Studies on singlet oxygen in aqueous solution. Part 3. The decomposition of peroxy-acids
The decompositions of peroxyacetic acid, peroxymonosulphuric acid (Caro's acid), and mono-peroxyphthalic acid have been studied in aqueous solution at pH values close to the pKa of the acids. When precautions are taken to eliminate metal-ion catalysis, essentially quantitative yields of 1O2, are obtained, as determined using the caesium salt of anthracene-9,10-bis(ethanesulphonate)(aes) as a trap for 1O2. With peroxypivalic acid the maximum yield of 1O2, was only ca. 80%, but in this case non-1O2 attack on the aes also occurred. Negligible amounts of 1O2 are formed when H2O2 reacts with monoperoxyphthalate or peroxyacetate anions at pH ca. 10.7 under normal ‘clean’ conditions. However, the presence of the chelating agent diethylenetriamine-NNN′N″N″-penta-(methylphosphonic acid) produces a dramatic decrease in the reaction rates. It is concluded that the reactions of H2O2 with peroxy-acid anions are extremely sensitive to catalysis, probably by transition-metal ions.