DOI:
10.1039/A704395C
(Paper)
Reference Section for:
J. Chem. Soc., Faraday Trans., 1997,
93, 4265-4268
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 Cr
IVAt pH 3 and 3.7, respectively, the values for
OH are
K=91±9 and 59±3;
k=(3.7±0.1)×10
6 and (5.8±0.1)×10
6, based on the assumption that free and complexed
OH react at the same rate with benzoic acid, which was used to measure
k(
OH+Cr
III) by the competition kinetics method; and for SO
4- they are
K=(7.6±1.9)×10
2 and (4.2±0.5)×10
3;
k=(2.5±0.3)×10
4 and (1.2±0.1)×10
4. The absorption spectrum of Cr
IV generated by
OH and SO
4- 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)×10
4 s
-1 at pH 3 and (6±1)×10
4 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=4.1×10
7 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
tetV and Cr
octIV in non-rate-determining steps.