Disproportionation of Cr V generated by the radiation-induced reduction of Cr VI in aqueous solution containing formate: a pulse radiolysis study

Abstract

The kinetics of the radiation-induced reduction of hexavalent CrVI to trivalent CrIII, through the unstable states CrV and CrIV, in N2O-saturated aqueous solution in the presence of formate, has been studied by pulse radiolysis with optical measurements in the pH range ca. 2–5 and conductometric measurements at pH 3. The absorption spectrum of CrV has been obtained in the pH range ca. 2–9 by comparing the observed and expected bleaching yields resulting from the one-electron reduction of CrVI by CO2˙– or eaq–. The observed second-order rate constant for the disproportionation of CrV, determined from the rate of partial regeneration of CrVI, shows a bell-shaped dependence on pH with a maximum value at pH 3.3. This pH dependence is consistent with a rearrangement of the coordination shell of CrV from tetrahedral, CrVtet, to octahedral, CrVoct, in two sequential proton assisted steps followed by disproportionation between CrVtet and CrVoct with k= 3.4 × 108 dm3 mol–1 s–1. The intermediate form of CrV generated in the first of these steps does not contribute to the disproportionation reaction. Based on the bell-shaped dependence on pH of the observed rate constant for disproportionation of CrV, the forms of CrV in acidic solution are assigned as H3CrO4(pKa= 2.7), H2CrO4–(pKa= 3.8), and HCrO42–. At pH 6.8 the spectral data indicate the presence of CrO43– and pKa= 7.0 for HCrO42–.