Promotion of the fenton reaction by Cu²⁺ ions: Evidence for intermediates

The promotion of the Fenton reaction by Cu²⁺ ions has been investigated using a wide range of [Cu²⁺]. Both the disappearance of Fe²⁺ and the evolution of O₂ were followed as a function of time by quenching the reaction mixture with o-phenanthroline or with excess Fe²⁺ ions, respectively. Two series of experiments were performed. In one series [H₂O₂] was 5 × 10^-4 mol dm^-3, and in the other [H₂O₂] was reduced to 5 × 10^-5 mol dm^-3. By stopping the reaction with excess Fe²⁺ ions, significant differences in the measured absorbance in the two series were observed. In the higher [H₂O₂] range, the absorbance decreased monotonically in time, due to O₂ formation during the reaction. In the lower range, an initial transient rise of the absorbance was observed, indicating the formation of spectroscopically distinct intermediates in the system. A mechanism involving the intermediates FeOCu⁴⁺ and FeOCu⁺⁵ has been set up. Rate constants of the mechanism have been determined.