Introduction to Experimental Inorganic Chemistry [Auth. transl. from German. Reprint 2020 ed.] 9783112356586, 9783112356579

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I N T K O D U C T I O N TO

EXPERIMENTAL INORGANIC CHEMISTRY AUTHORIZED

TRANSLATION

FROM

THE

GERMAN

OF

HEINRICH

BILTZ

BY

WILLIAM T. HALL AND JOSEPH W. PHELAN I N S T R U C T O R S IN C H E M I S T R Y , M A S S A C H U S E T T S

INSTITUTE

OP T E C H N O L O G Y

FIRS

T

FIRST

EDITION THOUSAND

NEW YORK JOHN W I L E Y & SONS LONDON : C H A P M A N

& HALL,

1909

LIMITED

COPYBIGHT,

1009,

BT

WILLIAM T. HALL AND JOSEPH W. PHELAN

Stanbopcpcm F. I . S I U O H BOSTON.

COUPANT U.S.A.

TRANSLATORS' NOTE. IN presenting this book the translators believe that it will find a place in the advanced courses of High and Preparatory Schools and in Colleges as a preparation for work in Analytical Chemistry. The modern student is in many cases deficient in actual knowledge of Descriptive Chemistry. This is due quite as much to a general objection to "test-tube reactions" as to the introduction of many experiments on principles and theory. The book aims to give the student a knowledge of Descriptive Chemistry and to explain reactions in the light of modern theories. In preparing the English text, and in reading the proof, much help and valuable criticism has been given by Dr. A. A. Blanchard, Dr. K. S. Williams, Mr. P. S. Fiske, and by Dr. H. Biltz himself, to all of whom many thanks are due. WILLIAM T. HALL JOSEPH W. PHELAN

iii

CONTENTS. PAGE INTRODUCTION

1

F I L T E R S AND F I L T R A T I O N

4

M A N I P U L A T I O N O F GLASS

6

U S E OF THE BLOWPIPE

10

CORK BORING

12

F I B S T G R O U P OF ACIDS AND NEGATIVE E L E M E N T S (ANIONS)

Hydrochloric Acid and Chlorine Chemical Reactions Sulphuric Acid Concentration of Solutions; Normal Solutions Nitric Acid Carbonic Acid Hydrogen Sulphide Sulphides Phosphoric Acid BASES

1. Alkali Metals Sodium Names of Inorganic Substances Potassium Ammonium 2. Alkaline Earth Metals Calcium Reversible Reactions — State of Equilibrium Strontium Barium Degree of Solubility Theory of Aqueous Solutions Theory of Solution and Precipitation 3. Magnesium Group Magnesium Zinc Cadmium v

14

16 20 21 26 27 30 33 36 37 42

43 43 49 52 55 58 59 64 65 65 68 69 77 80 80 83 85

vi

CONTENTS. Continued. Iron Group Aluminium Compounds of Alkali with Metal Oxides Hydrolysis Iron Iron Cyanogen Compounds Double Salts and Complex Substances Cobalt Nickel Chromium Group Chromium Molybdenum Uranium Manganese Oxidation Reduction Copper Group Copper Electro Affinity Mercury Mercuric Chloride and Mercuric Cyanide Silver Tin Group Tin Colloidal Solutions Lead Arsenic Group Arsenic Antimony Bismuth

BASES —

4.

4.

6.

2.

8.

SECOND G R O U P OF ACIDS

Hydrobromic Acid, Hydriodic Acid Hydrocyanic Acid Hydrofluoric Acid, Hydrofluosilicic Acid Chloric Acid Iodic Acid Silicic Acid Nitrous Acid Boric Acid Sulphurous Acid Thiosulphuric Acid

PAGE

87 89 92 93 96 101 103 108 112 114 114 121 122 123 129 130 132 133 137 138 100 144 148 148 151 154 157 157 165 170 172

172 174 176 178 179 179 180 182 183 184

EXPERIMENTAL INORGANIC CHEMISTRY. INTRODUCTION. THE student should have at hand in his desk the following articles: scissors for cutting filter paper, a threecornered file for cutting glass, a rat-tail file for smoothing and widening holes in cork stoppers, pincers, blowpipe, test-tube holder, 1 platinum wire, platinum foil, closed tubes prepared as described on page 8, test-tubes and a test-tube rack, funnels, flasks, glass stirring rods with rounded ends, small beakers, a wash bottle, porcelain crucibles, evaporating dishes, an iron tripod or a lamp stand with movable rings, wire gauze, funnel support, and a Bunsen or Tirrill gas burner. It is also well to have a spatula, — either of glass, porcelain, pure nickel, horn, or best of all, platinum; nickel-plated instruments are not practicable in a chemical laboratory. The platinum foil is easily kept clean in a small beaker containing water and a little hydrochloric acid. The platinum wire should be fused into the end of a glass rod, and kept immersed in a test-tube containing dilute hydrochloric acid. Glassware should always be scrupulously clean. Beakers are to be washed, wiped dry, and kept in an inverted position on filter paper with which the floor of 1 A strip of folded filter paper wrapped about the tube and this pinched together with the fingers makes a satisfactory holder.

