Experimental chemistry for junior students

PART. III. Med

FOR

JUNIOR STUDENTS.

BY

J. EMERSON REYNOLDS, M.D., F.R.S.

VICE-PRESIDENT CHEMICAL SOCIETY OF LONDON ; PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF DUBLIN.

PART III.

METALS AND ALLIED BODIES

{with an Analytical Appendix).

LONDON :

LONGMANS, GREEN, AND CO.

I.ONDON : PRINTED BY

SPOTTISWOODE AND CO., NEW-STKEET SQUARE AND PARLIAMENT STREET

Preface

The success which has attended the issue of Parts I. and II. of this work leads the author to hope that the present volume, which continues the same line of study, may also prove useful to a large class of students. His aim in this Part, as in its predecessors, is to place the student to some extent in the position of an independent investigator of chemical phenomena. This consideration has largely influenced the order of study pursued, which, though somewhat unusual in dealing with the metals and allied bodies, seems best adapted to lead the worker to recognise the natural affinities of the elements, as well as their distinctive characters.

Detailed descriptions of metallurgical processes would be obviously foreign to the character of the work, but an outline of each of the more important operations is given, and illustrative experiments are added whenever they are likely to elucidate the general statement.

Preface .

iv

The author trusts that the Analytical Appendix will be found equally useful to advanced as well as to junior students. The Synoptic Tables included in the Appendix have been employed with advantage for some years in the Dublin University Laboratory. They were arranged by the author with a view to enable students to compare the reactions of the metals, and to aid in planning methods for their recognition in simple and in complex mixtures presented for analysis. The ready-made tables often given for the separation of the metals, though generally good, possess little educational value, and their use often produces unintelligent analysts ; whereas the gradual development of such schemes by the student for himself is a most useful intellectual exercise. A special table for the analysis of a solution containing but one metal is given as an illustration of the many schemes which may be devised for similar purposes.

J. E. R.

Trinity College,

Dublin University.

Contents

CHAPTER XXVI.

PAGE

Experiments with arsenic and its compounds — Reduction of arsenic — Oxides — Arsenious and arsenic acids —

Appendix

Note on a classification of the elements. . . . 294

Note on crystalline forms 297

Note on spectrum analysis 299

Analytical tables and instructions for systematic qualitative analysis 303

FOR

JUNIOR STUDENTS.

PART III ;

CHAPTER XXVI.

Introduction

EXPERIMENTS WITH ARSENIC AND ITS COMPOUNDS.

The group of pentad or five-link elements, to which nitrogen and phosphorus belong, includes, as already stated , 1 three other important members, namely, Arsenic, Antimony, and Bismuth. Of these, the two former are in different degrees pseudometallic in character, and connect the great division of non-metals with that of metals, while bismuth more distinctly belongs to the latter class. The examination of these bodies therefore naturally precedes that of the elements usually classed as metals, and thus preserves the continuity of our line of study. Moreover, in experimenting with the metallic elements generally, the order followed will be found to nearly coincide with that of their atomicities and electro-

1 PartTl. page 270.

B chemical relations, those of higher atomicity or electro - negative characters being first examined, wherever this course does not interfere with the natural grouping of the metals.

Arsenic. Symbol, As v = 75. Weight op gaseous

molecule = 300.

We have already employed white oxide of arsenic (As 2 0 3 ) in Experiment 106, and as this is the compound of the element that can be most easily procured, we shall use it in the following experiments, but the student must handle the body with great care, and preserve it and all of its derivatives under lock and key, as most arsenical compounds are well known to be deadly poisons.

Experiment 350. — Introduce a small quantity of the powdered white oxide of arsenic into a narrow test-tube and gently heat. Note that the body is vaporised, without odour, and that small brilliant crystals, with triangular faces, as shown in fig. hi, deposit on the cooler sides of the tube. These crystals have the same composition and properties as the original powder; hence the latter volatilizes unchanged.

Experiment 351. — With a glass rod rub the crystals formed as above down the sides of the tube to the bottom ; then place over the oxide some dry charcoal in rough powder, and apply heat to that part of the tube where the charcoal lies. Note that an odour resembling garlic is developed, and soon the

Fig. hi.

Reduction of Arsenic Compounds .

