Effect of Chloride Ion on the Kinetics and Mechanism of the Reaction between Chlorite Ion and Hypochlorous Acid

The effect of chloride ion on the chlorine dioxide formation in the ClO2−−HOCl reaction was studied by following ·ClO2 concentration spectrophotometrically at pH 5−6 in 0.5 M sodium acetate. On the basis of the earlier experimental data collected without initially added chloride and on new experiments, the earlier kinetic model was modified and extended to interpret the two series of experiments together. It was found that the chloride ion significantly increases the initial rate of ·ClO2 formation. At the same time, the ·ClO2 yield is increased in HOCl but decreased in ClO2− excess by the increase of the chloride ion concentration. The two-step hydrolysis of dissolved chlorine through Cl2 + H2O ⇋ Cl2OH− + H+ and Cl2OH− ⇋ HOCl + Cl− and the increased reactivity of Cl2OH− compared to HOCl are proposed to explain these phenomena. It is reinforced that the hydrolysis of the transient Cl2O2 takes place through a HOCl-catalyzed step instead of the spontaneous hydrolysis. A seven-step kinetic model with six rat...