Mechanism of the oxidation of L-ascorbic acid by the bis(pyridine-2,6-dicarboxylate)cobaltate(III) ion in aqueous solution

Abstract

A detailed investigation of the oxidation of L-ascorbic acid (H₂A) by the title complex has been carried out using conventional spectrophotometry at 510 nm, over the ranges: 0.010 ≤ [ascorbate] _T ≤ 0.045 mol dm⁻³, 3.62 ≤ pH ≤ 5.34, and 12.0 ≤ θ ≤ 30.0 °C, 0.50 ≤ I ≤ 1.00 mol dm⁻³, and at ionic strength 0.60 mol dm⁻³ (NaClO₄). The main reaction products are the bis(pyridine-2,6-dicarboxylate)cobaltate(II) ion and l-dehydroascorbic acid. The reaction rate is dependent on pH and the total ascorbate concentration in a complex manner, i.e., k _obs = (k ₁ K ₁)[ascorbate] _T /(K ₁ + [H⁺]). The second order rate constant, k ₁ [rate constant for the reaction of the cobalt(III) complex and HA⁻] at 25.0 °C is 2.31 ± 0.13 mol⁻¹ dm³ s⁻¹. ΔH ^‡ = 30 ± 4 kJ mol⁻¹ and ΔS ^‡ = −138 ± 13 J mol⁻¹ K⁻¹. K ₁, the dissociation constant for H₂A, was determined as 1.58 × 10⁻⁴ mol dm⁻³ at an ionic strength of 0.60 mol dm⁻³, while the self exchange rate constant, k ₁₁ for the title complex, was determined as 1.28 × 10⁻⁵ dm³ mol⁻¹ s⁻¹. An outer-sphere electron transfer mechanism has been proposed.