An Electrochemical and ESCA Study on the Tarnishing of Silver in Solutions of Different pS and p H Values

The variation of the open circuit potential of Ag electrodes in solutions of varying p H values, and containing increasing concentrations of S exp 2-- , is followed as a function of time until steady state values, E sub st are reached. In all cases E sub st values are attained from positive potentials. The shift towards negative values depends on the ambient sulphide content, being more the higher the concentration. The E sub st --log(S exp 2-- ) plots are invariably S-shaped, denoting adsorption from solution. Before the inflexion linearity exists between E sub st and C sub S2-- , and this is explained on the basis of a Gibbs' type adsorption model. Evidence is presented to support the conclusion that the adsorbed species at all p H values in the SH exp md ion. The variation of E sub st after inflexion follows the general relationship: E sub st = E' exp 0 --0.059logC sub S2-- V. The relation indicates a 1e electrode process, despite the fact that bulk Ag sub 2 S forms quickly on the surface of the Ag electrodes. Galvanostatic reduction of the corrosion film formed on Ag electrodes in sulphidic solutions occurs in a single step corresponding to Ag sub 2 S. Similarly, ESCA spectrograms of the films exhibit peaks for Ag and sulfur. The S 2p peak shows two sublevels corresponding to S2p sub 1/2 and S2p sub 3/2 with a separation of 1.20 eV. The relative atomic concentrations of the Ag:S peaks is very near to 2:1, indicating that the film is formed of Ag sub 2 S. The results of the present investigation strongly suggest that Ag sub 2 S is formed chemically, consecutive to a 1e electrode process. Ageing of the Ag/AgSH/Ag sub 2 S electrode in sulphide-free solutions causes the electrode to respond theoretically to pS exp 2-- , with a Nerstian slope of 0.029 V/log C unit. 43 ref.--AA