Kinetics and Mechanism of the Decomposition of Chlorous Acid

The kinetics of decomposition of aqueous chlorous acid has been reinvestigated at pH 0.7−1.9, ionic strength 1.0 M (HSO4-/SO42-), 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, Cl2O2, Cl2O3, •ClO, •OH. The stoichiometric ratio of ClO2 produced to Cl(III) consumed varies with pH and [Cl-]. Reaction of Cl2O3 with Cl(III) yields chlorate exclusively. Reaction of Cl2O3 with Cl- favors ClO2 over chlorate, but does not entirely exclude chlorate, because it is produced by hydrolysis of Cl2O2. Invoking Cl2O3 explains the variation in stoichiometric ratio as well as the maximum observed in the initial rate of ClO2 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.