Kinetics of gold dissolution in thiosulfate electrolytes

Abstract

The kinetics of gold dissolution in solutions containing Na₂S₂O₃ with the concentration c from 0.025 to 0.2 M and different supporting electrolytes is studied by the voltammetric method on renewable electrodes and the quartz crystal microbalance. It is shown that in the range from the steady-state potential to E = 0.3 V (from hereon, the potentials are related to the normal hydrogen electrode), the polarization curves are well approximated by straight lines in semilogarithmic coordinates. The exchange currents i ₀ and the transfer coefficients α are calculated. It is shown that for c = 0.025 M, the values of i ₀ and α are about 4 × 10⁻⁶ A/cm² and 0.2. With the increase in the Na₂S₂O₃ concentration, the exchange current increases weakly and the transfer coefficient remains virtually unchanged. The reaction order of gold dissolution with respect to ligand is calculated to have the value p = $\left( {\frac{{\partial logi_a }} {{\partial logc}}} \right)_E$ = 0.25 which is independent of E. With the changeover of supporting electrolyte, the exchange current increases in the following sequence: Li⁺ < Na⁺ < K⁺, but α and p remains unchanged. Data in thiosulfate solutions is compared with analogous data obtained earlier for the gold dissolution processes in cyanide and thiocarbamide electrolytes in which complexes of the similar structure were also formed. In electrolytes under comparison, the kinetics of gold dissolution is shown to exhibit common features.