The lifetime of the ferryl ion (minutes at pH below 1) is reduced by a fac-2ϩ

the reaction involving formation of the manganese(II)-superoxide complex and reaction of H 2 O 2 with Mn(IV) species formed due to reversible disproportionation of Mn(III), is suggested.

Mn(II)aq−superoxide complex, MnO2 +, was formed in pulse radiolysis by three distinct routes:  Mn(I) + O2, Mn(II) + O2 - and Mn(III) + H2O2. The stability of this complex was found to be governed by the two equilibria:  Mn2+ + O2 - ⇌ MnO2 + (1,−1) and Mn2+ + HO2 ⇌ MnO2 + + H+ (6,−6). Both forward and reverse rate constants of the reactions involved in these equilibria were determined:  k 1 = (1.5 ± 0.2) × 108 M-1 s-1, k - 1 = (6.5 ± 1.0) × 103 s-1; k 6 = (1.1 ± 0.2) × 106 M-1 s-1, k - 6 = (6.5 ± 1.0) × 106 M-1 s-1, yielding K 1, - 1 = (2.3 ± 0.5) × 104 M-1 and K 6, - 6 = 0.17 ± 0.06. The metal−oxy complex MnO2 + decays by self-reaction with k(MnO2 + + MnO2 +) = (6.0 ± 1.0) × 106 M-1 s-1 and in acid solutions also by reaction with HO2, k(MnO2 + + HO2) = (1.0 ± 0.3) × 107 M-1 s-1. In both cases stoichiometric amounts of H2O2 are formed as the end product. Mn(I) was formed by reduction of Mn2+ with H atoms. It has an absorption spectra with maxima at 290 and 340 nm with ε290 = 1300 ± 200 M-1 cm-1 and ε340 = 1000 ± 150 M-1 cm-1. It reacts with oxygen with k(Mn(I) + O2) = (6.0 ± 1.0) × 106 M-1 s-1. Mn(III) reacts with hydrogen peroxide with k(Mn(III) + H2O2) = (2.8 ± 0.3) × 103 M-1 s-1.

Stock solution of ozone was prepared by bubbling the effluent from an Erwin Sander (V) ozonator through an aqueous HClO 4 solution. Ozone concentrations of (2.5Ϫ3.0) ϫ 10 Ϫ4 M were achieved without any special procedure. The ozone concentration was determined spectrophotometrically at its maximum absorption wavelength, max ϭ 260 nm, using an ApparatusFor the kinetic studies a Hi-Tech SF 51 Stopped-flow spectrophotometer (SFS) with UV and VIS lamps was used. This instrument has a mixing time of about 1 -2 ms, which enables us to measure rate constants in the range of 0.01 -700 s Ϫ1 . Both the ozone and ABSTRACT: The temperature dependence of the oxidation kinetics of Fe 2ϩ by O 3 at pH 0 -3 was studied by stopped-flow technique in the temperature range 5 -40°C. Activation parameters of the reactions involved in formation and decay of the ferryl ion (iron(IV)), FeO 2ϩ are determined. The reaction of Fe 2ϩ ϩ FeO 2ϩ was found to branch into two channels forming iron(III)-dimer, Fe(OH) 2 Fe 4ϩ , and Fe 3ϩ . The yield of the dimer, Fe(OH) 2 Fe 4ϩ , increases with temperature on the expense of the Fe 3ϩ yield. On the basis of the overall rate constant and relative yield of Fe(OH) 2 Fe 4ϩ the activation energy is determined for both channels. The activation parameters of the hydrolysis of the ferryl ion and its reaction with H 2 O 2 were also determined.