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in: if gold be present, a reddish or purplish brown precipitate or cloud will be formed, marking the passage of the stannous solution as it mixes with the liquid under examination. It is better, in performing this experiment, to operate upon a considerable quantity of an extremely dilute gold solution (a drop or two of auric chloride in a beaker of water). This precipitate is called “purple of Cassius”: when dry, and in fine powder, it has a dull blue tint. Its composition has been much discussed; but the formula Au Sn,03 + 2aq seems a probable one. This compound may be prepared by placing an aqueous solution of auric chloride in contact with pure granulated tin. The purple of Cassius, when moist, is soluble in hydrate of ammonium, forming a solution of a magnificent reddish purple colour; boiled in hydrate of potassium, however, or in water, it remains unchanged. Boiling concentrated nitric or hydrochloric acids, or dilute sulphuric acid, act upon it but slowly, removing the tin.
Most metals precipitate gold from its solutious in the metallic state, either as a brilliant metallic deposit, or in the form of a brown powder. Some metals yield a purple powder containing gold, oxygen, and the metal used as the precipitant. Phosphorus, sulphur, and many reducing agents, separate metallic gold from its solutions.
One of the most characteristic tests for the presence of gold in a solution is the reduction of the metal effected by the addition of ferrous sulphate (Fe, SO). If to a solution of gold containing 1 part of metal in 80,000 parts of water, a solution of ferrous sulphate be added, a bright blue tint is produced ; even if the mixture be diluted to 320,000 parts, a pale violet colour is apparent.
AUROUS SALTS, OR PROTO-SALTS OF GOLD.
There appear to be no soluble salts of this series; the insoluble salts known are these,—the chloride, the iodide, the oxide, and the sulphide.
THE CHLORIDE is obtained in variable quantity whenever auric chloride is evaporated to dryness. The latter salt is completely converted into this compound, with loss of chlorine, if it be heated for some time to the meltingpoint of tin (228° C.), with continued stirring. It is a yellowish white
powder. Its formula is Auci. By water, especially on ebullition, it is resolved into auric chloride and metallic gold.
THE IODIDE is produced by adding a solution of iodide of potassium, not in excess, to a neutral solution of auric chloride: it is a lemon-yellow cry. stalline powder. Its formula is Aul. It is very soluble in excess of its precipitant. Water and acids are without action upon it in the cold ; but upon raising the temperature, they decompose it into iodine and gold.
THE OXIDE is obtained by the action of boiling hydrate of potassium on a solution of auric chloride, or on the solid aurous chloride: it is a dark green powder. Its formula is Au,O. It is slightly soluble in hydrate of potassium ; hydrochloric acid converts it into auric chloride and metallic gold.
The Sulphide is obtained by passing hydrosulphuric acid gas through a boiling solution of auric chloride: it is a brownish black powder. Its formula is Au, S.
No other salts of this series are well known.
AURIC SALTS, OR PER-SALTS OF GOLD.
Solution for the reactions :—auric chloride (AuCl,) in water.
The principal insoluble auric salts are the iodide, the hydrate, and the sulphide.
THE CHLORIDE (AuCl,) is soluble.
THE IODIDE is produced when a solution of auric chloride is gradually added to a solution of iodide of potassium ; the liquid becomes green and deposits a green precipitate, which redissolves on agitation.
Its formula is Aul,; but it is speedily resolved into aurous iodide and iodine.
It is soluble in iodide of potassium solution.
THE CYANIDE is produced by adding cyanide of potassium, not in excess, to a solution of auric chloride : it is a yellow precipitate. Or it may produced by adding an excess of cyanide of potassium, and then acidifying with hydrochloric acid.
Its formula is probably AuCyz.
It is insoluble, or nearly so, in dilute hydrochloric acid, but dissolves in excess of its precipitant, forming a salt to which the formula KAuCy, has been assigned, and which is much employed in electro-gilding.
THE HYDRATE is produced by the action of hydrate of potas
sium, not in exeess, and of the ordinary temperature, upon a solution of auric chloride. It is a reddish yellow precipitate.
Its formula is unknown.
It dissolves in excess of its precipitant, forming aurate of potassium, probably KAu0. It is also soluble in hydrochloric acid, and in great excess of nitric or sulphuric acid.
Carbonate or hydrate of ammonium added (not in excess) to a tolerably concentrated normal solution of auric chloride produces a reddish yellow precipitate of the so-called aurate of ammonium, or fulminating gold: it detonates most powerfully after it has been carefully washed with hot water. Its constitution is obscure.
The Sulphide is obtained by the action of hydrosulphuric acid gas upon auric chloride at the ordinary temperature, or by the action of sulphide of ammonium. It is a black precipitate.
Its formula is Au, Sg.
It dissolves in the sulphides of potassium and ammonium, and in hydrate of potassium, forming sulphaurates and aurates. From these solutions it is reprecipitated by acids in which it is insoluble. In nitro-hydrochloric acid, however, it dissolves.
THE CARBONATE does not exist.
THE OXALATE does not exist. Oxalic acid acts upon solulutions of auric salts in a peculiar manner; and this action constitutes one of the most characteristic tests for the presence of gold: when added to the gold solution (which should be previously evaporated in the presence of hydrochloric acid to ensure the absence of nitric acid), and the mixture boiled, the metal separates in the form of an orange-brown powder, or in minute spangles ; for
2AuCl. +3H, C,0,=2Au + 6HC1+6CO,. TIE SULPHATE, THE FERROCYANIDE, THE FERRICYANIDE, and THE PHOSPHATE do not appear to exist.