Fig. i 12.

cool sides of the tube above the charcoal, as at a, fig. 1 1 2, become coated with a dark metallic-looking film, while above the latter some crystals of the oxide usually appear, unless air is excluded by nearly covering the mouth of the tube with the finger while heating. The dark metal-like body is the element Arsenic, thus reduced from its oxide by means of carbon.

Arsenic as thus obtained has the metallic lustre, and is a moderately good conductor of heat and electricity. It is converted into vapour at 180° C., and its specific gravity in that condition is 150 (H = 1); but, for reasons similar in kind to those already assigned in the case of phosphorus (vide Part II. page 269), the atomic weight of arsenic cannot be greater than 75 ; hence the gaseous molecule of the element, like that of phosphorus, includes four atoms. Moreover, the heat capacity of arsenic is such that 80 '9 c.grs. contain at ioo° C. the same quantity of heat as 108 c.grs. of pure silver at the same temperature (see Part I. page 58) ; and this value is so close to 75, and so far removed from 150, that it amply confirms the lower atomic weight.

Elemental arsenic occurs in very small quantity in nature ; it is chiefly prepared from arsenical pyrites, a native arsenio-sulphide of iron, FeSAs, termed mispickel by mineralogists. When this compound is strongly heated in vessels from which air is excluded, most of the arsenic sublimes, together with a little sulphur, and much of the crude ‘ metallic ’ arsenic of commerce is obtained by this method. It is commonly met with in the form of greyish-black lumps, and is used to a small extent as a poison for flies, but in much greater quantity in the manufacture of shot (see Lead).

Experiment 352. — Powder a few grains of the dark grey elemental arsenic — which is very brittle — and allow it to fall into a jar of chlorine gas ; note that the arsenic takes fire as it falls through the gas, and white fumes are produced, which can be condensed to an oily liquid. The latter has the composition indicated by the formula AsC 1 3 , and results from the direct union of the two elements. This trichloride is the only compound of chlorine and arsenic yet known ; the arsenic analogue of PC1 5 is therefore wanting.

When water is added to the contents of the jar the trichloride is decomposed thus : —

Tri-iodide and tribromide of arsenic are known, and decompose with water like the trichloride.

Experiment 353. — Heat a minute fragment of the elemental arsenic on the bottom of a wide test-tube; note that the element disappears and crystals of the oxide are deposited on the side of the tube.

Thus arsenic is easily oxidised when heated in air.

Experiment 354. — Obtain a small quantity of

Roasting A rsenical Ores.

native mispickel 1 — FeSAs — roughly powder it, and introduce a small quantity into a glass tube, open at both ends (see fig. 113). This tube should be about 30 c.ms. long Fig. 113.

and 5 to 7 m. ms. in diameter. The powdered mineral is deposited in the tube about 4 or 5 c.ms. from one end, and the glass on which the mineral rests is gently heated in the gas flame, as shown in the figure.

It is necessary to incline the tube so that the unheated end shall be well above that which is in the gas flame ; thus a current of air is established over the heated mineral, and oxidation of its constituents proceeds. Sulphur dioxide gas (S 0 2 ) issues from the top of the tube, and more or less of a nearly white or yellowish sublimate is observed in the tube above the ore, and throughout this the easily recognised crystals of As 2 0 3 can be detected, while the residue assumes a very dark brown colour, especially when allowed to cool ; this is due to the conversion of the iron into Fe 2 0 3 . The complete decomposition of the mineral is represented by the equation--

This mode of decomposing an ore is termed

1 Instead of this ore the mineral leucopyrite , F eAs 2 , a simple arsenide of iron, may be employed.

roasting , and the process just carried out is one of the best illustrations of this very/ common treatment of metallic ores.

On the large scale the glass tube is replaced by a long iron tube, or a brick furnace, in which mispickel, or other ore containing arsenic, is roasted, and the fumes are passed through long condensing chambers in which the impure oxide of arsenic is deposited. This crude material is purified by resublimation in iron pots, and then appears as the ‘white arsenic,’ or simply, though incorrectly, ‘ arsenic ’ of commerce.

This body is a deadly poison, and when sold as a white powder has been frequently mistaken for many innocuous white bodies. Thus it has been added to flour instead of ‘ bread soda,’ to soup instead of salt, &c., and has caused so many fatal accidents that the Sale of Poisons Act now requires the vendors to colour it with a small proportion of either soot or indigo.

