Auric hydroxide, AuOOH

A 5 per cent, yield of the pure Auric hydroxide, AuOOH, is obtained by precipitating auric chloride with basic magnesium carbonate, and treating the precipitate with 30 per cent, nitric acid, most of the auric hydroxide dissolving in this solvent. The poor yield is probably explicable on the assumption of the formation of insoluble complex salts which are precipitated with the hydroxide, and render necessary the drastic process of treatment with nitric acid, thus dissolving a large proportion of the hydroxide itself. The hydroxide has also been prepared in an impure state by several other methods.

In the pure condition auric hydroxide resembles ferric hydroxide in appearance, and has the colour of brown ochre, but impurities impart to it a yellowish or greenish tint. It is soluble in nitric acid and hydrochloric acid, and in hot potassium-hydroxide solution, but insoluble in solutions of the carbonates of the alkali-metals and ammonium. It dissolves in solutions of the chlorides of sodium, potassium, and barium with production of complex salts. Prolonged heating at 140° to 150° C. eliminates all its water, with formation of auric oxide, Au₂O₃. At 155° to 165° C. auric oxide is converted into gold monoxide, AuO.

Auric hydroxide behaves as a very weak acid having the anion AuO₂'. Evaporation of its solution in aqueous potassium hydroxide yields yellow needles of potassium aurate, KAuO₂,3H₂O. In accordance with the weak nature of the acid, the solution of this salt in water is strongly alkaline.

For the preparation of aurates, Meyer recommends fusing precipitated gold with sodium or barium peroxide, the corresponding aurate being formed. On extraction with water a greenish-yellow solution is produced, and addition of sulphuric acid liberates auric hydroxide, AuO•OH,H₂O, which is converted by alkalies into the corresponding aurate, M'AuO₂ or M''(AuO₂)₂, containing water of crystallization. The products are pale-green, acicular crystals, the dry salts being stable to heat. They are reduced by sulphurous acid and by alcohol to metallic gold. Dilute sulphuric acid and nitric acid yield the corresponding metallic nitrate and auric hydroxide; hydrochloric acid produces the metallic chloride and auric chloride. The calcium, barium, and magnesium salts are not readily soluble in water.

A complex derivative of auric oxide and higher oxides of manganese has been described.