The kinetics of the radiation-induced reduction of hexavalent CrV' to trivalent Cr"', through the unstable states CrV and CrIV, in N,O-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 CrV' by C02'-or eaq -. The observed second-order rate constant for the disproportionation of CrV, determined from the rate of partial regeneration of Cr", 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, Cr: , , to octahedral, Cr: ' , , in two sequential proton assisted steps followed by disproportionation between Crz, and CrrCt with k = 3.4 x lo8 dm3 mol-' s-'. 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 H,CrO, (pK, = 2.7), H2Cr0,-(pK, = 3.8), and HCr0,2-. At pH 2 6.8 the spectral data indicate the presence of Cr0,3-and pK, = 7.0for HCrO,' -.Although the radiolytic reduction of CrV' to Cr"' in acidic solution has been reported in a number of earlier studies,'-' the roles of the intermediate oxidation states CrV and Cr" have not been fully elucidated. In particular, the effect of pH, which we find to be one of the main parameters in the overall reaction kinetics, has not been studied in any detail. Tong and King6 suggested that the slow rate of Crv'-Crl'' and Cr"'-Crv' redox reactions may be attributed to the retarding influence of the rearrangement of the coordination shell from tetrahedral for CrV to octahedral for Cr'". To obtain information on the intermediate redox reactions of the Crv1-Cr"' couple, we have studied, by pulse radiolysis, the kinetics and mechanism of CrV' reduction initiated by its reaction with CO,'-. In the light of our findings, we propose a quite explicit mechanism for the reduction of CrV' to Cr"' and report rate constants for the disproportionation of CrV as a function of pH.
