An electrochemical and ESCA study on the tarnishing of silver in solutions of different pS and pH values

• Published in: Electrochimica acta 1985-01, Vol.30 (4), p.461-468
• Main Authors: Shams ElDin, A.M., Abd ElKader, J.M., Abd ElWahab, F.M., Saber, T.M.H., El Azhari, A.A., El Waraky, A.A.
• Format: Article
• Language: English
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/0013-4686(85)80035-9

The variation of the open circuit potential of Ag electrodes in solutions of varying pH values, and containing increasing concentrations of S 2-, is followed as a function of time until steady values, E st are reached. In all cases E 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 st—log[S 2-] plots are invariably S-shaped, denoting adsorption from solution. Before the inflexion linearity exists between E st and C 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 pH values is the SH − ion. The variation of E st after inflexion follows the general relationship ▪ The relation indicates a 1ē electrode process, despite the fact that bulk Ag 2S 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 2S. Similarly, ESCA spectrograms of the films exhibit peaks for Ag and S. The sulphur 2 p peak shows two sublevels corresponding to S2 p 1 2 and S2 p 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 2S. The results of the present investigation strongly suggest that Ag 2S is formed chemically, consecutive to a 1ē electrode process. Ageing of the Ag/AgSH/Ag 2S electrode in sulphide-free solutions causes the electrode to respond theoretically to pS 2-, with a Nerstian slope of 0.029 V/log C unit.