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(l)-ammonia-thiosulfate-hydroxide system in solutions of I M ammonium thiosulfate)solutions containing 0.5-2 M ammonia at pH 10-11 shows that although Au(S2O3) 2 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)((ads))(2-). This oxidation corresponds to a rate constant of k(Au)=2.8 x 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)(2)(3-). (c) 2004 Elsevier B.V. All rights reserved.