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

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-tet(V), to octahedral, Cr-oct(V), in two sequential proton assisted steps followed by disproportionation between Cr-tet(V) and Cr-oct(V) with k = 3.4 x 10(8) dm(3) 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 HCrO42-. At pH greater than or equal to 6.8 the spectral data indicate the presence of CrO43- and pK(a) = 7.0 for HCrO42-.