THERMODYNAMICS OF THIOSULFATE REDUCTION ON SURFACES OF IRON, NICKEL AND CHROMIUM IN WATER AT 25-DEGREES-C AND 300-DEGREES-C

The conditions of potential and pH in which thiosulfates dissolved in aqueous solution can be reduced to adsorbed sulfur by direct reaction with the surface of iron, nickel and chromium are calculated. The calc ulations are based on standard Gibbs energies of formation for sulfur adsorbed on Fe, Ni and Cr which have been reported in previous papers on E-pH diagrams for sulfur adsorbed on metal surfaces.[P. Marcus and E. Protopopoff, J. Electrochem. Sec. 137, 2709 (1990).; P. Marcus and E. Protopopoff, J. Electrochem. Sec. 140, 1571 (1993).; P. Marcus and E. Protopopoff, J. Electrochem. Sec. 144, 1586 (1997).] The E-pH relat ions associated with the equilibria between water, dissolved S2O32- or HS2O3- and adsorbed sulfur and oxygen are given at 25 and 300 degrees C. The corresponding E-pH diagrams are calculated for a sulfur concen tration of 10(-4) mol kg(-1) and superimposed to the usual Pourbaix di agrams for the S-M-H2O systems (M=Fe, Ni, Cr). The direct reduction of S2O32- and HS2O3- to form adsorbed sulfur monolayers on the metal sur faces is predicted in E-pH domains in which usual Pourbaix diagrams pr edict no metal sulfide stability. (C) 1997 Elsevier Science Ltd.