1

2

INORGANIC CHEMISTRY.

the cupboard is partly covered. Flasks, when clean and dry, should be protected from dust by means of corks or pieces of paper bent over the necks. Test-tubes must be cleaned immediately after the experiments. Water and a test-tube brush usually suffice for the cleaning, but sometimes a little precipitate adhering to the sides is best removed with the aid of a few drops of concentrated hydrochloric acid. It is comparatively easy to clean vessels immediately after they have been used; but the cleansing process is often tedious and difficult if left to the next day. I t is best to give test-tubes a final rinsing with distilled water, and then to allow them to drain in an inverted position on the pegs of the rack. A supply of clean, dry test-tubes should always be on hand. The beginner must, at the start, become accustomed to cleanliness in the laboratory work. Time is saved and better results obtained. Most of the experiments can be carried out in testtubes and as a rule but little of the substance should be taken for each experiment. When a reaction between solutions is to be studied, only from 0.5 to 1 cc. should ordinarily be used; not only is this in the interest of economy, but results are obtained more promptly. The student should become accustomed to estimating weights and volumes. It is well to weigh a test-tube empty, half full of water, and entirely full, in order to obtain an idea of its capacity. A convenient graduate may be prepared by sticking a long narrow label along the length of the tube, and marking upon it the height of 1, 2, 3, etc., grams of water. The most important requisite for success in carrying out laboratory experiments is adequate home study. No exercises should be attempted in the laboratory until the text has been studied at home in connection with a suit-

INTRODUCTION.

3

able text-book on Inorganic Chemistry. The reactions of the metals of the fourth and fifth groups especially are so complicated that they will be understood only by the aid of conscientious work by the student at his study table. In the following pages precise directions are given for carrying out the experiments, but in many cases modifications may be suggested as the result of the student's own observations. Such changes should be noted on the margin of the book. A number of theoretical discussions are interspersed to aid in explaining the operations; it is self-evident that there has been no attempt to obviate the necessity of a modern course in analytical chemistry, or of one on the theory of solutions and the mass-action law. Any one desirous of studying more closely these theoretical relations should refer to such books as OstwaldM'Gowan's Scientific Foundations of Analytical Chemistry, Ostwald-Findlay's The Principles of Inorganic Chemistry, or Alexander Smith's General Inorganic Chemistry.

FILTERS

AND

FILTRATION.

Filters can be purchased already cut to proper size, or they may be prepared from large sheets (55 cm. square). Twenty-five filters of a size suitable for an ordinary 7 cm. funnel 1 can be obtained from a large sheet, by dividing each side into fifths, creasing the paper from edge to edge, and cutting out the squares thus formed. To prepare a " smooth " or ordinary filter such a square is folded

FIG. 1.

twice at right angles (Fig.la 2 ) and the folded filter cut as indicated by the dotted lines in Fig. 1 b. The filter is then opened (Fig. lc) and inserted in a funnel (Fig. Id), the upper edge of which should be about 1 cm. higher than the filter. It is never permissible to have the filter paper extend above the funnel. Before beginning a filtration the filter is moistened with a little water and pressed tightly to sides of the funnel. 1 The size of a funnel is given as the diameter across the top. The above funnel will take an 11 cm. filter, i.e. one with the diameter of 11 cm. across the open sheet of paper. 2 In the drawing a represents the original sheet of filter paper and b one of the quarters. 4

FILTERS AND FILTKATION.

5

For the purposes of qualitative analysis, and in making preparations, a so-called " plaited " filter usually works to better advantage, although precipitates cannot be washed as well upon it. A plaited filter (Fig. 2) is made

Fig. 2. in the same way as before except that instead of folding the paper into quarters it is divided into sixteenths (or eighths in the case of small filters) with the folds all on the same side of the doubled paper (Fig. 2a). Then each division is folded again so that the new crease comes on the opposite side of the paper. In Fig. 2b the left half of such a filter has been folded in this way while the right half has not. The filter is then opened and placed in the funnel (Fig. 2c). After a little practice, one of these filters can be folded very quickly. In filtering, care should be taken not to fill the filter so full that the solution runs over the edge of the paper. The washing, except in special cases when the precipitate is acted upon by the atmosphere, should not begin until the original solution has all passed through the funnel, then the stream of water should be directed from the wash bottle to a line just below the upper edge of the filter paper; after each washing the filter should be allowed

6

INORGANIC CHEMISTRY.

to drain as completely as possible. The general rule is to wash frequently with little water, letting the filter drain after each washing. In the case of many precipitates the washing is so tedious, after the pores of the filter have become clogged, that it is advisable to avoid, as much as possible, transferring the precipitate to the filter; the precipitate should be allowed to settle, and the supernatant liquid poured off through the filter without disturbing the precipitate. The latter is then mixed with more water, allowed to settle, and the process repeated. This is called washing b y decantation and is very successful with some precipitates. MANIPULATION OF GLASS. The chemist, in fitting up apparatus for his experiments, must frequently bend glass tubing, seal tubes, round off broken ends, or break rods or tubing. It is desirable, therefore, that the student should acquire some skill in such manipulation. A few suggestions are given, but a great deal more can be learned b y watching a skilled glass blower and by consistent practice. More complete directions may be found in Shenstone's " The Methods of Glass Blowing." Cutting Glass Tubes. Tubing not over 1 cm. diameter can be cut as follows: Make a scratch with a sharp, threecornered file, or a glass-knife, across the glass at the point where it is desired to cut it: ordinarily the scratch should extend around the tube from one-fourth to one-fifth of the circumference. Then grasp the tube with both hands so that the points of the thumbs meet directly opposite the middle of the scratch, and break the tube by gently pulling it apart; if a slight pressure is not sufficient the scratch must be deepened. In the case of a large tube, a

