Ozone decomposition in water studied by pulse radiolysis. 1. Perhydroxyl (HO2)/hyperoxide (O2-) and HO3/O3- as intermediates

Abstract

Pulse radiolysis allows one to selectively enter the chain mechanism for ozone decomposition in water by OH or O₂^- and to study the elementary reactions involved. Details of the reaction mechanism of the first chain propagation step (O₂^- + O₃) in the Weiss model have been resolved to include the two additional transients O₃^- (λ_max = 430 ± 10 nm, ε_max = 2000 M^-1 cm^-1) and HO₃ (λ_max = 350 ± 20 nm, (ε_max = 300 ± 30 M^-1 cm^-1). The elementary steps are as follows: O₂^- + O₃ → O₃^- + O₂, <I>k</I> = (1.6 ± 0.2) × 10⁹ M^-1 s^-1; O₃^- + H⁺ ⇌ HO₃ (a, b), <I>k</I>_a = (5.2 ± 0.6) × 10¹⁰ M^-1 s^-1, <I>k</I>_b = (3.7 ± 0.3) × 10⁴ s^-1; HO₃ → OH + O₂, <I>k</I> = (1.1 ± 0.1) × 10⁵ s^-1. A p<I>K</I>(HO₃/O₃^-) = 6.15 ± 0.05 is derived. Phosphate buffer also takes part in the protonation/deprotonation of HO₃/O₃^-: O₃^- + H₂PO₄^- ⇌ HO₃ + HPO₄^2- (a, b), <I>k</I>_a = (2.1 ± 0.2) × 10⁸ M^-1 s^-1, <I>k</I>_b = (2.0 ± 0.3) × 10⁷ M^-1 s^-1. Consequences of these high rate constants on the possible chain termination processes and on the possibility of interactions of organic solutes with the chain reactions are discussed.