Kinetic Studies of the Oxidation of l-Ascorbic Acid by the Peroxodisulfate Ion, and of Copper(II)-catalysis

Abstract

Abstract Kinetic studies of the oxidation reaction of l-ascorbic acid by the peroxodisulfate ion(S2O82−) are carried out in an aqueous solution over the pH range of 3.4–4.6 at various ionic-strengths from 0.071 to 1.07 M (1 M= 1 mol dm−3) with NaClO4, and at four temperatures between 15 and 30 °C, at an ionic-strength of 1.07 M. The variations in the rate of the oxidation with the hydrogen-ion concentrations are consistent with the reaction schemes involving two pH-related species; ascorbic acid H2A (k1=0.032 M−1 s−1 at 25 °C, ΔH1\eweq=17 kJ mol−1, ΔS1\eweq=−220 J deg−1 mol−1) and the ascorbic anion HA− (k2=0.43 M−1 s−1 at 25 °C, ΔH2\eweq=45 kJ mol−1, ΔS2\eweq=−102 J deg−1 mol−1). A relationship of logk2=−1.47+2.17I1⁄2⁄(1+I1⁄2) is found with the ionic-strength (I) at 25 °C. The reaction rate is greatly catalyzed by the presence of trace amounts of the copper(II) ion; the mechanisms of the copper(II)-catalyzed reaction are discussed.