The kinetics of the arsenic(III) - chromium(VI) reaction on acetic acid-acetate buffers

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Virginia Polytechnic Institute

A kinetic study of the oxidation of arsenic(III) by chromium(VI) was made in acetic acid-acetate buffers at ionic strengths 1.5M and 3.0M. The reaction rate showed a first order dependence on the arsenic(III), total chromium(VI), and acetic acid concentrations. Two possible mechanisms for the reaction have been suggested. One possibility is the formation of an arsenic-chromium adduct in which either the acid chromate or dichromate ion is involved. A second possible mechanism postulates adduct formation involving only the acid chromate ion, followed by a CrO₄= catalyzed decomposition of the adduct. In either suggested mechanism, the acetate and hydroxide ion catalyzes the decomposition of the adduct. The rate dependence on total chromium(VI) concentration is contrary to previous reports which postulate an adduct or ester mechanism.

Induced reaction studies show the initial step involves the oxidation of one arsenic(III) species by chromium(VI) with the formation of tetravalent chromium as-a reactive intermediate.

The stoichiometry or the overall reaction was found to be

3 As(III) + 2 Cr(VI) = 3 Aa(V) + 2 Cr(III).