQL Modeling

IlaII

grw:

TOTM

TOTM

FIDeM

FleeM

loglH'1

pH

K+, K., KSI

_

TOT SOH _

K+, K., TOT SOH _

KSI

-~

TOT SOH +/. s KSI ./. S

Fig. 44-2. Exampleof FlTEQL applicationto ion adsorptiondataon soil.

model containsonly a limited numberof componentsand is designedto predict the sequenceof formation of evaporitemineralsin soil systems.

FITEQL The FITEQL model developedby Westall and Morel (1977) and Westall (1982a,b)is a nonlinearleast squaresoptimization programthat can be usedto adjust parametersin a chemicalequilibrium model to fit experimentaldata.The codeis basedon the Gaussmethod(Gaizer,1979; Dzombak& Morel, 1990),and is flexible with many potentialusesin soil chemistry.The adjustableparameters may take various forms including equilibrium constantsfor solubility, surface and solution complexation,and ion exchangereactionsas well as total concentrations of componentssuch as surfacesites (X-, SOH). The program FITEQL and other such programs(e.g., NONLIN; Felmy, 1989) offer a methodologyto estimate equilibrium constantsand their associateduncertainty (Dzombak & Morel, 1987, 1990).The FITEQL may be usedto optimize parametersor to perform equilibrium calculationswith fixed equilibrium constantsand component concentrations(Westall, 1982a). The FITEQL containsfour adsorptionmodels(CCM, DLM, TLM, and the StemLayer model), and can readily be formulatedto perform ion exchangecalculationsvia the half-reactionapproach.The Daviesconventionis employedfor single-ionaqueousphaseactivity coefficients.The FlTEQL hasbeenusedwidely for the calculationof KJK. of mineral solids from alkalimetric titration data, and sorptionconstantsfor surfacespecies,K si . A typical applicationof FITEQL to soil chemicalexperimentaldata is shown in Fig. 44-2 for the adsorptionof a metalcation (M) by soil material.Adjustableparameterssuchas TOTSOH or Ksi for multiple surface speciesmay be optimized separatelyor simultaneously, althoughexperiencehasshownthat FlTEQL convergenceis lesslikely for more than two adjustableparameters.The FITEQL will not convergeusing the TLM if KJK. andKeatlKAn are adjustedsimultaneously,becausethe reactionsare interdependent.A thoroughand insightful discussionof the useof FITEQL for extraction of equilibrium constants forsurfacecomplexationreactionsmay be found in

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1327

Dzombakand Morel (1990).A listing (Table 44-5) showssomesampleapplications of FITEQL to complexationand adsorptionproblemsrelevantto soil. The FlTEQL providesoutputon the goodnessof fit of the optimizedmodel parametersto the experimentaldata and the standarddeviationof eachadjusted parameter.An overall varianceof approximatelyone is indicative of good fit, with a rangeof 0.1 to 20 beinggenerallyacceptable(Westall, 1982a,b;Dzombak & Morel, 1990). The variance(Vy) is dependenton user input estimatesof the absolute[sCabs)]andrelative[s(rel)] experimentalerrorassociatedwith eachtype of input data(e.g., pH, free metal concentration,total metal concentration,etc.). Error estimatesrecommendedby Dzombakand Morel (1990) for applicationof FITEQL to acid-basetitration dataof solids and metal ion sorptionexperiments are summarizedin Table 44--6. EXAMPLES OF EQUILIBRIUM COMPUTATIONS SpeciationCalculationwith GEOCHEM·PC

All examplesof speciationcalculationswereconductedusingGEOCHEMPC Version2.0 (Parkeret aI., 1995).Typical examplesof speciationcomputations will be illustratedfor a systemof sevencationiccomponents(Ca, Mg, Na, K, Cu, Zn, and H) and six anionic components(C03, S04' CI, P04, N03, and OH). The total concentrationsof the componentshave beenselectedto representa typical soil solution composition.The examplesare intendedto show different types of equilibrium computationsrangingfrom simple solution speciationto more complex redox speciationand naturalorganiccomplexation. Solution PhaseSpeciation

