Reduction of the Tetrahydroxoargentate(III) Ion by Thiocyanate in Aqueous Alkaline Media

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The reduction of Ag(OH)4- by thiocyanate ion in the range 5 × 10-4 ≤ [SCN-] ≤ 0.25 M was studied in aqueous alkaline media at 25 °C and μ = 1.2 M by stopped-flow spectrophotometry. At [SCN-] ≤ 0.01 M, the reaction was found to be first-order in each reactant, with an apparent second-order rate constant of 21.7 ± 0.3 M-1 s-1. At higher thiocyanate concentrations, silver(III) disappearance continues to appear pseudo first order, but the thiocyanate dependence becomes complex. Oxidation products are SO42-, OCN-, and (at high SCN- only) cyanide ion. A mechanism is proposed consisting of a series of two-electron redox steps. Competition between SCN- and reaction intermediates for remaining silver(III) accounts for changes in both rate and product distribution. The activation enthalpy and entropy for the initial step are 28.3 ± 1.2 kJ mol-1 and -129 ± 7 J mol-1 K-1 respectively. Numerical simulation and reaction with potential intermediates have been used to support the conclusions. © 1991, American Chemical Society. All rights reserved.

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Inorganic Chemistry