Kinetics of Anodic Dissolution of Gold in Cyanide Solutions Containing Bismuth Hydroxy Compounds

Abstract

Effective values of order of the reaction of the gold dissolution process by cyanide ions p, transfer coefficient α, and exchange current i ₀ are measured at constant values of the coverage of the gold surface by bismuth adatoms θ. The constancy of θ is ensured by maintaining a constant value of the time t during which the electrode is in contact with solution after the renewal of its surface until the instant of taking measurements. The solutions under study contain 2.5 × 10^–6 to 7 × 10^–6 M bismuth hydroxy compounds, 0.1 M KCN, 0.01 M KAu(CN)₂, and 0.1 M KOH. With increasing t, quantities p, α, and i ₀ increase from 0.17, 0.1, and 10^–5 A cm^–2, respectively, which are the values typical for the gold dissolution process in plain potassium cyanide solutions, to p ≅ 1.1, α ≅ 0.45, and i ₀ ≅ 2 × 10^–4 A cm^–2. The increase in the quantity p by approximately a unity is interpreted as an evidence in favor of the assumption that the presence of bismuth adatoms alters the nature of the limiting stage: the latter starts involving cyanide ions localized outside an adsorption layer, rather than the adsorbed cyanide ions only, as is the case in plain solutions. A comparison of obtained results with the data of similar investigations performed earlier in thallium- and lead-containing solutions shows that possible mechanism of acceleration of anodic process suggested in the previous works may be used for explaining regularities of the gold dissolution process observed in the presence of bismuth adatoms as well.