Symbol, AuCl{3}. Atomic weight, 303.
This salt is formed by dissolving pure metallic gold in nitro-hydrochloric acid, and evaporating at a gentle heat. The solution affords deliquescent crystals of a deep orange color.
Chloride of gold, in a state fit for photographic use may easily be obtained by the following process:—Place a half-sovereign in any convenient vessel, and pour on it half a drachm of nitric acid mixed with two and a half drachms of hydrochloric acid and three drachms of water; digest by a gentle heat, but do not boil the acid, or much of the chlorine will be driven off in the form of gas. At the expiration of a few hours add fresh aqua-regia in quantity the same as at first, which will probably complete the solution, but if not, repeat the process a third time.
Lastly, neutralize the liquid by adding carbonate of soda until all effervescence ceases, and a green precipitate forms; this is carbonate of copper, which must be allowed several hours to separate thoroughly. The solution then contains chloride of gold in a neutral state, and free from copper and silver, with which the metallic gold is alloyed in the standard coin of the realm.
The weight of a half-sovereign is about 61 grains, of which 56 grains are pure gold. This is equivalent to 86 grains of chloride of gold, which will therefore be the quantity contained in the solution.
The following process for preparing chloride of gold is more perfect than the last:—dissolve the gold coin in aqua-regia as before; then boil with excess of hydrochloric acid to destroy the nitric acid, dilute largely with distilled water, and add a filtered aqueous solution of common sulphate of iron (6 parts in 1 part of gold); collect the precipitated gold, which is now free from copper; re-dissolve in aqua-regia, and evaporate to dryness on a water bath.
Avoid using ammonia to neutralize chloride of gold, as it would be liable to occasion a deposit of "fulminating gold," the properties of which are described immediately following.
Properties of Chloride of Gold.—As sold in commerce it usually contains excess of hydrochloric acid, and is then of a bright yellow color; but when neutral and somewhat concentrated it is dark red (Leo ruber of the alchemists). It gives no precipitate with carbonate of soda, unless heat be applied; the free hydrochloric acid present forms, with the alkali, chloride of sodium, which unites with the chloride of gold, and produces a double salt, chloride of gold and sodium, soluble in water.
Chloride of gold is decomposed with precipitation of metallic gold by charcoal, sulphurous acid, and many of the vegetable acids; also by protosulphate and protonitrate of iron. It tinges the cuticle of an indelible purple tint. It is soluble in alcohol and in ether.