The effect of thallium ions on the gold dissolution rate in thiosulfate electrolytes

The effect of TlNO3 additions in the concentration (c 1) range from 5 × 10−6 to 1 × 10−4 M on the anodic dissolution of gold in sodium thiosulfate solutions with the concentration (c 2) from 0.005 to 0.2 M is studied by voltammetry on the electrode surface renewed by cutting off a thin metal layer immediately in solution and also by the quartz-crystal microbalance method. For c 2 = 0.2 M, as c 1 increases from 5 × 10−6 to 1 × 10−4 M, the gold anodic dissolution rate is observed to increase from 0.02 (in the absence of TlNO3) to 0.75 mA/cm2 for c 1 = 7.5 × 10−5 M according to a nearly linear law. The dissolution accelerates because the effective values of the transfer coefficient and the exchange current density increase from 0.2 and 4 μA/cm2 (in the absence of TlNO3 admixtures) to 0.47 and 35 μA/cm2 (for c 1 = 1 × 10−4), respectively. Experiments with the renewal of the electrode surface in the course of electrolysis suggest that the gold dissolution is catalyzed in the presence of thallium ions by the adsorption mechanism and also as the result of the mixed kinetics of their adsorption on the electrode surface.