The OsO₄-catalysed decomposition of hydrogen peroxide

Abstract

The decomposition of H2O2 has been investigated in the presence of OsO4 as a catalyst. The rate of decomposition is proportional to the first power of the OsO4 concentration and to the power 1–1.2 of the hydrogen peroxide concentration. The pH dependence of the decomposition rate is quite characteristic; a high maximum is attained at pH 10.6 and a much lower one at about pH 8.3. With the aid of appropriate free-radical reagents and by e.s.r. spectrometry, it is shown that hydroxyl and superoxide radicals are formed during the catalysed decomposition. The rate of formation of the OH radical depends on the concentrations of H2O2 and OsO4, as well as on the pH. The rate of bleaching of N-dimethyl-p-nitrosoaniline and other dyes by the OH radical as a function of pH exhibits a maximum at pH 8.3. The OH radical is of both primary and secondary origin, and is involved in a chain reaction. The length of the chain is about 80–90 at pH < 8, while at pH > 9 this value drops to ca. 10 and is independent of the pH. The apparent activation energy of the reaction route involving the OH radical is 105–111 kJ mol–1 at pH < 9, 35–40 kJ mol–1 at pH > 10. Another decomposition route parallels that involving the OH radical, and predominates at pH > 9.5. In this case the catalysed decomposition can be approximated on the assumption that peroxo-osmate anion acts as a nucleophile towards the non-dissociated hydrogen peroxide molecule. The temperature dependence of this rate component gives an apparent activation energy of 60 kJ mol–1 at pH < 9, ca. 15 kJ mol–1 at pH > 10.