The sesquioxide of arsenic is met with in commerce in two forms; one of these is a white crystalline powder, the other is a glassy mass; the latter gradually undergoes change into the former, and becomes opaque.

Experiment 355. — Digest with gentle heat a small quantity of crystalline As 2 0 3 with some water in a test-tube. Note that the solid seems to be insoluble in water. Allow the powder to subside and pour off most of the liquid into a clean test-tube ; 1 note that

1 Although the specific gravity of the oxide is greater than that of water (37), some of the powder persistently floats on the latter, owing to adhesion of air which buoys up the particles. This is a point of some medico-legal interest.

Arsenical Wall Paper .

the solution is sensibly acid to blue litmus paper. Add to the liquid a few drops of solution of ammonia-silver nitrate, 1 a canary-yellow precipitate is obtained which has been found to consist of Ag' 3 As0 3 , evidently the silver salt of the acid H 3 As0 3 . We conclude, then, that arsenic sesquioxide is sensibly soluble in water (one part dissolves in forty parts if the mixture be boiled for an hour and allowed to cool, but if boiled for a few minutes only, about eighty parts of water are required for the solution of one of the oxide), and acts as the anhydride of the tribasic acid, whose formula is given above, which is termed arsenious acid.

Arsenious Arsenious

anhydride. acid.

When the solution of the acid is evaporated the anhydride separates out ; hence the acid is easily decomposed — so easily that it has not been isolated in a pure state.

Experiment 356. — Add to some of the solution of arsenious acid prepared in the last experiment a few drops of ammonia-copper sulphate. Note that a grass-green precipitate is formed. The composition of this is represented by the formula Cu^ITAsOg — an arsenite of copper and hydrogen commonly known as Scheele’s Green. 2 This compound is often, and

1 Prepared by adding to silver nitrate very dilute ammonia drop by drop, with agitation, until the precipitate first formed is just redissolved. The copper compound is prepared in the same way from copper sulphate solution.

2 Another pigment, termed Schweinfurt Green, is obtained by boiling equal parts of arsenic sesquioxide and copper acetate with water for some hours.

improperly, used as a pigment for wall-papers, and many cases of chronic arsenical poisoning have arisen in consequence, for the pigment, though not volatile, is easily detached from the paper and mechanically carried through the air of a room to the lungs and skins of the inhabitants. In order to test a piece of such paper for arsenic, dry the material well, scrape off some of the green pigment, introduce it into a narrow tube of hard glass and cover with a layer of powdered charcoal. Heat as in Experiment 351, when a dark ring of arsenic will be obtained, or crystals of the oxide on the cooler portions of the tube, if the pigment be arsenical.

Experiment 357. — Dissolve a small quantity of potassium carbonate, K 2 C0 3 , in half a test-tube of water and add a little arsenic sesquioxide : on boiling the liquid, solution of the arsenic takes place. At first solution is not accompanied by evolution of carbon dioxide gas, but if the boiling be long continued gas is evolved and a true potassium arsenite — K 3 As0 3 — is formed. When this solution contains four grains of the oxide and the same weight of potassium carbonate in one fluid ounce of water, coloured red with compound tincture of lavender, it constitutes Fowler 's solution or Liquor arsenicalis , B. P. For the action of free iodine on this solution see Experiment 363. The oxide is readily soluble in the caustic alkalies KOH and NaOH, and at once forms arsenites.

Experiment 358. — Boil some arsenic sesquioxide with water containing a few drops of hydrochloric acid. Solution is easily effected, and if the liquid contains much arsenic when hot it will deposit crystals of the

Tests for Arsenic Acid.

anhydride on cooling. A solution containing four grains of the oxide in one ounce of dilute hydrochloric acid constitutes the Hydrochloric solution of arsenic , B. P. This liquid does not contain arsenic trichloride — A sC 1 3 — as the latter is easily decomposed by water in the manner already pointed out under Experiment 352, though during solution a little of the trichloride is formed and volatilised from the boiling liquid.

