Reaction of Ferrate (VI)/Ferrate (V) with Hydrogen Peroxide and Superoxide Anion - a Stopped-Flow and Premix Pulse Radiolysis Study

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Abstract
The reduction of ferrate(VI) to ferrate(V) by superoxide ions was studied over the pH range 2.6–13.0 using the premix pulse radiolysis technique. The pH dependence indicates that only the unstable protonated forms of ferrate, H2FeO4 (pKa 3.5) and HFeO4− (pKa 7.3) are reactive, k(HFeO4−− + O2−−) = (1.7 ± 0.2) × 107 M-1s-1 The stable ferrate ion, FeO42-, showed no significant reactivity towards either hydrogen peroxide or superoxide anion. The rate constants for the spontaneous dimerization and decomposition of the protonated ferrates, e.g. k(HFeO4-+ HFeO4-) = 250M-1s-1, are orders of magmtude slower than their corresponding reduction by superoxide indicating an outer-sphere mode of electron transfer for the latter process. In contrast the ferrate(VI) species H3FeO4+ (pKa = 1.6 ± 0.2), H2FeO4, and HFeO4−− oxidize hydrogen peroxide, e.g. k(HFeO4−− + H2O2) = 170 M-1s-1), at rates which correspond closely to their dimerization rates suggesting an inner-sphere controlled mechanism.