One of the commonproblemsin soil chemistryinvolves computingspeciation of dissolvedspecies.The neededdata for suchcomputationsare the total concentrationsof all the componentsof interest.As a rule for a soil system,all the major cationic and anionic componentsalways should be included because thesecomponentscontributesignificantly to the ionic strengthand speciation.If equilibrium computationsinclude trace components,it also is necessaryto include all tracecomponentsthat are presentin concentrationscomparableto the tracecomponentsof interest.Becausespeciationinvolves equilibria betweenall components,omitting componentswill fail to provide a completeand accurate picture of speciation. The total concentrationsof componentsusedin theseexamplesare shown in Table 44-7. The datainput and outputformats are not describedherebecause they are describedin the manualsof eachof the equilibrium codes.Typically, the output may include: (i) a table that echoesinput data,the initial guessesfor ionic strengthand free ionic concentrations,the numberof metals,ligands, complexes and solids involved in the computation;(ii) a list of relevant thermochemical data, numberof iterations;(iii) computedionic strength,free and total concentrationsof eachcomponent;the chargebalanceof the solution; (iv) a tabulation of stoichiometryand concentrationsof various complexes;(v) free concentra-

Kaolinite

Surfacesoil Ultisol subsoil Fe203·nH20 CaC03

FeOOH

FeOOH

ZnS

Schindleret aI., 1987

Goldberg& GJaubig,1988b Zacharaet aI., 1989 Dzombak& Morel, 1990 Zacharaet aI., 1990

Goldberg,1991

Hayeset aI., 1991

Ronngrenet aI., 1991

Nat TLM

Adsorption model

H+, Zn2+

H+

Various anionic sorbates

OptimizedK.i for metal binding to edgeSOH, and K Mex for cation exchange

Optimized stability constantsand discreteligand concentrations Multicomponentadsorptioncalculations

Use of FITEQL

OptimizedKsi for severalAs surfacespecies Optimized SOHT for hypothesizedsoil aluminum-goethitesorbent K± for Fe203• nH20, extensiveself-consistentdatabasefor K.i Optimized half-reactionconstants(KMex) for exchangeagainstCa (Kcax) CCMandTLM OptimizedK si , use of Vy 2 to distinguishbetweenpotentialinnerand outer-spherecomplexationmechanisms Sensitivity of K± to different input values,error estimates,data TLM,CCM groupings CCM Optimized log~oh 10g~_Il' log~101§

CCM and halfreaction exchange CCM As04 TLM Cr04 Many metal cations/anions DLM Ni, Zn, Cd, Co,Ba, Sr Half-reaction

H+, Cu Cr01-, HCOj", S01-, Ca, Na Cu,Cd,Pb

Solute

§ Formationconstantsfor the generalequilibria pH< + qZn2+ + r(-SZn) =HpZnq(-SZn)p'+Zq)+

:j: Vy = SOS/DF

=not applicable.

Aquatic fulvic acid Fe203·nH20

Fish & Morel, 1985 Zacharaet aI., 1987

t NA

Sorbent

Citation

Table 44-5. Examplesof FITEQL applicationrelevantto soil chemistry.

~

~

Ro

t:I

8

~

§

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1329

Table 44-6. Error estimatesfrom acid-basetitration dataand sorption data(from Dzombak& Morel, 1990). Measurementt

S[rel]:t:

XH

0.05 0.10 0.01 0.05 0.01 0.05 0.01

XH TH

XM TM

XA TA

S[abs]§

Remarks

0.0 0.0 0.01 x min TH 0.0 0.01 x min TM 0.0 0.01 x min TA

±0.02 pH unit ±0.04 pH unit ±0.02 pM unit ±0.02 pA unit

t H = hydrogenion (W); M = cation; A = anion. :t: SH[rel] =0.05 usedfor pH measurements in sorptionexperiments;SH[rel] =0.10 usedfor pH measurementsin titration. § For b log", Sb[abs] 0.434Sa[ref]; seeWestall (1982a)or Skoogand West (1982, p. 75-80).

=

=

tions of componentsand the total concentrationof complexesbetweeneachmetal and ligand; (vi) a table that lists free and complexconcentrationsas the percentage of total concentrationof eachcomponent;(vii) tablesof interaction intensities, and capacities(Sposito & Mattigod, 1980); and (viii) a list of conditional constantsfor variousspecies. In this example,speciationwas computedin the solution phaseassuming that the systemwas closedregardingtransferof massbetweenthe solution phase and the solid and gas phases.Selectedoutput from GEOCHEM-PC,described under (i), (iii), (iv), (v), (vi), and (viii) from this computation are tabulated (Tables44-8 to 44-13).The first sectionof the output(Table 44-8) lists the input data and the numberof metals,ligands, complexes,and solids that are included in the computations.In this example,99 complexesresulting from the interactions betweenseven metals and six ligands were considered.No solids were included because it wasspecifiedthat the computationsmust consideronly aqueous speciation. The secondtabularoutput(Table 44-9) lists the total andthe free ionic concentrations,the free ionic activities of eachmetal and ligand, and the remainder values.Theseremaindervaluesare computedas the differencebetweenthe total input concentrationsand the sum of computedconcentrationsof all speciesof eachcomponentdivided by the total input concentration.This value must be less than 10-4 for the computationto be convergent.Additional data (Table 44-9) includes thecomputedionic strength,the chargebalance,and the alkalinity of the solution. Table 44-7. Total concentrationsof componentsusedin illustrative computations.t Component

pCT

Ca Mg K Na Cu (II)

2.43 3.22 3.00 2.38 5.76 4.90

Zn

t pH = 7.90; pCT = -log (molar concentration).

