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

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 (pK_a 3.5) and HFeO₄₋ (pK_a 7.3) are reactive, k(HFeO₄⁻₋ + O₂⁻₋) = (1.7 ± 0.2) × 10⁷ M^-1s^-1 The stable ferrate ion, FeO₄^2-, 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⁺ (pK_a = 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.

Key Words:

• ferrate(VI)
• ferrate(V)
• hydrogen peroxide
• superoxide radical
• kinetics
• pulse radiolysis