The other special reagents of the present and three preceding subdivisions give no characteristic reactions with auric salts.
The presence of gold is usually recognized by the formation of the malleable yellow globule, by the precipitation of the beau
tiful purple of Cassius, and by the reduction of the metal effected by ferrous sulphate or oxalic acid.
SALTS OF TUNGSTEN.
To this and the remaining metals of the present subdivision, many of the remarks made on the characteristics of arsenic apply. These elements do not yield true normal salts, but, uniting with several equivalents of acid. radicals, produce bodies which present the features of acid anhydrides ; they are nevertheless noticed here, inasmuch as (like the analogous compounds of antimony and arsenic) they form certain insoluble combinations, such as hydrates and sulphides, which may occur among the salts of the other metals.
Tungsten forms three combinations with oxygen, two only of which are represented by combinations with other acid-radicals. These oxides are the binoxide (W,02) with the corresponding chloride (WC1,), and the teroxide (W203) with the corresponding ter-chloride (WCly); the remaining oxide is W.Os, and seems to be formed by the union of the two former.
Some of the salts of tungsten are volatile; but those which are not, when heated before the blowpipe, are usually converted into tungstic oxide, which is yellow when cold, but becomes darker on heating ; in the reducing flame the oxide blackens, but does not fuse. When heated on platinum wire with carbonate of sodium, a dark yellow glass is obtained, which becomes paler, opaque, and crystalline, on cooling. Tungsten salts impart no colour to the flame. With nitrate of cobalt no reaction is obtained. Fused with borax, however, on platinum wire, a clear colourless bead is formed in the oxidizing flame; but this, if the tungstic oxide be increased in quantity, becomes slightly yellow, and even somewhat opaque while in the flame, and on cooling, milk-white. In the reducing flame with a moderate quantity of the oxide, the bead becomes dark yellow while hot, and brownish yellow on cooling. A more characteristic result is obtained if microcosmic salt be employed instead of borax; for in the oxidizing fame the bead becomes pale yellow, while in the reducing flame it is clear blue. If, however, iron were present, as in testing the mineral Wolfram (tungstate of manganese and iron), the bead would have been of a bright red.
With stannous chloride the salts of tungsten give a blue precipitate of the oxide W.0g. There are no tungsten compounds corresponding to the chloroplatinates, chloriridiates, and chloraurates.
TUNGSTOUS SALTS. These compounds are but few in number, and, with the exception of the sulphide, unstable; they are also comparatively unimportant, as the analyst is almost entirely concerned with the tungstic series.
The chief insoluble salts of this series are the oxide and the sulphide.
The Oxide is produced both by decomposing the chloride (WCI,) with water, and by acting on tungstic oxide (W, 0,) with nascent hydrogen : the latter method at first produces the blue oxide. This oxide may also be obtained by heating tungstate of ammonium in a closed vessel : it is a brown or violet brown powder. Its formula is W,0%. It dissolves in hydrate of potassium, with evolution of heat and formation of tungstate of potassium (KWO,). Hydrofluoric is the only acid which attacks it.
THE SULPHIDE is obtained by the action of heat on tungstic sulphide. It also occurs in nature. Its formula is W. S. Nitrohydrochloric acid converts it into tungstic oxide and sulphuric acid.
THE SULPHATE, THE CARBONATE, THE OXALATE, THE FERROCYANIDE, THE FERRICYANIDE, and THE PHOSPHATE are unknown.
These compounds are chiefly recognized by the insoluble oxide and sulphide.
THE CHLORIDE is a volatile liquid, decomposed by water.
The Oxide is obtained by decomposing the salts in which tungsten occurs as part of the acid-radical: thus the native tungstate of calcium, or the artificial tungstate of sodium, yield the oxide when acted upon by hydrochloric acid. It is a yellow powder, becoming dull green when heated. Freshly precipitated it is gelatinous. Its formula is W, 0g. This substance is soluble in the hydrates of potassium and ammonium, forming tungstates; from these solutions it is reprecipitated by nearly all acids, excess of which does not cause its re-solution (with the single exception of phosphoric acid). The organic acids, oxalic, citric, and tartaric, do not, however, effect the precipitation of tungstic oxide from tungstates, if added in excess. Tungstic oxide dissolves in the soluble sulphides of the alkaline metals.
The Sulphide is obtained by saturating a solution of an alkaline tungstate with hydrosulphuric acid, and the subsequent addition of hydrochloric acid. It is produced also when the oxide is dissolved in an alkaline sulphide, and the resulting solution reprecipitated by an acid. It is a livercoloured precipitate. Its formula is W,Sg. This compound is soluble in alkaline sulphides, forming a series of salts such as those described under antimony and arsenic: these solutions decompose in the air, finally yielding the tungstate and the sulphate of the alkaline metal. Tungstic sulphide dissolves in hydrate and carbonate of potassium, and less readily in hydrate of ammonium : in these cases it forms a mixture of sulphotungstate and tungstate, just as arsenic sulphide yields under the same conditions a mixture of sulpharseniate and arseniate. Tungstic sulphide is slightly soluble in water, colouring it yellow or brown; heat increases this solubility.
THE SULPHATE, THE CARBONATE, THE OXALATE, THE FERROCYANIDE, THE FERRICYANIDE, and THE PHOSPHATE are unknown.