Oxidation of CrIIIto CrVIinitiated by OHand SO4-in acidic aqueous solution A pulse radiolysis study

Abstract

The kinetics of the radiation-induced oxidation of CrIII to CrIV by OH and SO4- in aqueous solution saturated with N2O and argon, respectively, have been studied by pulse radiolysis with optical and conductometric measurements at pH 3 and 3.7. The mechanism of oxidation by both OH and SO4- is consistent with the rapid formation of a precursor complex, with a stability constant K (d mol-1), followed by electron transfer from the CrIII to the radical, with a rate constant k (s-1). min CrIVAt pH 3 and 3.7, respectively, the values for OH are K=91±9 and 59±3; k=(3.7±0.1)×106 and (5.8±0.1)×106, based on the assumption that free and complexed OH react at the same rate with benzoic acid, which was used to measurek(OH+CrIII) by the competition kinetics method; and for SO4- they are K=(7.6±1.9)×102 and (4.2±0.5)×103; k=(2.5±0.3)×104 and (1.2±0.1)×104. The absorption spectrum of CrIV generated by OH and SO4- appears as a weak rising band between 420 and 250 nm. The shape and intensity of the band is the same at pH 3 and 3.7 and the molar absorption coefficient increases from 4.3±0.4 mol-1 at 420 nm to 48±6 mol-1 at 250 nm. Conductivity measurements show that proton release follows the oxidation step with a rate constant of (4±1)×104 s-1 at pH 3 and (6±1)×104 s-1 at pH 3.7; proton release indicates that CrIV is more hydrolysed than CrIII. The final product, CrVI, is formed in a second-order process over a period of ca. 0.01 to 0.5 s; a mechanism is proposed in which the rate-determining step is 2CrIV→CrIII+CrV with k=4.1×107 d mol-1 s-1 at pH 3.7, followed by the rearrangement of the coordination shell of CrV from octahedral to tetrahedral and reaction between CrtetV and CroctIV in non-rate-determining steps.