Experiment 359. — We have already learned from Experiment 106 that As 2 0 3 is oxidised by moderately strong nitric acid, and a new acid of arsenic obtained containing an atom of oxygen more than arsenious acid, and termed Arsenic acid — H 3 As0 4 . Repeat the experiment in a test-tube, and boil off as much of the excess of nitric acid as possible; then dilute some of the solution with water and add a few drops of ammonia-silver nitrate. Note that a dull brick-red precipitate is formed whose composition is represented by the formula Ag 3 As0 4 or silver arsenate. Arsenious and arsenic acids are therefore easily distinguished by the colours of their silver salts (see Experiment 355).

Experiment 360. — Take some of the arsenic acid solution and evaporate it rapidly to dryness in a capsule; a white deliquescent mass is left which consists of arsenic anhydride — As 2 0 5 — and this, when dissolved in water, reproduces arsenic acid, as shown by the action on litmus and by the silver test.

Experiment 361. — To another portion of arsenic acid solution add sodium carbonate until effervescence ceases, then evaporate until crystals form. These crystals consist of Na 2 HAs0 4 , i2H 2 0, and therefore correspond in composition with the common sodium phosphate (Part II. page 259). Moreover, the crystals are almost identical in form with those of the phosphate; hence the two salts are said to be isomorphous . l The British Pharmacopoeia directs this salt to be prepared by the following indirect process.

Experiment 362. — Heat in a tube a mixture of about equal weights of arsenic sesquioxide and sodium nitrate with half the weight of dry sodium carbonate. The mixture fuses and begins to effervesce ; continue the heat until gas ceases to come off, then allow the residue, which consists of sodium pyro-arsenate — Na 4 As 2 0 7 — to cool and solidify. The following change takes place during fusion : —

The residue is boiled with water, in which the sodium pyro-arsenate dissolves; at the same time it unites with a molecule of water and forms the sodium and hydrogen ortho-arsenate prepared in the last experiment ; the latter can be crystallised out from the solution.

Sodium Sodium and hydrogen

Pyro-arsenate. Ortho-arsenate.

The crystallised salt contains twelve molecules of water of crystallisation, five of which are lost on mere exposure to dry air, and the remaining seven molecules of water are expelled on heating the body to 150° C.

1 See page 12.

Action of Iodine on Arsenious Acid. i t

Experiment 363.— Add solution of iodine in potassium iodide drop by drop to some of the solution obtained in Experiment 357. Note that the colour of the iodine disappears until a point is reached at which no further decolouration occurs. If the solution be examined it is now found to contain arsenic acid, or rather its sodium salt, the iodine having served as an indirect oxidising agent. Thus, assuming the acids to be free —

This is the foundation of process for the volumetric estimation of arsenic in the arsenious condition. For this purpose it is necessary to have a considerable excess of solution of sodium hydrogen carbonate 1 — NaHC0 3 — present, in order that the arsenic and hydriodic acids produced may be neutralised as fast as they are formed. The reaction is irregular and incomplete unless this precaution is adopted.

Experiment 364. — Dilute a few drops of arsenic acid solution, and add some magnesium sulphate solution followed by ammonium hydrate, as in Experiment 345 with a phosphate, a crystalline precipitate forms, which contains Mg // NH 4 / As0 4 , 6H 2 0. This compound is isomorphous with the corresponding phosphate, and affords magnesium pyro-arsenate — Mg 2 As 2 0 7 — when heated to low redness. Generally the arsenates and phosphates closely correspond in crystalline form and chemical constitution, and the analogies can be traced not only through the

1 The neutral carbonate Na 2 C0 3 cannot be used for this purpose, as it decolourises a very sensible quantity of iodine, even in the absence of arsenic.

ortho-derivatives, but through pyro- and meta-compounds, for meta - arsenates have been obtained, though these bodies, unlike the meta-phosphates, are not known in solution. Analogy would lead us to conclude that arsenic is a pentad like phosphorus, and the constitution of arsenates can best be represented on this view, but in most of the common compounds of the element, e.g. the trichloride, arsenious anhydride, &c., it acts as an apparent triad.

Experiment 365. — Add a few drops of lead nitrate solution to a liquid containing some alkaline arsenate. Note that a white precipitate is formed as in the analogous case of a phosphate. The composition of the body precipitated is represented by the formula Pb // 3 (As 0 4 ) /// 2 ) it therefore corresponds to lead ortho-phosphate, Pb // 3 (P 0 4 ) /// 2 , which is formed under similar conditions.

