The solubilities of gold in various solutions as calculated from electrode potential and free energy data agree well with experimental results reported in the literature. In acid solutions gold dissolves as the ion AuCl 4 (super -) . Formation of this ion requires that the solution contain excess Cl (super -) , and that it contain or be in contact with a fairly strong oxidizing agent. Naturally-occurring oxidizing agents whose effectiveness has been demonstrated both experimentally and theoretically are MnO 2 , O 2 , Fe (super +++) and Cu (super ++) . Neither H (super +) nor SO 4 (super --) is sufficiently strong as an oxidizing agent to affect gold appreciably at ordinary temperatures. In naturally-occurring alkaline solutions gold is not significantly soluble unless sulfide is present. In sulfide solutions gold is carried as a complex ion, probably AuS (super -) ; the fact that gold metal is soluble in HS (super -) solutions at ordinary temperatures indicates that this ion is an exceptionally stable one. The effect of temperature on the solubility of gold cannot be predicted from available thermodynamic data, but experiments indicate that the solubility increases with rising temperature in both acid and alkaline solutions.

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