Kinetics of anodic dissolution of gold in thiocarbamide solutions

The kinetics of gold dissolution in acid solutions of thiocarbamide is shown to depend on the duration of the electrode contact with solution. Gold whose surface was in contact with solution for a long time (one hour and longer) dissolves in thiocarbamide predominantly under diffusion control, which is presumably connected with a catalytic effect of adsorption of sulfide species, i.e., products of the thiocarbamide decomposition. The dissolution rate of renewed gold is limited by a kinetic stage and is characterized by small values of the exchange current and transfer coefficient (i(0) = 30 mu A cm(-2), alpha = 0.125). The effective reaction order by the ligand is also small and equals 0.2. The kinetic data on the anodic behavior of the renewed metal are analyzed with allowance made for the literature data about adsorption of thiocarbamide. It is shown that these can be made consistent assuming that the elementary act of anodic dissolution of the metal involves only the adsorbed ligands.