A surface adsorption/reaction mechanism for gold oxidation by copper(II) in ammoniacal thiosulfate solutions

Literature data for gold dissolution in ammoniacal copper(II) thiosulfate solutions is reinterpreted on the basis of adsorption and mixed potential theory. The dissolution reaction appears to take place via the adsorption of copper(II)-ammonia-thiosulfate onto the gold surface, forming the adsorbed species ⊢Au(S2O3)nCu(NH3)−(2n−2)p. Equilibrium constants for the formation of these species from Cu(NH3)2+m are in the range Kads=172-510 (molar units) for m=4, n=1 or 2, and p=2 or 3. These complexes decompose with a rate constant of kAu=1.7×10−4molm−2s−1, to produce Au(S2O3)3−2 and Cu(NH3)+3 or Cu(NH3)+2, where the copper(I) complexes in solution are re-equilibrated to the more stable species Cu(S2O3)3−2 and Cu(S2O3)5−3.