Isomorphism . — Lead arsenate and phosphate both occur in nature in combination with lead chloride as well crystallised minerals ; the mineralogical name of the arsenate is mimetesite , and of the phosphate Fig ii pyromorphite. When well-defined natural p crystals of the two minerals are exam-

ined, they are found to be very similar in form — that is to say, they are six-sided prisms resembling fig. 114 , whose base is a regular hexagon, more or less modified by the planes p p derived from a six-sided pyramid.

Crystals differing but little from the above type, and from each other in form, have been found to vary in composition to the very wide extent shown by the following analyses : —

Law of Isomorphism.

Arsenate of Lead

II

hi.

Phosphate of Lead .

—

19*00

89-91

Chloride of Lead

9’93

9 ’ 3 °

10*09

100*00

100*00

100*00

The formulae derivable in the usual way from analyses I. and III. are

III.— Pyromorphite . 3[Pb // 3 (P0 4 ) //// 2 ] + PbCl 2 .

Hence these bodies are strictly analogous in chemical constitution as well as almost identical in crystalline form. So close is this relationship that, as in Analysis II. above, a portion of the arsenate of lead can be replaced by the phosphate without any change of structure, but such a replacement always takes place in equivalent , or rather molecular , proportions. Thus, in the above instances the molecular weights of lead arsenate and phosphate are respectively 899 and 813, and it is in the ratio of these weights that the compounds replace each other in the crystalline group.

Facts of the order just indicated led Mitscherlich in 1819 to enunciate the general principle that analogous groups of elements can replace each other in composition without essential alteration of crystalline form ; but it must be remembered that any such replacement takes place in the proportions already pointed out.

Mitscherlich ’s £ law of isomorphism 5 is often used in chemistry to determine the class relationships of particular elements, and incidentally to settle questions affecting atomicity, &c. In this way, as we have seen, the natural affinities of arsenic and phosphorus can be traced; and we shall presently refer to the rare element vanadium, which can be similarly connected with the phosphorus and arsenic group; while later on, in the cases of iron and aluminium and other metals, the law will be found to prove a valuable aid in solving problems which are otherwise difficult of solution.

An intelligent student will scarcely fail to recognise in the facts on which the law of isomorphism rests so many apparent contradictions to the ‘ law of constant composition,’ which he has been rightly taught to regard as the foundation of the science of chemistry. The apparent conflict between these two great principles admits of a satisfactory explanation, if we suppose (and we have good reason to do so) that the crystalline molecule is a much more complex system than the chemical molecule, and is made up of a large number of the latter. Thus the chemical molecule of mimetesite may be represented by the formula above given, whereas a crystal of the mineral is probably built up of many such groups, arranged symmetrically about the crystalline axes, just as bricks build up a pillar. It is very easy to understand, then, that during the growth of such a crystal in a liquid containing not only arsenate but the phosphate of lead, some of the chemical molecules of the latter may seize the opportunity afforded to them of entering as chemical units into the formation of a structure similar to that which they could produce unaided, and the crystal, when complete, would necessarily exhibit the mixed

Reduction of Arsenic Acid.

character of the mineral of the second analysis, because some of the bricks (i.e. chemical molecules) consist of arsenate and others of phosphate of lead. The substitution necessarily takes place in the ratio of the molecular weights of the bodies concerned.

Having sufficiently traced the products of oxidation of arsenic, we shall now examine the effect of reducing agents on arsenic acid and other compounds.

Experiment 366. — Add some sulphurous acid solution to arsenic acid contained in a test-tube, and boil until all smell of the former body completely disappears. Now test the residual liquid with ammonia-silver nitrate, and note that yellow silver arsenite is formed; therefore sulphurous acid reduces arsenic acid 1 to arsenious ; thus—

Sulphurous acid is unable to carry the reduction further; but we already know from Experiment 114 that nascent hydrogen is a reducing agent of great power, hence we shall try its effect on arsenious acid.

