Kinetics and mechanism of the carbonate ion inhibited aqueous ozone decomposition

The carbonate ion inhibited aqueous decomposition of ozone was studied by t he stopped-flow method at 25.0 +/- 0.1 degrees C in 0.5 M NaClO4. It was sh own that the rate of decomposition sharply decreases and reaches a limiting value by increasing the carbonate ion concentration. A detailed kinetic mo del was developed for the interpretation of the results. The corresponding set of rate constants was calculated by simultaneously fitting kinetic trac es obtained at the absorption maxima of O-3 (260 nm), O-3(-) (430 nm), and CO3- (600 nm). It was confirmed that the inhibition is mainly due to the re moval of two dominant chain carrier radicals, OH and O-3(-), via the follow ing reaction steps: CO32- + OH --> CO3- + OH-, k = (1.0 +/- 0.1) x 10(8) M- 1 s(-1) and CO3- + O-3(-) --> CO32- + O-3, k = (5.5 +/- 0.5) x 10(7) M-1 s( -1). The kinetically less significant reactions of other transient species are also discussed in detail. The mechanism gives proper description of ozo ne decay, the formation and subsequent disappearance of ozonide ion and car bonate ion radicals, as well as the concentration change of other intermedi ates over the pH range 10.7-12.8. The model predicts that ozone decompositi on occurs in a simple first-order process at high CO32- concentration, in a greement with the experimental data.