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

Abstract

The kinetics of the radiation-induced oxidation of Cr^III to Cr^IV 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 Cr^III to the radical, with a rate constant k (s^-1). min Cr^IVAt 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+Cr^III) 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 Cr^IV 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 Cr^IV is more hydrolysed than Cr^III. The final product, Cr^VI, 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 2Cr^IV→Cr^III+Cr^V with k=4.1×10⁷ d mol^-1 s^-1 at pH 3.7, followed by the rearrangement of the coordination shell of Cr^V from octahedral to tetrahedral and reaction between Cr_tet^V and Cr_oct^IV in non-rate-determining steps.