Note 33 — Chapter XXIV · COPPER, SILVER, AND GOLD

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.