OXIDATION OF COPPER SULFIDES IN AQUEOUS AMMONIA .2. ELECTROCHEMICAL ASPECTS

The oxidation of covellite, CuS, to copper(II) and sulfur or sulfate ions in aqueous ammonia is slowed by the formation of an equilibrium thickness of a copper polysulfide CuyS (y < 1). The polysulfide is thermally and electrochemically unstable when in contact with CuS. A surface coating of sulfur, although formed, is not responsible for the semi-passivation of CuS during oxidation. Cyclic volt- ammetry, controlled potential electrolysis, linear sweep voltammetry, and various amperiometric and coulometric experiments with electrodes of pressed CuS, of carbon paste containing CuS and or a carbon ring with a CuS disc have been used to study the formation or decomposition of CuyS and its effect on rates of oxidation or CuS in aqueous ammonia. CuS-carbon paste electrodes offer some advantages over pressed discs of CuS for such studies. © 1979 ASEG.