Studies on singlet oxygen in aqueous solution. Part 4. The ‘spontaneous’ and catalysed decomposition of hydrogen peroxide

Abstract

The decomposition of H2O2 in alkaline solution has been studied in the presence of the chelating agent diethylenetriamine-NNN′N″N″-penta(methylphosphonic acid). A minimum rate constant of 4.7 × 10–7 dm3 mol–1 s–1 at pH 11.6 and 35 °C was obtained. This is the slowest rate yet reported, but still probably represents a catalysed reaction. When the decomposition was carried out using the caesium salt of anthracene-9,10-bis(ethanesulphonate)(aes) as a trap for 1O2 the production of small but significant amounts of 1O2 was observed. If it is assumed that the ‘spontaneous’ reaction produces 100%1O2, then reasonably consistent corrected rate constants were obtained under different conditions, leading to a tentative ‘true’ value of 3.7 × 10–8 dm3 mol–1 s–1 at 35 °C. The I– catalysed decomposition of H2O2 produces 0.4%1O2 and an explanation of this low yield is given. In the Br–-catalysed decomposition (pH-independent region), the yields of 1O2 varied from 11 to 80%, the upper value being consistent with that of 76% previously obtained for the direct reaction of HO2– and HOBr. The corrected rate constants are again in reasonable agreement. Catalysis by Cl– in the pH-independent region is exceedingly slow. A yield of ca. 8.4% of 1O2 was obtained in one run at 50 °C. The corrected rate constant (assuming 100% production of 1O2 for the uncatalysed reaction) is approximately an order of magnitude less than the literature value. In the decompositions of H2O2 catalysed by Fe3+, catalase, the Fe3+–triethylenetetramine complex, and heterogeneously by Ag, Pt, or MnO2, no 1O2 could be detected, although with Fe3+ evidence is presented for the involvement of ˙OH radicals.