Gold Protobromide, AuBr, is a yellowish-green powder obtained by heating the tribromide to about 140°. It is insoluble in water, but is decomposed by it, metalic gold and the tribromide being formed; the change is especially rapid on boiling, and is hastened by the presence of hydrobromic acid.
Auro-auric Bromide, Au2Br4, is produced by the action of bromine on finely-divided gold in the cold, some tribromide being simultaneously formed. Water breaks up this bromide into AuBr and AuBr3, but, according to some authorities, it is only a mixture of these bodies.
Gold Tribromide, AuBr3, is produced by the action of a mixture of bromine and water on gold, particularly on the application of heat. Auric tribromide resembles the trichloride in most of its properties. It crystallises in blackish needles or scarlet plates. It is deliquescent, but is far less soluble in water than gold trichloride. Concentrated solutions which may contain about 1 per cent of the tribromide are nearly black in colour. Auric tribromide suffers decompositions similar to those noted in describing AuCl3, its solutions being still less stable than those of the chloride. A solution of gold tribromide is gradually decolorized by sulphur dioxide, being completely reduced to the state of monobromide before any precipitate of metallic gold is formed. It is prepared in a pure state by heating finely-divided gold in sealed tubes with bromine and arsenic bromide, AsBr3, to 126°. Gold tribromide forms intensely coloured brownish-red aqueous solutions, the presence of a mere trace of the salt in a solution being observable in this way. Double bromides exist analogous to the chlor-aurates.
The Iodides of Gold are of little interest to the metallurgist. They are prepared with difficulty, and decompose more readily than the chlorides and bromides, and are not likely to become of practical importance. The tri-iodide is formed if gold is acted on by a solution of iodine in potassium iodide.