Aurous oxide, Au₂O

This oxide is produced by the action of potassium hydroxide on soluble auric salts or on the comparatively slightly soluble aurohalides, but the product is contaminated with metallic gold. It can be prepared in the pure state by reducing potassium auribromide, KAuBr₄, to potassium aurobromide, KAuBr₂, by the action of sulphurous acid, and adding dilute potassium hydroxide to the solution. Aurous hydroxide is precipitated as a dark-violet substance, converted into a greyish-violet product by drying over phosphoric oxide. On heating, it loses water up to 200° C. At 205° to 210° C. oxygen is also evolved, and at 250° C. there is rapid decomposition into gold and oxygen. When freshly precipitated, aurous hydroxide dissolves in water, yielding a colloidal solution of indigo-blue colour, and characterized by a brownish fluorescence in reflected light. It also dissolves in alkali-metal hydroxides with formation of complex anions. It has the character of a very weak base.