6

INORGANIC CHEMISTRY.

to drain as completely as possible. The general rule is to wash frequently with little water, letting the filter drain after each washing. In the case of many precipitates the washing is so tedious, after the pores of the filter have become clogged, that it is advisable to avoid, as much as possible, transferring the precipitate to the filter; the precipitate should be allowed to settle, and the supernatant liquid poured off through the filter without disturbing the precipitate. The latter is then mixed with more water, allowed to settle, and the process repeated. This is called washing b y decantation and is very successful with some precipitates. MANIPULATION OF GLASS. The chemist, in fitting up apparatus for his experiments, must frequently bend glass tubing, seal tubes, round off broken ends, or break rods or tubing. It is desirable, therefore, that the student should acquire some skill in such manipulation. A few suggestions are given, but a great deal more can be learned b y watching a skilled glass blower and by consistent practice. More complete directions may be found in Shenstone's " The Methods of Glass Blowing." Cutting Glass Tubes. Tubing not over 1 cm. diameter can be cut as follows: Make a scratch with a sharp, threecornered file, or a glass-knife, across the glass at the point where it is desired to cut it: ordinarily the scratch should extend around the tube from one-fourth to one-fifth of the circumference. Then grasp the tube with both hands so that the points of the thumbs meet directly opposite the middle of the scratch, and break the tube by gently pulling it apart; if a slight pressure is not sufficient the scratch must be deepened. In the case of a large tube, a

MANIPULATION

OF

GLASS.

7

scratch will not insure a clean break. The tube must be filed to some depth, half-way, or even all around it. Should the break be uneven, the projecting parts may be chipped off by means of a pair of pliers; but care should

FIG. 3.

be taken not to chip off too much glass at one time. The pliers are held as shown in (Fig. 4) and the pull should be made in the direction indicated by the arrow.

FIG. 4 .

Rounding the Ends of Tubes. Before setting up an apparatus the sharp ends of all glass tubing should be well rounded. This is accomplished very readily by heating 2 to 4 cm. of the tube while constantly revolving it in the luminous flame of the blast lamp (i.e. without using the air blast) and then softening the very end by means of the non-luminous flame (i.e. with the air blast). If too wide a strip is heated, the tube is likely to become restricted. Large tubing is treated in exactly the same manner except t h a t the preliminary heating must be done very cautiously or the tube will crack.

8

INORGANIC CHEMISTRY.

Preparation of Closed Tubes. A closed tube usually signifies one t h a t is closed a t one end, whereas one closed at b o t h ends is designated as a sealed tube. Closed t u b e s are used for carrying out ignition and sublimation experiments. Some glass tubing of about 0.6 cm. outside diameter is cut into 10 cm. lengths. Such a piece is softened in t h e middle b y constantly being r o t a t e d in t h e flame of a blast l a m p ; when the glass has become soft it is t a k e n out of t h e flame and immediately drawn out so t h a t a narrow capillary is made, a b o u t 10 to 15 cm. long, between the wider p a r t s of the tube. The capillary is now heated at t h e center for a m o m e n t (Fig. 5a) a n d then drawn a p a r t . One of the halves is next heated a t the place where the bore narrows, and, when sufficiently softened, t h e adhering t h r e a d of glass is removed so t h a t a small closed t u b e is obtained as shown in Fig. 56, I n

Fig. 5. order to round off t h e pointed end, t h e rotating t u b e is softened, t a k e n from the flame, a n d blown out gently. The pressure f r o m the lungs should be applied very slowly at first, increasing it little b y little until the glass begins to give way. If necessary t h e heating and blowing can be repeated until finally the end is round and uniformly thick. Similarly much larger ignition tubes can be prepared. Bending Glass Tubing. To bend a piece of glass t u b i n g t h a t is not too wide in diameter, use the luminous flame of

MANIPULATION

OF

GLASS.

