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

By G. Senanayake


Literature data is re-interpreted to show that the anodic oxidation of gold in ammoniacal thiosulfate solutions is dependent upon the concentration product [M 2S 2O 3][NH 3] to the order 0.8 (M=Na or NH 4). Analysis of chemical speciation in solution shows that the concentration of NH 4S 2O 3 - ion-pair is equal or larger than that of the unassociated S 2O 3 2- 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(S 2O 3) 2 3- is the predominant complex (>99%), a small fraction of Au(S 2O 3)(NH 3) - is present. It is proposed that ammonia facilitates gold dissolution via oxidation of the surface adsorbed species Au(S 2O 3)(NH 3) 2- (ads). This oxidation corresponds to a rate constant of k Au=2.8×10 -4 mol m -2 s -1 to produce Au(S 2O 3)(NH 3) - (ads/aq) followed by desorption and re-equilibration in solution to Au(S 2O 3) 2 3-

Publisher: 'Elsevier BV'
Year: 2005
DOI identifier: 10.1016/j.hydromet.2004.12.003
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Provided by: Research Repository
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