Auric chloride, AuCl₃

Under certain conditions gold is converted by chlorine into auric chloride. At ordinary temperatures such reagents as chlorine-water, aqua regia, and the higher chlorides of manganese, nickel, and cobalt effect this change. At 300° C. the dry gas transforms the metal into the chloride, the product subliming in reddish, bulky crystals. An aqueous solution can be prepared by the action of water on the dichloride, or on aurous chloride:

3AuCl₂ = 2AuCl₃+Au.
3 AuCl = AuCl₃+2Au.

Spontaneous evaporation of the solution of the chloride yields dark, orange-yellow crystals of the dihydrate, AuCl₃,2H₂O, which changes to the anhydrous salt on exposure to the air. The monohydrate, AuCl₃,H₂O, is supposed to exist in solution in the form of an acid, H₂AuCl₃O. With silver carbonate this acid yields a yellowish silver salt, Ag₂AuCl₃O. The constitution of the acid and of its salts is a matter of uncertainty.

On rapid evaporation of a solution of auric chloride in an acidic solution capable of generating chlorine, and subsequently drying the product at 150° C., the salt is obtained in anhydrous form as a dark-brown crystalline mass, reducible to a reddish-brown powder. Its melting-point in an atmosphere of chlorine or in a sealed tube is 287° to 288° C., and its density is 3.9. On heating in a closed space it dissociates, the reaction beginning at 190° to 200° C.:

AuCl₃ ⇔ AuCl + Cl₂.

Under a pressure of two atmospheres in contact with chlorine there is no separation of metallic gold, even at the melting-point.

Auric chloride is reduced by carbon in accordance with the equation

4AuCl₃ + 6H₂O+3C = 4Au +12HCl+3CO₂.

It is also reduced to metallic gold by phosphorous acid and sodium hypophosphite, and by cupric sulphide in accordance with the equation

CuS+2AuCl₃+3H₂O+O = 2Au+CuSO₄+6HCl.

A test of the purity of the salt is its complete solubility in ether. It is unaffected by the prolonged action of radium bromide.

In dissolving alloys of gold and silver in aqua regia in presence of ammonium chloride and nitrate, purplish-brown crystals of the formula

3AgCl, 4AuCl₃, 8NH₄Cl

are sometimes obtained as a by-product.