9

an ordinary fish-tail gas burner. It requires considerable practice to accomplish the bending with the blast lamp, although this is necessary in the case of wide tubes. In the first case the tube is held so that the wide part of the flame is parallel to the length of the tube which is revolved continuously in the flame, a stretch of about 4 cm. being heated uniformly. As soon as the glass is soft enough, the tube is very gradually bent while the heating is continued. If the bend is made too rapidly a constriction forms (Fig. 66) and the tube is so weakened at this point 9

9

a FIG. 6.

that it is likely to break subsequently. Bends of more than a right angle should be made very carefully, and the middle portion must not be overheated. Instead of the above flame, a flame spreader on the top of a Bunsen burner with a non-luminous flame may be used. The glass is then softened more quickly, and the bend is made while the tube is outside the flame. More practice is required in order to get good results by this method. The student should prepare a right-angled tube with arms about 4 and 12 cm. long respectively. Glass tubing of greater diameter than 0.8 cm. should be bent with the aid of the blast lamp. The tube is closed at one end with a stopper and the formation of a constriction is prevented by blowing into the tube while bending. It

10

INORGANIC

CHEMISTRY.

requires considerable skill to make satisfactory bends with large tubes. To Draw out Tips. In making a tip for a wash bottle or a burette it is not correct to draw out the tubing as described for making sealed tubes, as the walls of the drawn out portion are then far too thin. The tube must be heated in the blast or Bunsen flame until the walls have become thick at the heated part (Fig. 7a). It is somewhat difficult to keep the softening mass of glass uniformly revolving, as it should be, without causing any deformation. When the walls of the tube at the heated spot have become of about twice the normal thickness, the tube is withdrawn from the flame and slowly drawn out until the desired shape is obtained. The tube is then

FIG. 7.

allowed to cool, after which it is cut and the ends rounded in the flame. The preparation of such tips is excellent practice before undertaking more complicated problems of glass blowing, as it teaches the proper heating of a softened mass of glass, which, as has been said, requires considerable skill. USE OF THE BLOWPIPE. The blowpipe, although formerly much used in all chemical laboratories, has unfortunately fallen more and more into disuse, except in metallurgical laboratories where it is still regarded as indispensable. By this instrument a powerful, pointed flame may be directed in a

10

INORGANIC

CHEMISTRY.

requires considerable skill to make satisfactory bends with large tubes. To Draw out Tips. In making a tip for a wash bottle or a burette it is not correct to draw out the tubing as described for making sealed tubes, as the walls of the drawn out portion are then far too thin. The tube must be heated in the blast or Bunsen flame until the walls have become thick at the heated part (Fig. 7a). It is somewhat difficult to keep the softening mass of glass uniformly revolving, as it should be, without causing any deformation. When the walls of the tube at the heated spot have become of about twice the normal thickness, the tube is withdrawn from the flame and slowly drawn out until the desired shape is obtained. The tube is then

FIG. 7.

allowed to cool, after which it is cut and the ends rounded in the flame. The preparation of such tips is excellent practice before undertaking more complicated problems of glass blowing, as it teaches the proper heating of a softened mass of glass, which, as has been said, requires considerable skill. USE OF THE BLOWPIPE. The blowpipe, although formerly much used in all chemical laboratories, has unfortunately fallen more and more into disuse, except in metallurgical laboratories where it is still regarded as indispensable. By this instrument a powerful, pointed flame may be directed in a

USE OP THE BLOWPIPE.

11

horizontal direction so that substances can be heated upon a non-conducting support, — usually a piece of wood charcoal. B y regulating the supply of air it is easy, with a little practice, to provide either a flame with an excess of the unburned illuminating gas, or at other times one in which an excess of air prevails. The former is the socalled reducing flame and the latter the oxidizing flame. I t requires considerable practice to maintain at will either flame steadily for a long time: it is necessary to breathe through the nose in such a way that during the breathing the blast of air from the mouth is not noticeably affected: the lungs and cheeks should be well filled and the supply of air must never become exhausted. The flame from an oil lamp burning a flat wick is admirably suited for blowpipe work, although the barely-luminous flame of a Bunsen burner is satisfactory. To produce an oxidizing flame, the point of the blowpipe is held in the midst of the flame 1 or 2 cm. above the top of the burner, and by a strong blast of air through the blowpipe a small Bunsen flame is blown out sideways: the typical inner cone can be easily distinguished and also the outer mantle containing the true oxidizing zone. A reducing flame is obtained, on the other hand, by keeping the tip of the blowpipe outside the luminous flame of the Bunsen burner, in which no inner cone or outer mantle is discernible, so that the greater part of the flame is blown to one side in the desired direction. In many cases, as in the preparation of beads from microcosmic salt, the flame from the ordinary Bunsen burner can be used to advantage without the aid of the blowpipe. In this flame is a large inner cone consisting of a mixture of illuminating gas and air; there is a slight luminous tip to this cone when the supply of air is so •limited that it is not quite sufficient to completely burn the gas. This luminous tip, as well as the entire border

12

INORGANIC CHEMISTRY.

between the inner cone and the outer flame mantle, is a reducing flame; but the outer flame mantle is an oxidizing zone because there is a constant supply of air reaching it from the outside. The outer mantle is the hottest part of the Bunsen flame; the inner zone, on the contrary, is cold; in it the combustion of the gas has not yet begun to take place. It is easy to show this by holding a match across the flame for a few seconds; that part of the match which was held in the inner cone will remain unchanged, whereas those portions in contact with the outer flame mantle will have become scorched and charred. B y holding beads in the oxidizing flame, oxidation beads are obtained; similarly b y holding the beads in the reducing flame reduction beads are formed; the latter should always be cooled in the inner cone rather than in contact with the oxygen of the air, which, in many cases, would cause oxidation to take place within the hot bead. I t is perfectly true that more satisfactory oxidizing and reducing flames can be obtained with the aid of the blowpipe than without it.

