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

The initiation of ozone decomposition in "pure water" is first order in O<sub>3</sub> and OH<sup>-</sup> concentration and <I>k</I><sub>O<sub>3</sub>,OH<sup>-</sup></sub> becomes 70 ± 7 M<sup>-1</sup> s<sup>-1</sup> when a sequence of reactions is assumed by which three molecules of O<sub>3</sub> 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. H<sub>2</sub>O<sub>2</sub> also reacts with O<sub>3</sub> when present as an anion, HO<sub>2</sub><sup>-</sup>. <I>k</I><sub>O<sub>3</sub>,OH<sup>-</sup></sub> is (2.8 ± 0.5) × 10<sup>6</sup> M<sup>-1</sup> s<sup>-1</sup> when it is assumed that 'two molecules of O<sub>3</sub> are eliminated per primary event. Therefore, whenever [H<sub>2</sub>O<sub>2</sub>] > 10<sup>-7</sup> M (pH <12), HO<sub>2</sub><sup>-</sup> has a greater effect than OH<sup>-</sup> on the decomposition rate of O<sub>3</sub> in water. The high reactivity of HO<sub>2</sub><sup>-</sup> explains that H<sub>2</sub>O<sub>2</sub> is observed as a significant intermediate in the ozonated water only if the pH is low, e.g., <6.