Kinetics of Anodic Dissolution of Gold in Alkali–Cyanide Solutions: Effect of Lead Ions

Abstract

Effective order of the gold dissolution reaction by cyanide ions, p, transfer coefficient α, and exchange current i0are measured at constant coverages of the gold surface by lead adatoms, θ. Constancy of θ is ensured by maintaining the time period Δt, during which the electrode is in contact with solution after the renewal of its surface and before taking measurements, constant. Solutions under study contain (0.5–2) × 10–5M lead acetate, 0.05–0.2 M KCN, 0.1 M KOH, and 0.01 M KAu(CN)2. With increasing Δt, quantities p, α, and i0increase from, respectively, 0.17, 0.1, and 10–5A dm–2(in 0.05 M KCN) to p≅ 1.2, α ≅ 0.45, and i0≅ 10–4A dm–2. The increase in psuggests that the limiting stage alters in the presence of lead adatoms: in addition to adsorbed cyanide ions, as is the case with pure alkali–cyanide solutions, it involves cyanide ions located outside the adsorption layer. A feasible mechanism for the acceleration of gold dissolution by lead adatoms is offered.