CORK BORING. T o make a hole in a cork stopper, a cork borer is chosen which is a little smaller than the diameter of the desired hole. The borer is warmed a little in the flame of a Bunsen burner (in no case should it be heated very hot) and then placed in the position shown in Fig. 8. The borer is

12

INORGANIC CHEMISTRY.

between the inner cone and the outer flame mantle, is a reducing flame; but the outer flame mantle is an oxidizing zone because there is a constant supply of air reaching it from the outside. The outer mantle is the hottest part of the Bunsen flame; the inner zone, on the contrary, is cold; in it the combustion of the gas has not yet begun to take place. It is easy to show this by holding a match across the flame for a few seconds; that part of the match which was held in the inner cone will remain unchanged, whereas those portions in contact with the outer flame mantle will have become scorched and charred. B y holding beads in the oxidizing flame, oxidation beads are obtained; similarly b y holding the beads in the reducing flame reduction beads are formed; the latter should always be cooled in the inner cone rather than in contact with the oxygen of the air, which, in many cases, would cause oxidation to take place within the hot bead. I t is perfectly true that more satisfactory oxidizing and reducing flames can be obtained with the aid of the blowpipe than without it.

CORK BORING. T o make a hole in a cork stopper, a cork borer is chosen which is a little smaller than the diameter of the desired hole. The borer is warmed a little in the flame of a Bunsen burner (in no case should it be heated very hot) and then placed in the position shown in Fig. 8. The borer is

CORK BORING.

13

held in the right hand against the palm and the cork is held by the fingers of the left hand. The cork is bored by rotating it always in one direction, meanwhile exerting a slight pressure against the borer. Should it be found difficult to make the boring all at one time, the borer is removed, any cork that has remained within it is taken out, the borer is once more heated slightly in the flame and the boring continued as before. Should the borer become dull or injured in any way, it may be sharpened by a special sharpener, or by means of a knife and three-cornered file. Corks for flasks should always be chosen a little larger than at first seems necessary; then by carefully softening the cork in a cork-press, while turning it frequently, it can be made to fit the neck of the flask tightly. If such a cork is to be perforated it should be softened first, then the hole bored, and the cork, that has been widened a little by the boring, pressed again while the hole is filled with the round file, or something similar.

ACIDS. Acids are compounds containing hydrogen; and the latter, in the presence of water, is either wholly or partly replaceable by metal. Monobasic acids are those that contain but one replaceable hydrogen atom in the molecule (e.g. hydrochloric acid, HC1; nitric acid, H N 0 3 ) . Similarly dibasic, tribasic and tetrabasic acids contain respectively two, three and four replaceable hydrogen atoms (e.g. sulphuric acid, H 2 S 0 4 ; phosphoric acid, H 3 P 0 4 ; pyrophosphoric acid, H 4 P 2 0 7 ) . The replacement of the hydrogen atoms in acids by atoms of metal gives rise to the formation of salts (first definition of salts). Neutral salts are formed when all of the acid hydrogen atoms have been replaced by metal (e.g. potassium chloride, KC1; sodium sulphate, N a 2 S 0 4 ; sodium phosphate, N a 3 P 0 4 ) . Acid salts are those in which the hydrogen has been partially but not wholly replaced by metal (e.g. sodium bisulphate, N a H S 0 4 ; disodium phosphate, N a 2 H P 0 4 ) . Another series of acid salts is formed by the addition of acid molecules to molecules of the neutral salts (e.g. sodium hydrogen fluoride, NaHF 2 ; potassium trinitrate, K H 2 ( N 0 3 ) 3 ) . In the case of the tribasic acids there are two series of acid salts and these may be distinguished by the use of the words primary and secondary; the corresponding neutral salts are then designated as tertiary salts. primary sodium phosphate, NaHjPO, primary calcium phosphate, Ca(H 2 P0 4 ) 2 secondary sodium phosphate, Na,HP0 4 secondary calcium phosphate, CaHP0 4 tertiary sodium phosphate, Na 3 P0 4 tertiary calcium phosphate. Ca 3 (P0 4 ) 2 14

ACIDS.

15

As shown by the above examples, the tertiary salts are those in which all three hydrogen atoms of a tribasic acid molecule have been replaced by metal; the secondary salts those in which two have been replaced; and the primary salts those in which but one hydrogen atom has been replaced. Aqueous solutions of acids turn blue litmus paper red, decolorize a solution of Phenolphthalein t h a t has been colored red by a little alkali, and change the yellow color of a methyl orange solution t o pink: acid reaction.

