Mechanism of the Fenton reaction. Evidence for a new intermediate

Abstract

For the first time, the time dependence of the amount of O₂ evolved in the reaction of Fe²⁺ with H₂O₂ (the Fenton reaction) was measured by following the decrease of the total [Fe³⁺] formed upon quenching the reaction mixture with an excess of a strongly acidic solution of Fe²⁺. The disappearance of Fe²⁺ in the reaction was followed by quenching it with o-phenanthroline and measuring the absorbance of the Fe²⁺-phenanthroline complex formed at λ=510 nm. At [H₂O₂]<10^-4 mol dm^-3, and quenching the reaction with Fe²⁺ ions, the formation of a new intermediate was observed. It was identified as the mixed valence binuclear species {FeOFe}⁵⁺. A new mechanism for the reaction has been proposed in which FeO²⁺ acts as the key intermediate. Several rate constants and ratios of rate constants have been determined. The existence of free radicals in the system does not seem to be compatible with the data.