A strategy to determine the potential interval for selective silver electrodeposition from ammoniacal thiosulfate solutions

A strategy based upon a thermodynamic analysis, followed by a microelectrolysis study, was used to determine the potential interval for the selective electrodeposition of silver from a typical thiosulfate leaching solution (0.001 M Ag(I), 0.05 M Cu(II)/Cu(I), 0.2 M S2O32−, 0.025 M EDTA, 0.6 M NH3). In the thermodynamic analysis presented in this work, the system behavior was shown to be highly sensitive to changes in the solution pH, potential and concentration of some of its components. This fact, in addition to the similarity between the silver and copper species, obligates a strict control of each of the system variables in order to achieve efficient separations only under these conditions can the direct electrodeposition of silver on an industrial level be a viable alternative. In the first stage of the microelectrolysis study, it was determined that stainless steel 304 (SAE 304) and aluminum (Al) promote the electroactivity of the ammoniacal thiosulfate solution and therefore, are not recommended as substrate for the silver reduction process. In contrast, the vitreous carbon (VC) and titanium (Ti) do not, and for that reason these two substrates were used in the study of the electrodeposition of silver and copper. In the second stage, it was shown that the potential window for selective silver electrodeposition is 50 mV larger on titanium than on the vitreous carbon electrode, and this window is smaller and shifted to more negative potentials, as the thiosulfate concentrations increases, just as the thermodynamics analysis predicted.