HYDROCHLORIC ACID AND CHLORINE. Hydrochloric acid, HC1, is a colorless gas with a suffocating odor; when exposed to the air it attracts water and fumes. It is very soluble in water. The so-called concentrated hydrochloric acid of the laboratory is an aqueous solution containing about 39 per cent by weight of the gas; the dilute acid contains about 23 per cent by weight and the 2-normal acid contains 72.9 grams of the gas in a liter of solution, or about 7.05 per cent by weight. Commercial hydrochloric acid contains, among other impurities, a little ferric chloride and is colored yellow thereby. Hydrochloric acid gas is more soluble in pure, cold water than in hot water, or in solutions of its salts, or in other acids. For this reason it is possible to prepare small amounts of the gas by allowing concentrated sulphuric acid to drop into concentrated hydrochloric acid; large amounts are prepared by heating sodium chloride, (common salt) with concentrated sulphuric acid. Hydrochloric acid dissolves many metals (e.g. iron, zinc and aluminium) with evolution with hydrogen gas. The chlorine in hydrochloric acid may be set free by heating the acid with oxidizing agents such as lead dioxide or manganese dioxide. Chlorine decomposes many dyestuffs and consequently decolorizes or bleaches them. It acts upon iodides and bromides, replacing the iodine and bromine and setting these elements free. Hydrochloric acid and all of its salts, the chlorides, give a white precipitate of silver chloride when treated in aqueous solution with silver nitrate or any soluble silver salt.

Place one or two cubic centimeters of concentrated hydrochloric acid (10 to 20 drops) in a test-tube and heat it under the hood; moist hydrochloric acid gas is evolved. 1 1 In heating liquids in test-tubes, especially when the liquids are saturated with gas or contain solid particles in suspension, the test-tube should be kept in gentle motion; this shaking serves to prevent any sudden boiling out of the liquids from the tube, and prevents superheating of the walls of the tube by keeping the liquid in contact with them. The tube should be pointed away from any person to prevent injury in case the liquid should be suddenly boiled out from the tube.

ltt

HYDROCHLORIC ACID AND CHLORINE.

17

To the same amount of hydrochloric acid in another test-tube, add about twice its volume of concentrated sulphuric acid (Hood); a considerable stream of hydrochloric acid gas is given off and the liquid foams. In carrying out this experiment do not pour the concentrated sulphuric acid directly from the reagent bottle into the hydrochloric acid, for in that case there is danger of the reagent becoming contaminated by the escaping gas. Heat about 2 grams of sodium chloride with about 1 cubic centimeter of concentrated sulphuric acid in a test-tube under the hood. Hydrochloric acid is evolved which, in this case, is perfectly dry: 2 NaCl + H 2 S0 4 = 2 HC1 + Na 2 S0 4 . Place about 4 grams of granulated zinc in a 50 cc. flask, moisten with a few drops of water, and add just enough concentrated hydrochloric acid to cover the metal. At once place an inverted funnel and testtube over the neck of the flask as shown in Fig. 9, holding the test-tube so that it does not quite rest upon the funnel. At the end of a minute raise the tube slowly, close it immediately with the thumb, invert it and open with the mouth of the tube near a flame. The hydrogen gas takes fire and burns in the test-tube with a colorless, scarcely-visible flame. After the flame has entirely gone out, place the tube once more over the funnel, but remove it when the air has been only partially replaced by hydrogen. This time on lighting the contents of the tube there is an explosion varying in intensity according to the Fig" 9" proportion of air and hydrogen in the mixture (detonating gas). In precisely the same way iron, aluminium and tin will

18

INORGANIC CHEMISTRY.

dissolve in concentrated hydrochloric acid with evolution of hydrogen. Next heat 0.5 gram of lead dioxide with about 1 cc. of concentrated hydrochloric acid in a test-tube under the hood. This time there is evolved chlorine,—a yellowish green gas with a characteristic, unpleasant odor. Since it attacks the mucous membrane, care should be taken not to inhale this gas. There remains in the test-tube, besides the excess of hydrochloric acid, a white crystalline solid, — lead chloride. 4 HC1 + P b 0 2 = 2 H 2 0 + PbCl3 + Cl2. For the technical preparation of chlorine, crude manganese dioxide, also called pyrolusite, is used instead of the much more expensive lead dioxide. Prepare a small evolution flask as shown in Fig. 10: the capacity of the flask is about 50 cc., the glass tube is bent to an angle of from 65° to 75°, one arm being 6 cm. and the other about 16 cm. long; the ends of the tube should be rounded in the flame. When the apparatus is ready, test it by blowing to see if it is tight. In the flask place about 2 grams of pyrolusite and pour upon it from 5 to 7 cc. of concentrated hydrochloric acid. Stopper the flask, suspend it from a ring stand, and introduce the delivery tube into a test-tube filled with water and held in the hand. Heat the flask gradually over a small gas flame. First air is expelled and escapes in bubbles through the water; then comes the chlorine gas, which is partially dissolved by the water, forming a yellowish-colored solution. This is the so-called chlorine water and may contain as much as 0.4 % of chlorine. After a few minutes take hold of the flask at the cork stopper, remove the test-tube and then withdraw the flame. If the flame were removed before taking away the test-tube,

HYDROCHLORIC

ACID

AND

CHLORINE.

19

water would suck back into the flask. The experiment must be carried out under the hood. Clean the evolution flask and tubing and put it aside for future use. 4 HC1 + Mn0 2 = 2 H 2 0 + MnCl2 + CI,. Hold pieces of red and of blue litmus paper in the upper part of the test-tube; in each case the litmus is bleached.