Component

pC, 2.59 3.00 2.70 4.66 2.60

MATIIGOD & ZACHARA

1330

Thble ~. Listing of input dataandcomputationalconditionsfor GEOCHEM-PC. The conditionsfor the different casesare: Metal Ca

Mg K Na

eu2• Zn

Ligand C03 S04 Cl P04 Fixed pH

Case1 -

Codeno.

GUESS

1 2 4 5 9 12

3.500 3.700 3.500 3.000 6.000 6.000

Codeno.

GUESS

Case1

1 2 3 9

4.000 3.000 3.000 5.000

2.590 3.000 2.700 4.660 7.900

2.430 3.220 3.000 2.380 5.760 4.900

t All simple solid phasesare disallowedfor this run. Maximum iterations= 50; Convergencecriterion =1.oooE-04.

The concentrations(-log molar) and the stoichiometryof complexesare listed in Table 44--10. For instance,the first line of the table lists two Ca-C03 complexes.The stoichiometryof the first complex (1 1 0) indicates that this complexcontainsone mole of Ca and one mole of C03 but doesnot containany H or OH. Thereforethe complex is CaC03° and the concentrationis 4.596 (-log molar). The stoichiometry of the second complex (1 1 1) indicates that this speciescontains one mole of each of Ca, C03, and H thus identifying it as CaHCOj. The stoichiometryof hydrolytic speciessuch as CaOH+ are indicated asl 0-1. Another tabularoutput(Thble 44--11) lists free concentrations(-log molar) of eachmetal and ligand (this is the repetitionof datacontainedin Table 44--11) Thble 44-9. Free metalsand ligandswith ionic strength=1.553 x 10-2 (computed)and fixed pH of 7.900. -log total Total concenIonst,:j: concentration tration Ca Mg K Na Cu2• Zn C03 S04 Cl P04 N03

3.715 x 10-3 6.026x lQ-4 1.000X 10-3 4.169x 10-3 1.738x 1~ 1.259x 10-5 2.570x 10-3 1.000X 10-3 1.995x 10-3 2.188x 10-5 2.512X 10-3

2.430 3.220 3.000 2.380 5.760 4.900 2.590 3.000 2.700 4.660 2.600

Free activity 2.059x 10-3 3.360x lQ-4 8.762x lQ-4 3.641 x 10-3 1.955x lo-B 3.235 x 1~ 7.863 x 1~ 4.613 x lQ-4 1.740x 10-3 2.069x 10-10 2.182x 10-3

-log -log free free Free concenactivity concentration tration 2.686 3.474 3.057 2.439 7.709 5.490 5.104 3.336 2.760 9.684 2.661

Remainder

3.395 x 10-3 2.469 1.070x 10-10 5.541 x lQ-4 3.256 2.352x 10-1l 9.929x lQ-4 3.003 1.319x 10-1l 4.126x 10-3 2.385 2.302x 10-10 3.224x 10-8 7.492 1.139x 10-12 5.335 x 1~ 5.273 6.612x 10-13 1.297x 10-5 4.887 2.401 x 10-9 7.607x lQ-4 3.119 7.076x 10-1l 1.971 x 10-3 2.705 -1.021 x 10-10 6.375 x 10-10 9.196 1.492x 10-11 2.473 x 10-3 2.607 1.081 x 10-10

t The solution contains1.315 x 10-2 equivalentsper liter of cationic species,-8.426x 10-3 eq/L of anionicspecies,andthus hasa computednet chargeof 4.727x 10-3 eq/L. This representsan error equalto 35.94%of the total chargeof cationicspeciesin solution. :j: The computedalkalinity for this solution is 2.554x 10-3 eq/L. The predictedpartial pressureof CO2 in equilibrium with this solution is 1.749x 10-3 atm. (pC02=2.757).

