The outer-sphere oxidation of ascorbic acid by the thioureapentacyanoferrate(III) ion

Abstract

The oxidation of ascorbic acid by thioureapentacyanoferrate(III) has been investigated over the pH range 0.8–7.2 at an ionic strength of 1.0 M (NaClO4). The variations in the rates of oxidation with hydrogen ion concentration are consistent with a reaction scheme involving three pH-related species; ascorbic acid (H2A) (k1 = 0.111 M−1 s−1, ΔH1≠ = 5.1 kcal mol−1, ΔS≠ = −46 cal deg−1 mol−1), the ascorbic anion, (HA−) (k2 = 66 M−1s−1, ΔH2≠ = 5.4 kcal mol−1, ΔS2≠ = −32 cal deg−1 mol−1) and the ascorbic dianion, (A2−) (k3 = 8.78 × 107 M−1 s−1, ΔH3≠ = 3.1 kcal mol−1, ΔS3≠ = −12 cal deg−1 mol−1). The low reduction potential of Fe(CN)5TU2− (E0 = 0.35 V) permits the measurement of the rate of oxidation of the ascorbate dianion, not previously investigated with metal complexes. The second acid dissociation constant of ascorbic acid [Formula: see text] has been measured in 1.0 M NaClO4 over the temperature range studied. A relationship between the rate of oxidation and the reduction potentials of the ascorbic acid species is shown.