FIG.

10.

Add a little of the chlorine water to 1 cc. of indigo solution: the deep blue color of the indigo disappears immediately and a yellowish red color appears, caused by decomposition products of indigo. Add a drop of the chlorine water to a few drops of potassium iodide solution, and another to a few drops of potassium bromide solution; in the first C8IS6J di brown coloration due to free iodine is obtained, in the second a yellowish coloration caused by free bromine. 2 KI + CI2 = 2 KC1 + I 2 2 KBr + Cl2 = 2 KC1 + Br2. Mix a few drops of dilute hydrochloric acid with a little water and add some dilute silver nitrate solution; a white precipitate of silver chloride is formed which on shaking

20

INORGANIC CHEMISTRY.

collects in the form of curds. The precipitate is insoluble in nitric acid but is readily dissolved by ammonia water. HC1 + AgN0 3 = AgCl + HNO a . Dissolve a pinch of sodium chloride in distilled water, acidify with nitric acid, and add a little silver nitrate. Silver chloride is again formed. To test a solution for the presence of chloride, always acidify with nitric acid because there are many other salts of silver which are precipitated from neutral solutions but are soluble in nitric acid. Take, for example, a drop of sodium carbonate solution, add water and a little silver nitrate. A heavy precipitate of silver carbonate is formed which dissolves on the addition of a little nitric acid. If this solution becomes perfectly clear, the sodium carbonate was free from chloride; if a turbidity remains, then the carbonate contained chloride as impurity. Test some of the tap water for chlorine by partially filling a test-tube with it, adding a few drops of nitric acid, and then a little silver nitrate solution. A turbidity shows the presence of a trace, and a precipitate indicates the presence of a considerable amount of a chloride. All these experiments should be performed in test-tubes that have been carefully rinsed in distilled water. Closely related to hydrochloric acid are hydrobromic acid, HBr, hydriodic acid HI, and hydrocyanic acid, HCN; they behave like hydrochloric acid in most of their reactions. CHEMICAL REACTIONS.

By a chemical reaction is understood a process in which new substances are formed. Thus, in the reaction between hydrochloric acid and silver nitrate, silver chloride and nitric acid are formed. Between solid substances, such chemical changes take place only very slowly, if at all; reactions between a solid substance on the one hand and a liquid or gas on the other take place more rapidly; the

S U L P H U R I C ACID.

21

fastest reactions, as a rule, take place between substances which are both either liquid or gaseous. In analytical chemistry, the great majority of the reactions used take place in aqueous solutions. In order to identify substances by means of chemical reactions, it is necessary that the chemical change should be readily perceptible, as is t h e case when a precipitate or a colored substance is formed or a gas evolved; when the new substance is thus easily recognized the reaction is called characteristic. A reaction is said to be sensitive when it can be carried out with very minute quantities of the substance. Thus silver nitrate is a sensitive reagent for detecting the presence of chloride or hydrochloric acid, because a very small amount of these substances will give a noticeable precipitate when treated with silver nitrate; the precipitate in this case is white, insoluble in nitric acid, and dissolves readily in ammonia. Silver cyanide, or silver thiocyanate, are the only other substances likely to be mistaken for silver chloride and it is easy to distinguish between these three silver salts by other chemical reactions. The analytical chemist desires to know what chemical reactions can be used for the identification of a substance, the extent to which they are reliable, and the conditions under which they take place. In the following parts of this book the reactions are for the most part represented by equations. The water which is almost invariably present as solvent is left out of consideration. In many cases the atomic formula instead of the molecular formula is given, e.g. H instead of H 2 ; and this is justifiable because the reactions take place with atoms rather than with molecules; and, moreover, the size of the molecule is in many cases unknown. As examples of simple equations: NaCl + AgN0 3 = AgCl + NaNO a , 2 NaCl + H 2 S0 4 = 2 HC1 + Na 2 S0 4 , HgO = Hg + O.

SULPHURIC ACID. Sulphuric acid is a thick, oily liquid which is colorless and odorless. The concentrated sulphuric acid of the laboratory contains between 97 and 98.5 per cent, by weight of pure H 2 S0 4 , the dilute acid (2-normal) 9.25 per ccnt by weight. Commercial sulphuric acid contains a little lead sulphate. Concentrated sulphuric acid decomposes most organic substances, usually with carbonization. Care should be taken, there-

22

INORGANIC CHEMISTRY.

fore, not to spill the acid, and also that all vessels into which it is poured are perfectly clean. Drops of the concentrated acid burn holes in clothing; the dilute acid produces red spots which can be removed by treatment with ammonia, even after some time. Dilute sulphuric acid is a solvent for many metals (iron, aluminium, zinc, etc.), forming sulphate of the metal with evolution of hydrogen: H 2 S0 4 + Fe = FeS0 4 + H2. Concentrated sulphuric acid does not dissolve the above-mentioned metals in the cold; at higher temperatures the sulphates are formed, as with the dilute acid; but the hydrogen that is set free acts upou the excess of acid and reduces it to sulphur dioxide: Fe + H 2 S0 4 = FeSO, + 2 H, 2 H + H 2 S0 4 = 2 H 2 0 + S0 2 . If zinc is used instead of iron, the reduction process goes still further and sulphur, or even hydrogen sulphide, is formed: 6 H + HjSO, = 4 H 2 0 + S, 8 H + HjSO, = 4 H 2 0 + H2S. The sulphur atom in sulphuric acid is in all probability hexavalent. It is connected by double bonds with two atoms of oxygen and furthermore with two hydroxyl groups. If each bond is represented by a dash, we may write the constitutional formula of sulphuric acid thus: O^"