Ca Ca Ca Ca Ca Ca Mg Mg Mg Mg Mg Mg K K K K K K Na Na Na Na Na Na

Metal

C03 S04 Cl P04 N03 OlL C03 S04 Cl P04 N03 OlL C03 S04 Cl P04 N03 OlL C03 S04 Cl P04 N03 OlL

Ligand

4.596 3.728 4.794 5.170 4.695 7.433 5.684 4.615 5.682 5.758 4.983 7.021 7.210 5.442 6.118 10.629 5.920 9.657 6.892 4.623 5.300 9.810 5.701 8.739

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0

0 0 0 1 0 -1 0 0 0 1 0 -1 0 0 0 0 0 -1 0 0 0 0 0 -1

Table 44-10. Concentrationof solublecomplexes.

1

2

1

(continuedon next page)

1

8.521

1

0 0

1 1

2 2

6.366

1 1

9.040 9.976 6,718

1 1

4.947 11.679

0

2

2 2

1

1 1

1

7.084

1

1 0 -2 1 1 1 1 1 1

15.675 5.866 12.398

1

0

0 0

10.523 7.955

1

7.903

1 1

1 1

6.602

2

1 1 1 1

7.116 7.413 14.888 5.119 5.815

1 2 2 0 0 -2 1 1

1

4.232

pC, and stoichiometry

... ... ~

-3

§!=

n

t'"

0

til

:z:

~

:z:

-

t::

Cl l'"l

0

3:

~

-e=~

S

C03 S04 Cl P04 N03 OHC03 S04 Cl P04 N03 OHC03 S04 Cl P04 N03

H+ H+ H+ H+ H+

Zn Zn

Zn Zn Zn Zn

eu2+

Cu2+

eu2+

Ligand

Metal

Cu2+ Cu2+ Cu2+

6.119 8.650 10.017 9.793 9.918 7.756 5.800 6.432 8.398 7.474 7.700 6.537 2.623 9.185 19.061 5.021 11.963

Table 44-10. Continued.

1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0

1 0 1 0 1 0 1 1 1 0 0 -1 1 0 1 0 1 0 1 1 1 0 o -1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0

7.896 13.059 13.632 11.839 13.435 9.210 9.577 10.840 10.813 9.820 11.116 6.591 4.219 27.440 5.877

1

1 1

o

2 2 2 1 2

o

2 2 2 1 2

2

0 0 0 2 0 -3 0 0 0 2 0 -2 2 2

1 1 3 1

1 1 0 0

1

0 1

11,683

o 3

-3

1 o -3 1 1 1 1 1 1 1 3 0 1 1 0

1 1 1 1

10.136

11.754 5.336 13.667 15.119 7.118

6.055 15.886 17.737 7.537

pC, and Stoichiometry

14.870

1 0 -4

0 0

1 4 1 2

17.814 11.593

0 0

1 0 -4

1 4 1 2

15.489

22.533 10.113

8.452 12.845

9.802

11.071 15.264

o

-2 1 1 -1 2 1 2

2

1 1 -1 2 1 2

~

~

Ro

~

8 CI

m

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1333

Table 44-11. Concentrationsoffree metal, free ligand, and total metal complexedby eachligand. Free ligand

Free metal

C03

S04

CI

P04

N03

OH-

2.47 3.26 3.00 2.38 7.49 5.27 7.85

4.89 4.08 5.01 5.85 4.94 5.78 5.21 2.60

3.12 3.73 4.62 5.44 4.62 8.65 6.43 9.18

2.71 4.79 5.68 6.12 5.30 9.98 8.12 19.06

9.20 5.08 5.70 7.08 6.36 7.53 6.96 4.91

2.61 4.60 4.98 5.92 5.70 9.92 7.70 11.96

6.05 7.43 7.02 9.66 8.74 7.73 6.26

Free ligand

Ca Mg K Na Cu2+

Zn H+

andthe sumof concentrations(-log molar) of all complexesinvolving eachmetal and ligand. For examplethe sumof the concentrationsof complexesinvolving Ca and C03 (CaC03': 4.596and CaHCO;: 4.232) is listed as 4.08 (-log molar). Free concentrationsof each metal and ligand, and the sum of concentrations of complexesof eachmetal and ligand as a percentageof the total concentration also are listed (Table 44-12). The data provided in this tabulation helps assesspromptly the significantforms of metalsand ligandsat equilibrium. In this example,we can quickly notice that a major fraction (>90%) of the alkaline earth and alkali cationsexist in free ionic forms whereasa major fraction of Cu (>95%) and nearly one-halfof Zn are complexedwith C03• The secondpart of this tabulation shows that nearly all of Cl and N03 (>98%) exist in free ionic forms, whereasabout 95% of C03, and about 50% of P04 exist in protonatedforms. About 20% of S04, and 10% of P04 are predictedto be complexedwith Ca. This tabulationprovidesa helpful summaryof major interactions,while other outputs provide detailedinformation aboutthe concentrationsand stoichiometryof various species. Finally, the thermodynamicconstantscorrectedfor the ionic strengthof the solution (conditionalconstants)are tabulated.Here only a part of the tabulation is reproduced(Table 44-13). Every line includes the identification number of eachmetal and ligand involved in the interaction,the conditional constantsand stoichiometryof three solids and six aqueouscomplex species.In this example, Table 44-12. Primary distribution of metalsand ligands,no solids are allowed. Component

