The decomposition of hypochlorous acid in the neutral pH region was studied in 1.0 M NaC104 from 15 to 50 °C. The pAa of HOC1 was also determined under these conditions. Hypochlorous acid has a maximum decomposition rate at pH 6.89. The decomposition is a third-order process. The values of AH* and AS* are 64.0 ± 0.6 kJ/mol and -67 ± 2 J/mol K, respectively. A mechanism for the decomposition of HOC1 is proposed involving C120-H20 and C102~a s intermediates. Rate constants for the rate-determining steps of the mechanism are presented. Above pH 6, the rate-determining step is proposed to be as follows: OCh + C120-H20 -HC1202~+ HOC1. Below pH 6, this process is proposed to be in competition with a parallel pathway: HOC1 + 020• 20 -» H2C1202 + HOC1.The proposed mechanism was tested by mathematical simulation of the experimental data using the GEAR algorithm. The simulation gives additional support for the proposed mechanism.

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The kinetics of the redox reactions of the peroxomonosulfate ion (HSO(5)(-)) with iron(II), vanadium(IV), cerium(III), chloride, bromide, and iodide ions were studied. Cerium(III) is only oxidized upon illumination by UV light and cerium(IV) is produced in a photoreaction with a quantum yield of 0.33 +/- 0.03. Iron(II) and vanadium(IV) are most probably oxidized through one-electron transfer producing sulfate ion radicals as intermediates. The halide ions are oxidized in a formally two-electron process, which most likely includes oxygen-atom transfer. Comparison with literature data suggests that the activation entropies might be used as indicators distinguishing between heterolytic and homolytic cleavage of the peroxo bond in the redox reactions of HSO(5)(-).

Kinetic traces for the redox decomposition of iron(III)-sulfito complexes exhibit a peculiar break in the presence of oxygen. A detailed kinetic analysis of this feature as a function of [Fe(III)], [Fe(II)], [S(IV)], and [02] at pH 2.5 indicated that this step is a result of the sulfite-induced autoxidation of produced iron(II) in the presence of oxygen. The so observed redox cycling of iron comes to a dead end at the point in time when all the oxygen in the solution has been used up. The kinetic traces can be interpreted in terms of a first-order decay of the iron(III)-sulfito complexes and a reverse pseudo-zero-order oxidation of iron(II) by S05*-, HS05-, and SO4•-. The latter species are generated in solution via the reaction of S03•-, produced during the reduction of iron(III) by sulfite, with oxygen. Radical scavengers do not affect the first-order decay but inhibit the pseudo-zero-order step. The results reveal no evidence for the formation of an intermediate oxygenated complex, and clearly indicate the important role of the sulfite-induced redox cycling of iron(II/III) in the presence of oxygen. Computer simulations based on the proposed reaction mechanism are in good agreement with the observed experimental kinetic traces and indicate that the formation of the SO5•-radical is the main oxygen-consuming step during the overall redox process.

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