The role of sulfide ions in the electrode processes involving gold in sulfite-thiocarbamide electrolytes

Abstract

The regularities of electrochemical deposition and dissolution of gold in the mixed sulfite-thiocarbamide electrolytes in the absence and in the presence of sodium sulfide additive are studied by using the voltammetric measurements on a renewable electrode and quarts microgravimetry. It is shown that, in the cathodic metal deposition, an addition of sodium sulfide promotes the depolarization effect, which is caused by the presence of thiocarbamide in the solution. Under the anodic polarization of gold in the mixed sulfite-thiocarbamide solution with pH < 10, the gold dissolution rate is insignificant. An addition of 10⁻⁵ M Na₂S to this solution dramatically accelerates the process. At pH > 10, the gold dissolution in the sulfite-thiocarbamide electrolyte is observed even in the solution free of Na₂S additive. It is evidenced that this is associated with spontaneous accumulation of sulfide-containing species in the solution, probably, as a result of thiocarbamide hydrolysis; the rate of hydrolysis steeply increases with increasing pH value.