Free metal

C03

S04

91.36 91.95 99.29 98.97 1.86 42.38

2.26 1.60 0.14 0.27 95.26 49.25

5.03 4.02 0.36 0.58 0.13 2.94

CI

P04

N03

0.23 0.33

0.68 1.72 0.12 0.05

OH

% of total

Ca Mg K Na Cu(lI)

Zn

Ca Component Freeligand 3.26 0.50 C03 18.71 76.07 S04 98.80 0.80 CI 9.19 P04 38.22 1.01 98.45 N03

Mg 0.38 2.43 0.10 0.38 0.41

K 0.06 0.36 0.04 0.38 0.05

0.43 0.35 0.08 0.12 0.06 Na 0.44 2.40 0.25 1.98 0.08

0.01 1.68 0.87

0.16

0.02 1.06 4.34

Cu(II) 0.06

Zn 0.24 0.04

H 95.05

0.13

0.50

49.59

Ligand

1 2 3 9 57 99 1 2 3 9 57 99

Metal

1 1 1 1 1 1 2 2 2 2 2 2

.00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

.00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00

Solids

Table 44-13. Computedconditionalconstants.

.00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00

0 0 0 0 0 0 0 0 0 000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

2.76 1 1.86 0 .38 1 14.34 1 .48 1 -28.11 1 2.46 1 1.76 1 .28 1 14.54 1 .88 1 -11.61 1

1 0 0 0 1 0 1 1 1 0 0 -1 1 0 1 0 1 0 1 1 1 0 0 -1 10.97 1 1 1 .00 0 0 0 .0000 0 20.24 1 1 2 .27 1 2 0 28.11 1 1 -2 10.87 1 1 1 .0000 0 .00 0 0 0 20.24 1 1 2 .0000 0 -28.11 1 0 -2

.00 .00 .00 5.85 .00 .00 .00 .00 .00 5.85 .00 .00

0 0 0 1 0 0 0 0 0 1 0 0

0 0 0 1 0 0 0 0 0 1 0 0

0 0 0 0 0 0 0 0 0 0 0 0

Complexes

.00 0 0 0 .00 0 0 0 .00 0 00 .00 0 0 0 .000 () 0 .00 0 0 0 .00 0 0 0 .00 0 0 0 .00 0 0 0 .00 0 0 0 .00 0 00 .00 0 0 0

.00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

.00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

~

> ~

N

1(0

t:l

~

C'l 0

....

...~

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1335

the first line indicatesthe interactionsinvolving Ca (metal no. 1) and C03 (ligand no. 1). The identification numbersfor variousmetalsand ligandsare tabulatedin the manualsthat accompanythe codes.The constantsfor solids are not listed becausethis specimencomputationinvolved only solution speciation.Two soluble complexes(CaC03° and CaHCOj) are listed with conditional logarithmic associationconstantsof 2.75 and 10.97,respectively.

Speciationand MassTransfer In this calculation,all other computationalconditionsare the sameexcept that the mass transfer between solution and the solid phasewill be allowed throughprecipitationof solid phasesif the solution is found to be supersaturated with respectto the secondarysolid phasesincluded in the database. Thereare two ways to interpretthe resultsthat predict a significantamount of solid precipitation. First, the systemmay be kinetically supersaturated with respectto that precipitatedphase.This condition may arise becausethe solid phasemay thermodynamicallybe the most stable phasein the soil systemand may form slowly. Becausethe computationalschemeallows the precipitationof the most stablephase,the resultsmay indicatea significant amountof precipitation. Such a possibility may be examinedby allowing metastablefast-forming solid phases,and disallowing the precipitationof the most stablephase.Second, the soil solution may contain colloidal particlesthat are included as part of the dissolvedphase.It is necessarytherefore to make sure that only the solution phaseis being analyzed. In the following example(Table 44-7) any solid is allowed to precipitate without restrictionfrom the soil solution consideredas a closedsystem.The section of the resultingoutput that lists only the percentagedistribution of the componentsis tabulated.The resultsof computationindicatethat among20 potential solid phases,the soil solution is supersaturated with respectto calciate(CaC03), hydroxyapatite [Ca5(P04)30H], malachite [Cu2C03(OH)2], and hydrozincite [Zn5(C03)z(0H)6](Table 44-14). About 20% of Ca, 50% of Cu, and 5% of Zn are predictedto precipitateas solids. Becauseno specific conditions have been imposedon the nature of precipitatingsolids, the most stable phasesare being precipitatedin this setof calculations.However,the soil solution may be in equilibrium with solids that precipitatemore rapidly (metastable)than the most stable solids. In such cases,additionalcomputationsare necessaryto identify these setsof metastablesolids. Assuming that the identities and the thermochemical data of these metastablephasesare known computationscan be made with imposedsets of metastablephasesto obtain the equilibrium speciation.

