A Study of Anodic Dissolution of Gold in Aqueous Alkaline Cyanide

Abstract

The rate of dissolution of gold in aqueous alkaline cyanide solutions was studied as a function of potential within the range −0.9 to +0.4V vs. SCE. The dissolution rate was found to exhibit three maxima at −0.65, +0.04, and +0.38V vs. SCE. These maxima corresponded to three current peaks found in anodic potential sweep measurements. Data from weight loss measurements were used for the determination of the stoichiometry of the electrochemical dissolution reaction and showed that for the region −0.9 to +0.6Vvs. SCE, ; for the region −0.1 to +0.15V vs. SCE, ; for the region +0.2 to +0.38V vs. SCE, . The dependence of the dissolution rate on the concentration of potassium hydroxide and potassium cyanide was determined in the two more anodic regions. At +0.38V vs. SCE the dissolution was directly proportional to the cyanide concentration and independent of hydroxide concentration. At +0.04V vs. SCE the dissolution rate was approximately linear with cyanide for low concentrations (<0.1M) and decreased with increased hydroxide concentration. At moderate hydroxide concentrations (0.1M) the dissolution rate decreased for cyanide concentrations greater than 0.2M. Several of the mechanisms suggested in the literature to account for the reactions in these two regions were shown to be incorrect in some way; instead, the following sequence was proposedThe dissolution rate was controlled by step 2 in the region of −0.1 to +0.15Vvs. SCE and by step 3 in the region of +0.2 to +0.38V vs. SCE. The above reaction sequence appeared to account for the dissolution over the complete potential region investigated. The decrease in current between the peaks in the potential sweep measurements was discussed in terms of the formation of films which interrupt the dissolution process. At potentials anodic to +0.38Vvs. SCE dissolution by the above sequence was suppressed by a gold oxidation reaction with .