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

Effective reaction order by cyanide ions, p, effective transfer coefficient αef, and exchange current i0of the gold dissolution process are measured at a constant surface coverage by thallium adatoms θTl. The constancy of θTlis 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, αef, and i0increase from values typical for pure KCN solutions (0.17, 0.1, and 10–5A/dm2, respectively) to p≃ 1.1, αef≃ 0.4, and i0≃ 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.