CarbonateEquilibrium Oneof the previousexamples(solution phasespeciation)showedhow carbonateequilibrium is computedfor closed systems(Table 44-9). For a closed system, the measuredtotal dissolved carbonateconcentrationwas used as an input without specifyingthe partial pressureof CO2. For opensystems,a number of computationsare run with different partial pressuresof COz. The partial pres-

MATIIGOD & ZACHARA

1336

Table 44-14. Primary distribution of metalsand ligands,solids precipitationallowed. Free metal

Component

C03

CI

S04

P04

N03

OH

% of total 73.81

Ca Mg K Na Cu(II)

92.29 99.31 99.01 1.22

Zn

45.86

1.38 18.53t 1.21 0.11 0.21 47.12 50.85§ 40.17

4.38

0.36

0.57 0.98*

4.35 0.38 0.61 0.09

0.36 0.08 0.12

3.42

0.07

1.77 0.12 0.05

0.02 0.71

0.18

4.85

5.46~

Component Free ligand 0.36 C03 S04 CI P04

78.17 98.93

N03

98.60

Ca 1.99 26.79 16.26 0.67 0.04 99.89 0.84

Mg 0.28

K 0.04

Na 0.33

2.62 0.11 0.01

0.38 0.04

2.53 0.26

0.43

0.05

Cu(II) 0.03 0.02

Zn 0.20 0.01 0.04

H 69.94

0.06 0.08

tCaC03· 0 H. * Cas(P04)3 § CU2C03(OHh· ~ Zns(C03)z(°H)6·

sure at which CO2 either barely dissolvesinto or degassesout of solution is the equilibrium partial pressure.Open system computationsindicate that the soil solution is in equilibrium with the partial pressureof CO2 at 2.89 (PC02). Oxidation/ReductionReactions Redox reactionsshould only be included in equilibrium computationsif there are indicationsthat both the speciesof a redox couple exist in significant concentrations.The type of dataneededfor redox computationsincludeeither: (i) the concentrationsof eachspeciesof eachredox couple,or (ii) the total concentrations of both speciesof each redox couple and the electron activity. Typical availabledatafor redox speciesconsistsof total concentrationswith no measured electronactivity. If the availabledata are of type (i), i.e., the measuredtotal concentrations of each redox couple are available,the data input is similar to other previously describedspeciationproblems.For Type (ii) data, the input must include both redox species,with total measuredconcentrationbeing assignedto one of the species,while the other speciesis assigneda nominal total molar concentration of 1 x 1Q-8. Additionally the input must include all relevantredox couples.The electron activity either measuredor estimatedmust be specified. The electron activity (pe) is a function of redox potential(Eh) and the relationshipis specified (Stumm & Morgan, 1981) as pe = Eh (measuredin millivolts)/59.155 (mv)

[24]

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1337

Table 44-15.Total concentrationsof componentsusedas input for redox calculations(with pH = 7.90 and pe =-4.(0).t Component

pCT

Component

pCT

Ca

2.43 3.22 3.00 2.38 5.76 8.00 4.90

C03 S04 S(II) Cl P04 NH3 N03

2.59 3.00 8.00 2.70 4.66 2.60 8.00

Mg K Na Cu(II) Cu(I) Zn

t Redox couples:SOJS- 2, NHy'N03, Cu+/Cu2+.

