Ferryl for real. The Fenton reaction near neutral pH

Abstract

According to the literature, the Fenton reaction yields HO˙ and proceeds with 53 M⁻¹ s⁻¹ at 25 °C and low pH. Above pH 5, the reaction becomes first-order in HO⁻, and oxygen atom transfer has been detected, which indicates formation of oxidoiron(2+), FeO²⁺. These observations, and the assumption that the intermediate [FeHOO]⁺ decays approximately iso-energetically to FeO²⁺, allow one to estimate an Gibbs energy of formation FeO²⁺ of +15 ± 10 kJ mol⁻¹, from which follows the one-electron E°′(FeO²⁺, H₂O/[Fe(HO)₂]⁺) = +2.5 ± 0.1 V and the two-electron E°′(FeO²⁺, 2H⁺/Fe²⁺, H₂O) = +1.36 ± 0.05 V, both at pH 7. In the presence of HCO₃⁻, formation of FeCO₃(aq) occurs which may facilitate formation of the [FeHOO]⁺ intermediate, and leads to CO₃˙⁻. At pH 7, the product of the Fenton reaction is thus FeO²⁺, or CO₃˙⁻ if HCO₃⁻ is present.