OH

In this connection it is worth mentioning that our present knowledge of the way the atoms are combined in inorganic substances is extremely limited: such relations have been worked out to a much greater degree of certainty in the case of organic substances. Since the molecule of hydrochloric acid contains no oxygen it cannot give an anhydride. Sulphuric acid, on the other hand, can be converted into sulphuric acid anhydride, also called sulphur trioxide. Sulphur trioxide exists in two modifications: as a colorless oil, and as a white crystalline substance; both forms attract moisture when exposed to the atmosphere, and fume strongly. By dissolving sulphur trioxide in concentrated sulphuric acid, the so-called fuming sulphuric acid is obtained. Theoretically this should not be regarded as a solu-

SULPHURIC ACID.

23

tion, but as a new compound, pyrosulphuric acid, which is formed by the union of two molecules of sulphuric acid with loss of water: s o

>m one containing hydrochloric acid. In the last case the addition cf sodium acetate solution will diminish the concentration of the H ion; so t h a t the zinc will be precipitated.

80

INORGANIC CHEMISTRY.

The solubility product of cupric sulphide is smaller than that of either of the above sulphides. Even in the presence of considerable H ions, as in a one per cent solution of hydrochloric acid, the concentration product of the Cu and S ions exceeds the solubility product of cupric sulphide, and the latter is precipitated. When the original S ions are removed by the precipitation of the sulphide, more of the hydrogen sulphide dissociates so that it is possible to precipitate a large amount of copper by passing hydrogen sulphide into the solution, even although the amount of S ions present in the solution at a given instant is extremely small.

3. MAGNESIUM GROUP. The metals of the magnesium group include beryllium, magnesium, zinc and cadmium. These elements are stable in the atmosphere because they become covered with a thin film of oxide which protects them from further oxidation. In a glowing condition they combine readily with oxygen; thus ignited magnesium will burn without further application of heat. The oxides and carbonates are very slightly soluble in water; the chlorides, nitrates, and sulphates are readily soluble. Of the sulphides, cadmium is the most stable, as well as the most insoluble. It is dissolved by hot dilute sulphuric acid, but is more stable than zinc sulphide, which is dissolved by cold dilute acid of the same concentration. Magnesium sulphide, as well as beryllium sulphide, which has never been prepared pure, are decomposed by the action of water; they can be prepared only in the dry way. MAGNESIUM. H o l d a short piece of m a g n e s i u m r i b b o n w i t h pincers a n d b r i n g i t in c o n t a c t w i t h a B u n s e n flame; t h e m e t a l t a k e s fire a n d burns w i t h o u t f u r t h e r a p p l i c a t i o n of h e a t , s e n d i n g o u t a blinding, w h i t e l i g h t a n d f o r m i n g a w h i t e s m o k e of finely d i v i d e d m a g n e s i u m o x i d e . P l a c e some of t h e ash in a porcelain e v a p o r a t i n g dish. Moisten a piece of red l i t m u s a n d t o u c h i t to t h e p o w d e r ; the p a p e r t u r n s blue a f t e r some t i m e , due to t h e m a g n e s i u m h y d r o x i d e f o r m e d w h i c h is n o t a b s o l u t e l y insoluble in w a t e r .

MAGNESIUM.

81

Dissolve the remainder of the magnesium oxide in dilute hydrochloric acid, using as little as possible; dilute the solution with water and filter off any undissolved particles. These consist chiefly of silicon dioxide formed from a small amount of silicon present as impurity in the magnesium ribbon. To separate portions of the magnesium chloride solution add the following reagents: Sodium hydroxide gives a white flocculent precipitate of magnesium hydroxide. MgCl2 + 2 NaOH = Mg(OH)2 + 2 NaCl. Ammonia likewise forms white magnesium hydroxide; the precipitation is incomplete at best (cf. pp. 57, 75) and upon the addition of ammonium chloride the precipitate dissolves. If too much hydrochloric acid was used in dissolving the magnesium oxide, no precipitate will be formed upon the addition of the ammonia. In such a case, repeat the experiment with a neutral solution of a magnesium salt. MgCl2 + 2 NH,OH = Mg(OH)2 + 2 NH4C1, ID the absence of ammonium salts.

Mg(OH)¡¡ + 2 NH4C1 = MgCl2 + 2 NH 4 OH, In the presence of ammonium salta.

The above equations are identical except that they are written in the reverse order. The reaction, therefore, takes place according to the external conditions, either in the direction left to right, or conversely right to left. Such " reversible reactions " are expressed by using a double arrow, in place of the equality sign (cf. p. 64). MgCl, + 2 NH4OH