In soil systems,the electronactivity may rangefrom -7 to +14 (Sposito& Mattigod, 1980). Typically in soils, the redox potentials exhibit seesaweffects (Bartlett, 1986);thereforemeasuredpe may not be in equilibrium with redox couples. The input data for a typical computationinvolving redox reactionsare tabulated below (Table 44-15). Three redox couples,S041'S-2, NHYN03, Cu+/Cu+2 were included and the measuredtotal concentrationsof both speciesof a redox couple(Type ii data)were assignedto one of the redox couples.The redoxpotential was set at -237 mV correspondingto a pe value of -4.00. The resultsof the redox computation(Table 44-16) show that at specified pe, all of Cu(II) will be reducedto Cu(I) and precipitatedas CU2S, A major fraction of reducedS (95.5%)also is predictedto precipitateall of dissolvedZn in the Table 44-16. Primary distribution of metals and ligands, redox reactionswith solid precipitation allowed. Cl

Free metal

C03

S04

Ca

74.71

4.44

0.36

Mg K Na Cu(II) Cu(I)

93.96 99.44 99.07

1.39 18.12t 1.23 0.11 0.20

4.44 0.38 0.60

0.36 0.08 0.12

Component

S(II)

P04

NH3

N03

0.98:1:

OH

0.02

100.0§ Zn 100.~

Component C03 S04 S(II)

Free ligand Ca 0.36 2.00 26.19t 75.60 16.29

Cl P04 NH3 N03 t CaC03·

:I: Cas(P04hOH.

§ CU2S, .ZnS.

Mg 0.29

K 0.04

0.33

2.64

0.37

2.44

0.11 0.01

0.04

0.26

Na

Cu(II)

Cu(I)

Zn

H 69.08

6.53§ 93.97. 98.92 99.89:1: 6.59

0.67

0.06 93.41

MATIIGOD & ZACHARA

1338

Table 44-17. Input datafor computingspeciationusing the mixture model (with pH =7.90). Component

peT

Organiccomponent

peT

Ca Mg K Na Cu (II) Zn C03 S04 Cl P04 N03

2.43 3.22 3.00 1.04 5.76 4.90 2.59 3.00 2.70 4.66 1.05

Citrate Salicylate Phthlate Arginine Ornithine Lysine Valine Maleate Benzene-sulfonate

4.14 4.27 3.97 4.49 4.36 4.36 4.36 3.97 4.36

form of ZnS. Even though the model predictionsfor this pe indicatethat NH3 is the dominantspecies,observationsof soil systemsindicate that under reducing conditions (flooded soils) the dominant speciesthat result from N03 reduction are gaseousN2 and N20, with minor amountsof NH3 and organic N (Reddy et aI., 1978). Speciationof the nonredoxspeciesdo not significantly differ from the results of previous examples where solid precipitation was allowed (Table 44-14).

Natural Organic Interactions In this final exampleof speciationcalculationswe will compareresults obtainedfrom the Mixture Model (MM) and the ScatchardQuasiparticleModel (SQM) (Sposito,1981a)with identical input data.The concentrationsof all inorganic componentswere the samein both cases,and the dissolvedorganic acid concentrationsin both models were set to 226 g of C/m3• Becausethe SQM model is relevantfor an ionic strengthof 0.1 mol!L, comparableionic strengths were usedin both models.The input datafor thesemodelsand the organiccomponentsin the MM that are selectedto simulate the behaviorof natural fulvic material are listed in Table 44-17. Similar organiccomponentsusedin the SQM are listed as FULl and FUL2 in Table44-18.The concentrationsof organiccomponentsin MM and in the SQM werecomputedas describedby Sposito(1981a). The resultsof outputsfrom the MM and the ScatchardQuasiparticleModel are listed in Tables44-19 and 44-20. Thereare no significant differencesin speciation of Ca, Mg, K, and Na predictedby both models.However,totally different speciationpatternsof Cu are predictedby thesemodels.The MM predictsthat Table 44-18. Input data for computingspeciationusing the Scatchard Quasiparticle model (with pH

=7.90).

Component

peT

Component

peT

Ca Mg K Na Cu (II) Zn

2.43 3.22 3.00 1.04 5.76 4.90

C03 S04 Cl P04 N03 FULl FULZ

2.59 3.00 2.70 4.66 1.05 3.73 2.73

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1339

Table 44--19. Primary distribution of metals and ligands, mixture model for organic interactions, solids precipitationallowed. Component

Free metal

C03

S04

Cl

80.80

1.02

1.39

0.23

P04

N03

OH

Organic interactions

% of total

Ca

13.66

1.93 1.04

0.98t

Mg K Na Cu(II) Zn

69.05 96.54 98.06 0.02 42.24

0.62 0.09 0.18 0.47 25.31

0.94 0.18 0.37

0.15 0.05 0.09

28.18 3.13 1.27

0.91

0.03

3.47

2.78

0.04 99.50 25.23

t Ca4(P04)JOH. Table 44-20. Primary distribution of metalsand ligands,quasiparticlemodel for organicinteractions, solids precipitationallowed. Component

