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Reduction Potential of the tert -Butylperoxyl Radical in Aqueous Solutions

1998; American Chemical Society; Volume: 102; Issue: 1 Linguagem: Inglês

10.1021/jp972903t

1520-5215

Tomi Nath Das, T. Dhanasekaran, Zeev B. Alfassi, P. Neta,

Advanced oxidation water treatment

Rate constants for oxidation of tert-butyl hydroperoxide anion (t-BuO2-) by several oxidants were determined by pulse radiolysis. Rapid oxidation was found with N3• (4.4 × 108 L mol-1 s-1) and •O- (2.6 × 108 L mol-1 s-1), but others (Br2•-, I2•-, ClO2•, CO3•-, C6H5N(CH3)2•+) reacted much more slowly. Since the reduction potentials of these oxidants are much higher than that estimated for t-BuO2•, the observed rate constants suggest that the self-exchange rate for t-BuO2•/t-BuO2- is very slow and that it would be difficult to establish rapid equilibrium by pulse radiolysis with a suitable redox pair. Therefore, to determine the reduction potential for this peroxyl radical, the forward and reverse rate constants for reaction 6, t-BuO2• + Fe(CN)64- ⇄ t-BuO2- + Fe(CN)63-, were measured independently. The forward rate constant was measured by kinetic spectrophotometric pulse radiolysis in N2O-saturated solutions containing high concentrations of N3- (1 mol L-1), t-BuO2- (0.4 mol L-1), and varying concentrations of Fe(CN)64- (0.012−0.036 mol L-1) at pH 13.7. The rate constant k6 was found to be 2.5 × 102 L mol-1 s-1. Such a low rate constant was possible to measure by pulse radiolysis only because the self-decay of t-BuO2• is very slow compared to those of other peroxyl radicals. The reverse reaction rate constant was measured by mixing the reactants and following the disappearance of Fe(CN)63- in the presence of spin traps to remove the peroxyl radicals from the equilibrium. A rate constant of k-6 = 1.4 × 10-2 L mol-1 s-1 was derived. By taking the reduction potential of Fe(CN)63-/Fe(CN)64- as 0.459 V vs NHE for the conditions used in these experiments, we calculate E(t-BuO2•/t-BuO2-) = 0.71 V and E(t-BuO2•/t-BuO2H) = 1.05 V at pH 7 and 1.47 V at pH 0. The rate constants for oxidation of Fe(CN)64- and other compounds by various peroxyl radicals were then utilized for the estimation of the reduction potentials of these peroxyl radicals.