Kinetics of Anodic Dissolution of Gold in Cyanide Solutions Containing Thallium Ions: Potentiostatic Measurements

Effective reaction order by cyanide ions, p, effective transfer coefficient _ef, and exchange current i ₀of the gold dissolution process are measured at a constant surface coverage by thallium adatoms _Tl. The constancy of _Tlis ensured by constant time tof the electrode's contact with solution after renewal of its surface and before starting the measurements. Solutions containing 2.5 × 10^–6to 10^–5M TlNO₃, 0.1 M KCN, and 0.1 M KOH are studied. With an increase in t, quantities p, _ef, and i ₀increase from values typical for pure KCN solutions (0.17, 0.1, and 10^–5A/dm², respectively) to p 1.1, _ef 0.4, and i ₀ 3 × 10^–4A/dm². The increase in pby nearly unity is interpreted as indicating a change in the limiting stage in the presence of thallium ions: in the latter case, it involves not only adsorbed cyanide ions (as in pure cyanide solutions), but also those located beyond the adsorption layer. A possible mechanism explaining the acceleration of gold dissolution in the presence of thallium adatoms is put forward.