[33] Stannous chloride as a reducing agent also acts on auric chloride, and gives a red precipitate known as purple of Cassius. This substance, which probably contains a mixture or compound of aurous oxide and tin oxide, is used as a red pigment for china and glass. Oxalic acid, on heating, reduces metallic gold from its salts, and this property may be taken advantage of for separating it from its solutions. The oxidation which then takes place in the presence of water may be expressed by the following equation: 2AuCl3 + 3C2H2O4 = 2Au + 6HCl + 6CO2. Nearly all organic substances have a reducing action on gold, and solutions of gold leave a violet stain on the skin.
Auric chloride, like platinic chloride, is distinguished for its clearly-developed property of forming double salts. These double salts, as a rule, belong to the type AuMCl4. The compound of auric chloride with hydrochloric acid mentioned above evidently belongs to the same type. The compounds 2KAuCl4,5H2O, NaAuCl4,2H2O, AuNH4Cl4,H2O, Mg(AuCl4)2,2H2O, and the like are easily crystallised in well-formed crystals. Wells, Wheeler, and Penfield (1892) obtained RbAuCl4 (reddish yellow) and CsAuCl4 (golden yellow), and corresponding bromides (dark coloured). AuBr3 is extremely like the chloride. Auric cyanide is obtained easily in the form of a double salt of potassium, KAu(CN)4 by mixing saturated and hot solutions of potassium cyanide with auric chloride and then cooling.
[34] If ammonia be added to a solution of auric chloride, it forms a yellow precipitate of the so-called fulminating gold, which contains gold, chlorine, hydrogen, nitrogen, and oxygen, but its formula is not known with certainty. It is probably a sort of ammonio-metallic compound, Au2O3,4NH3, or amide (like the mercury compound). This precipitate explodes at 140°, but when left in the presence of solutions containing ammonia it loses all its chlorine and becomes non-explosive. In this form the composition Au2O3,2NH3,H2O is ascribed to it, but this is uncertain. Auric sulphide, Au2S3, is obtained by the action of hydrogen sulphide on a solution of auric chloride, and also directly by fusing sulphur with gold. It has an acid character, and therefore dissolves in sodium and ammonium sulphides.
[35] Many double salts of suboxide of gold belong to the type AuX—for instance, the cyanide corresponding to the type AuKX2, like PtK2X4, with which we became acquainted in the last chapter. We will enumerate several of the representatives of this class of compounds. If auric chloride, AuCl3, be mixed with a solution of sodium thiosulphate, the gold passes into a colourless solution, which deposits colourless crystals, containing a double thiosulphate of gold and sodium, which are easily soluble in water but are precipitated by alcohol. The composition of this salt is Na3Au(S2O3)2,2H2O. If the sodium thiosulphate be represented as NaS2O3Na, the double salt in question will be AuNa(S2O3Na)2,2H2O, according to the type AuNaX2. The solution of this colourless and easily crystallisable salt has a sweet taste, and the gold is not separated from it either by ferrous sulphate or oxalic acid. This salt, which is known as Fordos and Gelis's salt, is used in medicine and photography. In general, aurous oxide exhibits a distinct inclination to the formation of similar double salts, as we saw also with PtX2—for example, it forms similar salts with sulphurous acid. Thus if a solution of sodium sulphite be gradually added to a solution of oxide of gold in sodium hydroxide, the precipitate at first formed re-dissolves to a colourless solution, which contains the double salt Na3Au(SO3)2 = AuNa(SO3Na)2. The solution of this salt, when mixed with barium chloride, first forms a precipitate of barium sulphite, and then a red barium double salt which corresponds with the above sodium salt.
The oxygen compound of the type AuX, aurous oxide, Au2O, is obtained as a greenish violet powder on mixing aurous chloride with potassium chloride in the cold. With hydrochloric acid this oxide gives gold and auric chloride, and when heated it easily splits up into oxygen and metallic gold.
APPENDIX I
AN ATTEMPT TO APPLY TO CHEMISTRY ONE OF THE PRINCIPLES OF NEWTON'S NATURAL PHILOSOPHY
By PROFESSOR MENDELÉEFF