Disproportionation of CrV generated by the radiation-induced reduction of CrVI in aqueous solution containing formate: a pulse radiolysis study

Abstract

The kinetics of the radiation-induced reduction of hexavalent Cr^VI to trivalent Cr^III, through the unstable states Cr^V and Cr^IV, 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 Cr^V 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 Cr^VI by CO₂˙^– or e_aq^–. The observed second-order rate constant for the disproportionation of Cr^V, determined from the rate of partial regeneration of Cr^VI, 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 Cr^V from tetrahedral, Cr^V_tet, to octahedral, Cr^V_oct, in two sequential proton assisted steps followed by disproportionation between Cr^V_tet and Cr^V_oct with k= 3.4 × 10⁸ dm³ mol^–1 s^–1. The intermediate form of Cr^V 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 Cr^V, the forms of Cr^V in acidic solution are assigned as H₃CrO₄(pK_a= 2.7), H₂CrO₄^–(pK_a= 3.8), and HCrO₄^2–. At pH 6.8 the spectral data indicate the presence of CrO₄^3– and pK_a= 7.0 for HCrO₄^2–.