Peculiarities of cathodic reduction of gold from sulfite-thiocarbamide electrolytes

The cathodic reduction of gold from mixed sulfite-thiocarbamide electrolytes is studied as a function of the electrolyte composition. In the absence of thiocarbamide in the gold-plating sulfite solutions, gold is deposited at the cathode at high overpotentials. The equilibrium composition of the electrolyte is calculated at various ratios between the amounts of sulfite and thiocarbamide; it is shown that an addition of 10(-5) to 0.5 M thiocarbamide does not change considerably the solution composition, and gold ions are present, predominantly, in the form of the complex with sulfite ions. However, an addition of thiocarbamide to the solution leads to a decrease in the overpotential of metal deposition by approximately 0.5 V. A possible mechanism of the catalytic effect of thiocarbamide on the cathodic reduction of gold from mixed sulfite-thiocarbamide electrolytes is proposed. It is shown that sulfite ions have a stabilizing effect on the decomposition of thiocarbamide in the alkaline solutions.