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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].