Free metal

C03

S04

Cl

77.05

0.98

1.33

0.22

P04

N03

OH

Organic interactions

% of total

Ca

13.03

6.42

28.01 3.13 1.27 0.16

1.67 0.78

0.92

4.11

3.29

11.53

0.98t Mg K Na Cu(II)

68.61 96.54 98.03 2.00

Zn

49.98

0.62 0.09 0.18 51.61 44.48:j: 29.95

0.94 0.18 0.37 0.04

0.15 0.05 0.09

1.08

0.04

t Cas(P04)JOH. :j: CU2C03(OH)z.

almost all dissolvedCu (99.5%) is complexedwith organics,whereasthe SQM predictsthat almost the samefraction (96%) of Cu is complexedand precipitated with carbonate.Also, significantly different speciationof Zn is predictedby thesetwo models. For instance,the MM predictsup to 25% of total Zn bound with organics,whereasthe SQM estimatesthat about 12% of Zn is complexed with organics. These large discrepanciesindicate that the organic interaction modelsusedhere should be usedwith caution.

Adsorption Models Calculationswith FITEQL. Two examplesof adsorptioncalculationswill be presentedusing the generalizedequilibrium and optimization code FITEQL. The data to be modeledinclude: (i) Cd adsorptionto a clay-sizedisolate «2.0 !lm) from a Mollisol subsoil and, (ii) CrOi- adsorptionon an Ultisol subsoil and its clay fraction. Disk copies of the input files used in these examplesmay be obtainedfrom the secondauthor. The input files will not be fully discussed,the reader is directed to the FITEQL Version 2.0 manual (Westall, 1982a,b)for a line-by-line descriptionof the input format. The FITEQL providesthe following output: (i) input data for verification, (ii) serial data and estimatedstandarddeviations,(iii) valuesof adjustablepara-

MATIIGOD & ZACHARA

1340 100

.. ,

¢rIJ

BO 1J Q)

.0

0

'"

OIl

60 [)

E Q) 40 !! Q)

~

a.

Il/I

•

• • .-

20 0 4

il

OIl

.;

•

•

I!b

o

Experiment

•

Model 1

Model2

•

Model3

00

6

pH

Fig. 44-3. Predictiveandbest-fit modelcalculationsof Cd sorptionto a smectiticsoil clay isolatewith both ion exchangeand edgecomplexationreactions.

metersat eachiteration (if any), (iv) descriptionof chemicalequilibrium including total concentrationanderror in the materialbalanceequation,(v) negativelog free concentrationof all speciesat eachtitration point, and free concentrationof all speciesat all titration points, (vi) descriptionof surfaceelectrostaticterms at equilibrium, (vii) a listing of the experimentaldata, (viii) estimatesof standard deviationin the experimentaldata,(ix) detailsof the optimizationprocedure,(x) error propagationdata,and (xi) termsin weightedsumof squaresand derivatives for the normal and optimizationmatrices. The following representsgeneralguidancepoints for performing adsorption modelingin soil materials. 1. Model building must proceed from simple to increasingcomplexity (i.e., multiple surfacespecies).The degreeof complexity shouldbe adequateto describethe data. 2. Chemically realistic sorbentsand surface speciesshould be selected. Wheneverpossible these choices should be basedon direct analysis (e.g., mineralogicalor spectroscopic). 3. Optimizationof the chosenmodelshouldbe conductedon aminimal set of selectedparametersthat are justifiable. Predictivecalculationsmust be constrainedto demonstrateapplicability under different chemicalor experimentalconditions. 4. Equilibrium constants fromdifferent SCM are not interchangeable.

Appropriatestepsnecessaryfor successfulSCM modelingof cationic and anionic adsorptionis illustratedas a flowchart (Fig. 44-1). Cation Adsorption.The adsorptiondata to be modeledare shown in Fig. 44-3 and involve the fractional adsorptionof Cd2+ (10-6 mollL) on a smectitic soil clay isolate (2.3 gIL) in a NaCI04 electrolyte(I =0.1 mollL) as the pH was incrementally raised from approximately4.5 to 8.5. The sorbentwas isolated from the BC horizon of a Mollisol by sedimentationandcontainedsmectite asthe dominantmineral phaseand a minor amountof mica. Prior to performingadsorption studies,the isolate was treatedwith dithionite-citrate-bicarbonate (DCB) to removefree iron oxidesand dilute hydrogen-peroxideto oxidize residualreductant, and then dialyzed againstdeionizedwater. The resulting sorbentcontained

EQUILIBRIUM MODELING IN SOIL CHEMISTRY

1341

someresidualorganicmatter(