Determination of fast ozone reactions in aqueous solution by competition kinetics

Abstract

High ozone rate constants (in water) cannot be determined directly by stopped-flow techniques. Competition experiments with buten-3-ol (k = 7.9 × 10⁴ dm³ mol⁻¹ s⁻¹; Dowideit and von Sonntag, Environ. Sci. Technol., 1998, 32, 1112), based on reasonably long-lived intermediates, have been set up with 1,4-dimethoxybenzene (DMB; intermediate at 250 nm measured at 3 s) and 1,3,5-trimethoxybenzene (TMB; intermediate at 260 nm, measured at 10 s). They yielded k(O₃ + DMB) = 1.3 × 10⁵ and k(O₃ + TMB) = 9.4 × 10⁵ dm³ mol⁻¹ s⁻¹, respectively. A competition of 1,3,5-trimethoxybenzene with indigotrisulfonate, monitored by the bleaching of the indigo derivative at 600 nm, yielded a rate constant of 9.4 × 10⁷ dm³ mol⁻¹ s⁻¹ for this compound. This convenient and highly reactive competitor now allows us to determine other very high ozone rate constants. As an example, its reaction with DABCO (k = 1.7 × 10⁶ dm³ mol⁻¹ s⁻¹) has been determined. Its direct reaction with ozone (by following the decay of the absorption at 260 nm) can only be determined in a pH range where DABCO is still largely protonated [k(O₃ + DABCO/H₂²⁺) < 30 dm³ mol⁻¹ s⁻¹; k(O₃ + DABCO/H⁺) = 3.5 × 10³ dm³ mol⁻¹ s⁻¹], and the rate constant of the fully deprotonated amine can only be arrived at by extrapolation [k(O₃ + DABCO) = 3.2 × 10⁶ dm³ mol⁻¹ s⁻¹].