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.