An Upper Limit to O₂ Evolution as Test for Radical and Nonradical Mechanisms for the Fenton Reaction

The origin of an upper limit to the amount of O₂ evolved in the rapid reaction between Fe²⁺ and H₂O₂ was investigated at a high concentration of H₂O₂. Using a nonradical model, including the formation of a primary Fe²⁺–biperoxy complex with a diminished rate of formation of the active intermediate FeO²⁺, agreement has been reached for the first time with the experimental data obtained by Barb et al. A limited formation of O₂ requires that a finite concentration of H₂O₂ should be present in the reaction mixture when [Fe²⁺] falls to zero. It has been shown that in Barb et al.’s model the condition for such a state ([Fe²⁺] = 0, [H₂O₂] > 0) does not exist. Free radical based models fail as mechanisms for the Fenton reaction.