Conductometric pulse radiolysis study of the interconversion Cr(III)/Cr(VI) redox reactions in acidic solution: Atmospheric pollution relevance

Hexavalent chromium (VI) is a potent carcinogen for humans, while trivalent chromium (III) is an important element in glucose metabolism. In atmospheric water droplets, oxidation of Cr(III) to Cr(VI) by free radicals is a major environmental issue. Laboratory simulation studies of the aqueous phase oxidation-reduction of the couple Cr(III)/Cr(VI) by OH and HO₂/O₂^- by applying conductometric pulse radiolysis techniques were carried out in the ph range 3.5-5.5 chosen as characteristic values of atmospheric water droplets acidity. A pK_alpha of 3.9 was found for the first hydrolysis of Cr(IV). The bimolecular second-order decay of the Cr(V) transient species shows a strong ph dependency consequence of the change of the coordination sphere of the tetrahedral Cr(VI) to the octahedral Cr(III)