Catalytic role of ammonia in the anodic oxidation of gold in copper-free thiosulfate solutions

Literature data is re-interpreted to show that the anodic oxidation of gold in ammoniacal thiosulfate solutions is dependent upon the concentration product [M2S2O3][NH3] to the order 0.8 (M=Na or NH4). Analysis of chemical speciation in solution shows that the concentration of NH4S2O3− ion-pair is equal or larger than that of the unassociated S2O32− ion. A species distribution diagram for the gold(I)–ammonia–thiosulfate–hydroxide system in solutions of 1 M ammonium thiosulfate solutions containing 0.5–2 M ammonia at pH 10–11 shows that although Au(S2O3)23− is the predominant complex (>99%), a small fraction of Au(S2O3)(NH3)− is present. It is proposed that ammonia facilitates gold dissolution via oxidation of the surface adsorbed species Au(S2O3)(NH3)2−(ads). This oxidation corresponds to a rate constant of kAu=2.8×10−4 mol m−2 s−1 to produce Au(S2O3)(NH3)−(ads/aq) followed by desorption and re-equilibration in solution to Au(S2O3)23−.