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Mechanism and Kinetics of the 2,2′-Bipyridyl Catalysis of the Chromium(VI) Oxidation of Malic Acid

J.H. Shan*, L. Fan, S.Y. Huo, J.J. Zhu, H.M. Liu and S.G. Shen

Key Laboratory of Analytical Science and Technology of Hebei Province, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China

A spectrophotometric study of the kinetics and mechanism of the oxidation of malic acid (Mal) by chromium(VI) catalyzed by 2,2′-bipyridyl (bpy) in aqueous acidic medium was conducted in a temperature range of ~298 to 313 K. This reaction was found to be pseudo-first order with respect to Cr(VI) and first order with respect to malic acid. Under the conditions of the pseudo-first order ([Mal]_o >> [Cr(VI)]_o ), the observed rate constant (k_obs) increased with the increase in [H⁺] and [bpy]. There was a weak negative salt effect. Based on the experimental results, a possible reaction mechanism for this oxidation catalyzed by bpy is proposed. The rate equation derived from this mechanism can explain all the experimental phenomena.

Keywords: Chromium(VI), Malic acid, 2,2′-Bipyridyl, Catalytic kinetics and mechanism