Kinetics of the Reduction of the Tetrahydroxoargentate(III) Ion by Arsenite

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The reaction of [formula-omitted] with arsenite ion to give Ag(I) and arsenate in strong base was studied by stopped-flow spectrophotometry. The reaction was found to fit a second-order rate law in which all three arsenite species present, [formula-omitted], [formula-omitted], and [formula-omitted], participate. Second-order rate constants for the three paths at 25 °C and μ- 1.2 M are respectively K1, = (6.2 ± 1.0) × 103 M-1 s-1, k2 = (3.7 ± 0.3) X 104 M-1 s-1 and k3 = (1.8 ± 0.5) × 104 M-1 s-1. In order to evaluate the activation parameters, the temperature dependence of the deprotonation of arsenite by [OH-] was studied. The equilibrium parameters are ΔH2(for [formula-omitted]) = 5 ± 1 kcal/mol and Δh3(for [formula-omitted]) = -2 ± 7 kcal/mol (μ = 1 M). The redox reaction appears to proceed by a one-step oxygen atom transfer. Two kinetically indistinguishable mechanisms are discussed: one involving initial attack of an As(III) oxygen at an axial silver(III) site and the other involving initial interaction between the arsenic and a bound hydroxyl. Differences in activation parameters are indicative of a larger barrier to redox for [formula-omitted] compared to those for the other two arsenite species. The dependence of the reaction rate on ionic strength supports the formulation of the Ag(III) species as the mononegatively charged ion [formula-omitted]. © 1983, American Chemical Society. All rights reserved.

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Inorganic Chemistry