Reaction of [Fe2(CN)10]4– with L-ascorbic acid

Abstract

The oxidation of L-ascorbic acid by the iron(III) dimeric complex [Fe₂(CN)₁₀]^4– in acidic aqueous solution at an ionic strength of 1.0 mol dm^–3(NaClO₄) has been studied by stopped-flow spectrophotometry. The overall reaction, which entails reduction of both iron(III) centres followed by aquation resulting in the formation of 2 mol of [Fe(CN)₅(OH₂)]^3–, takes place in three stages. The first is a one-electron reduction to form the mixed-valence iron(III, II) dimer followed by a second one-electron reduction of the other iron(III) centre to form the iron(II) dimer. The third stage which occurs with a very small absorbance change was not studied. The mechanisms for both stages of the reaction are similar and involve formation of an ion triplet [Fe₂(CN)₁₀]^n–·M⁺·A^p– where n= 4 or 5 and A^p– is the reacting ascorbate species with p= 1 or 2. The rate law was found to be of the form (i) where [A]_T is the total concentration of added ascorbate, k₁ and k₂ the acid dissociation constants Rate =k′K₁[H⁺]+k″K₁K₂//_{K1K2+K1[H⁺]+[H⁺]²}[A]_T[complex](i), of ascorbic acid and k′ and k″ the pseudo-second-order rate constants. They were found to be 6.44 × 10³, 2.48 × 10⁹ and 1.25 × 10³, 2.98 × 10⁸ dm³ mol^–1 s^–1 at 19 °C for HA^– and A^2– for the first and second stages respectively. The reaction is affected by alkali-metal cations, increasing rate with increasing size of the metal ions: Li⁺ < Na⁺ < K⁺.