Photochemistry of Sodium Chlorite in Solution, with Reference to Chlorite Bleaching

At wavelengths less than 3700 A, the absorption spectrum of an aqueous solution of sodium chlorite is that of the chlorite ion. This spectrum has two maxima, at 2600 and 2900 A, and is strongly affected by exposure of the solution to ultraviolet radiation from the quartz mercury discharge lamp. The intensity of the maximum at 2600 A decreases rapidly, whereas that at 2900 A increases, and a new absorption maximum appears at 3600 A, indicating that photochemical changes have occurred. Comparison of these spectra with those of chlorine dioxide and hypochlorite ion indicates that these two compounds are the primary photochemical decomposition products of chlorite ions. The amount of these products formed is strongly dependent on pH, chlorine dioxide being the chief product in an acid medium and hypochlorite ion in an alkaline one. Kinetic studies show that the same empirical rate equation (—d[ClO]/dt = kI0[ClO]0.5) holds in both acid and alkaline media. The rate constant, however, is greater in an acid medium. The quantum yield, which was determined actinometrically, is 0.9 at pH 6 and 0.6 at pH 9. Reaction mechanisms are proposed and the kinetic equations derived from them are found to agree well with the experimental results.