Kinetic model for anodic oxidation of gold in thiosulfate media based on the adsorption of MS2O3− ion-pair

It has been the general practice amongst researchers to use the molar concentration of thiosulfate salts to be the same as that of S2O32− ions involved in various equilibrium or rate expressions for interpreting gold oxidation kinetics. However, chemical speciation analysis based on the association of thiosulfate ion with monovalent cations (ion-pair formation) shows that the concentration of free S2O32− ions is lower than that of associated MS2O3− ions (M=Na, K, NH4). Analysis of literature data on electrochemical oxidation rates of gold in non-ammoniacal alkaline sodium thiosulfate solutions based on the adsorption of NaS2O3− shows that the rate constant for gold oxidation is 4.2×10−4 mol m−2 s−1.