Chloroauric acid
chemical compound
Chloroauric acid is an inorganic compound with the chemical formula H[AuCl4]. It forms hydrates H[AuCl4]·nH2O. Both the trihydrate and tetrahydrate are known. Both are orange-yellow solids consisting of the planar [AuCl4]− anion. Often, chloroauric acid is handled as a solution, such as those obtained by dissolution of gold in aqua regia. These solutions can be converted to other gold complexes or reduced to metallic gold, or gold nanoparticles.
Production
Chloroauric acid is produced by dissolving gold in aqua regia (a mixture of concentrated nitric and hydrochloric acids) followed by careful evaporation of the solution:
Au(s) + HNO3(aq) + 4 HCl(aq) → H[AuCl4](aq) + NO(g) + 2 H2O(l)
Under some conditions, oxygen can be used as an oxidant. For higher efficiency, these processes are conducted in autoclaves, which allows greater control of temperature and pressure. Alternatively, a solution of H[AuCl4] can be produced by electrolysis of gold metal in hydrochloric acid:
2 Au(s) + 8 HCl(aq) → 2 H[AuCl4](aq) + 3 H2(g)
To prevent the deposition of gold on the cathode, the electrolysis is carried out in a cell equipped with a membrane. This method is used for refining gold. Some gold remains in solution in the form of [AuCl2]−.
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