Experiment 367. — Fit up the hydrogen evolution bottle as shown in fig. 115, and have tlje bent tube t drawn out to a fine jet. 2 Introduce some pure zinc and water into the bottle, then pour in a little pure sulphuric acid. If the materials are pure, gas will be slowly evolved, but the addition of a few drops of copper sulphate will lead to brisk evolution of gas, owing to electro- chemical action. When air has been

1 See further experiment 375.

2 This should be narrow, and made of glass which is free from lead and difficult of fusion, or * hard, ’

1 6

Experimental Chemistry.

expelled from the bottle, and the test recommended in Experiment 20 applied, kindle the gas at the jet and note that the burning hydrogen, if the materials employed are pure, does not cause any stain on a piece of white porcelain when the latter is depressed in the flame. Now pour in through the funnel tube a few drops of solution of arsenious acid ; presently the flame changes to a pale bluish colour, and when

allowed to impinge on FlG * IIS ’ the white porcelain a

dark stain is left on the latter. The dark substance deposited from the flame is arsenic and is easily identified, as it is very soluble in a solution of ‘ bleaching lime ’ or other hypochlorite . 1

Apply a gas flame to the part of the tube marked #, and note that a mirror-like deposit forms just beyond the heated portion. This can also be shown to be arsenic when the tube is removed and washed with the bleaching lime solution. Before doing so, however, remove the flame from #, turn the point of the jet downwards, and make the issuing gas bubble through solution of silver nitrate contained in a test-tube. Note that a dark 1 The solution of the stain is due to the oxidation of the arsenic to arsenious acid or an arsenite.

Marsh's Test for Arsenic. 17

precipitate forms in the solution, and this can easily be shown to consist of finely divided metallic silver.

It is evident that arsenic is given off from the evolution apparatus in some gaseous form, and the particular form is that of an arsenide of hydrogen, whose formula is AsH 3 . This body is the result of the complete deoxidation of arsenious acid by nascent hydrogen, thus —

This hydride is easily decomposed by heat into hydrogen and arsenic, as in the heated tube and in the flame. Moreover, as we have seen, it acts as a reducing agent for silver nitrate, and this action is explained by the following equation —

The action of nascent hydrogen generated as above serves as a most convenient and delicate test for arsenic, as the effects of heat and of silver nitrate on the arsenical gas are quite characteristic; this mode of recognising the element is termed Marsh's test. 1

The hydride of arsenic obtained in this experiment, mixed with hydrogen, is when pure a colourless and highly poisonous gas, which the operator must be careful to avoid inhaling. It is evidently analogous in composition to phosphuretted hydrogen PH 3 and ammonia NH 3 , though it does not possess the spon-

1 In applying this test to the search for minute quantities of arsenic it is necessary to bear in mind that the zinc and acid employed may contain traces of arsenic ; hence the materials should all be carefully tested in the first instance.

III. C taneous inflammability of the former or the alkaline characters of the latter. A solid hydride is known whose formula is As 2 H or As 4 H 2 .

Experiment 368. — We already know (from Experiment 1 1 4 ) that zinc and strong sodium or potassium hydrate solution when boiled together afford hydrogen. Repeat this experiment, adding a few drops of arsenious acid to the contents of the tube, and cover the mouth of the latter with a piece of bibulous paper moistened with silver nitrate solution. The paper is soon discoloured owing to the action of the AsH 3 evolved on the silver salt, as already explained. This mode of testing for arsenic is termed Fleitmann's test.

In the above two instances of reduction of arsenious acid by nascent hydrogen we do not obtain the free element but its hydride, and this occurs when many other as highly electro-positive metals as zinc are used for the reduction; we shall therefore next try the metal copper, which is electro- negative to zinc though electro-positive to arsenic.

Experiment 369. — Introduce into a test-tube a few bright copper turnings, cover them with water and add a few drops of strong hydrochloric acid; no gas is evolved even when the mixture is gently warmed. Now add a few drops of solution of arsenious acid and boil for a minute, the copper quickly acquires a grey metal-like coating ; pour off the acid liquid and wash the coated copper repeatedly with pure water, then shake the turnings out on a piece of filtering paper, and dry the copper by pressure between the folds of paper. On examination the coating is seen to resemble arsenic in appearance. Next introduce

Reinsctis Test for Arsenic .

the turnings into a narrow hard glass tube closed at one end and heat; note that the copper regains its colour and a sublimate is obtained which is of a grey mirror-like character, if the tube be very narrow and air excluded, or crystalline and recognisable as arsenic sesquioxide if the tube be moderately wide. In the first part of the experiment, then, the copper displaces the arsenic from solution, and the latter separates as a coating on the undissolved copper, thus —