Relationships between the pourbaix diagram for Ag-S-H₂O and electrochemical oxidation and reduction of Ag₂S

Abstract

The potentialvs pH diagram or Pourbaix diagram for the Ag-S-H₂O system has been calculated from available thermodynamic data for 25‡ and 100 ‡C. The diagrams developed show the dependence of the electrochemical equilibria on the concentration of [Ag⁺] and the total dissolved sulfur. The appearance of the diagram is significantly altered when the solubility product for Ag₂SO₄ is exceeded. Various electrochemical experiments have been performed in acid solutions using Ag₂S electrodes for comparison between anticipated reactions from the diagrams and experimental results. The rest potential in acid solution shows a dependence of 0.055 V/decade change in [Ag⁺] concentration. Mass balances performed at constant cathodic potentials suggest the reaction Ag₂S + 2H⁺ +2e ^- = 2Ag + H₂S. Hydrogen evolution is a competing reaction which becomes less important as the surface becomes more completely covered with elemental silver due to the large hydrogen overvoltage on Ag. Similar anodic experiments have shown that part of the dissolution can be attributed to Ag₂S = 2Ag⁺ + S +2e ^- with some of the mineral forming SO ⁼₄ or HSO ^-₄ directly. The sulfur which forms is not completely protective, since the current observed during anodic potential sweeps is stirring dependent. This is consistent with the formation of Ag₂SO₄ on the electrode surface, which was confirmed by SEM and X-ray diffraction techniques.