Kinetics and Mechanism of the Decomposition of Chlorous Acid

Abstract

The kinetics of decomposition of aqueous chlorous acid has been reinvestigated at pH 0.7−1.9, ionic strength 1.0 M (HSO 4 - /SO 4 2- ), and temperature 25.0 ± 0.1 °C. Optical absorbances were collected in the 240−450 nm wavelength range for up to ∼90% decomposition for time series lasting as long as 2 days. The number of absorbing species was investigated by matrix rank analysis; no absorbing intermediate was formed in significant concentration during the decomposition. Of the many mechanistic models tested, the one that fit best included the following reactive intermediates: HOCl, Cl 2 O 2, Cl 2 O 3, • ClO, • OH. The stoichiometric ratio of ClO 2 produced to Cl(III) consumed varies with pH and [Cl - ]. Reaction of Cl 2 O 3 with Cl(III) yields chlorate exclusively. Reaction of Cl 2 O 3 with Cl - favors ClO 2 over chlorate, but does not entirely exclude chlorate, because it is produced by hydrolysis of Cl 2 O 2 . Invoking Cl 2 O 3 explains the variation in stoichiometric ratio as well as the maximum observed in the initial rate of ClO 2 formation as a function of pH. The kinetics of chlorous acid decomposition cannot be quantitatively fit through the last stages of the reaction without postulating a first-order decomposition. Scission of chlorous acid to give short-lived hydroxyl and chlorine(II) monoxide is a plausible route for this process. A set of best-fit and literature-derived parameters is presented for the complete mechanism.