Decomposition of ozone in water: rate of initiation by hydroxide ions and hydrogen peroxide

Abstract

The initiation of ozone decomposition in "pure water" is first order in O₃ and OH^- concentration and <I>k</I>_O₃,OH^- becomes 70 ± 7 M^-1 s^-1 when a sequence of reactions is assumed by which three molecules of O₃ are eliminated per primary event. The difference between this value and higher values reported in the earlier literature may be explained by interferences by radical chain reactions which have not been totally inhibited in those studies. H₂O₂ also reacts with O₃ when present as an anion, HO₂^-. <I>k</I>_O₃,OH^- is (2.8 ± 0.5) × 10⁶ M^-1 s^-1 when it is assumed that 'two molecules of O₃ are eliminated per primary event. Therefore, whenever [H₂O₂] > 10^-7 M (pH <12), HO₂^- has a greater effect than OH^- on the decomposition rate of O₃ in water. The high reactivity of HO₂^- explains that H₂O₂ is observed as a significant intermediate in the ozonated water only if the pH is low, e.g., <6.