Mechanism of aqueous ozone decomposition in alkaline solution.

Non-inhibited and carbonate ion inhibited ozone decomposition in aqueous so lution was studied by the stopped-flow method in the pH 10.4 - 13.2 range a t 25 +/- 0.1 degreesC and I = 0.5 M NaClO4. The absorbance change of the re action mixture was monitored at the characteristic absorbance maximum of oz one (260 nm), ozonide ion radical (430 nm) and carbonate ion radical (600 n m). A detailed radical type chain mechanism is proposed for the interpretat ion of the results. The kinetic role of previously ignored reaction steps o f the chain carriers was tested by comparing experimental and simulated kin etic traces. The rate constants for the initiation and the kinetically domi nant reaction steps were estimated by simultaneous evaluation of the experi mental traces using a GEAR-algorithm-based non-linear least squares method. The proposed mechanism predicts the lifetime of ozone as well as the kinet ics of the formation and subsequent decay of the ozonide ion radical (O-3(- )) and (in case of carbonate ion inhibition) carbonate ion radical (CO3-) r easonably well. The kinetically less significant reactions of other transie nt species are also discussed in detail. It was shown that the rate of ozone decomposition sharply decreases and rea ches a limiting value by increasing the carbonate ion concentration: It was confirmed that the inhibition is mainly due to the removal of two dominant chain carrier radicals, OH and O-3(-). In agreement with the experimental data, the model predicts that ozone decomposition occurs in a simple first- order process at high CO(3)(2-)concentration. This provides a tool for the direct determination of the initiation rate constant. Other factors affecting ozone stability and the decomposition rate are disc ussed in detail. It is shown that some of the unique phenomena and irreprod ucibilities of the kinetic traces are probably related to hydrogen peroxide formation during. the preparation of ozone stock solution under acidic con ditions. On the basis of the results, some of the discrepancies found in ea rlier literature were resolved.