THE EFFECT OF TEMPERETURE AND pH ON THE KINETICS OF LASCORBIC ACID OXIDATION CATALYZED BY COPPER(II)

Abstract

In the present work, the kinetics of uncatalyzed and Cu(II) ion-catalyzed oxidation of L-ascorbic acid, [ H(2)A], by molecular oxygen in citrate buffer was studied at different temperatures (20 degrees-35 degrees C) and pH levels (3.35-6.50). The H2A concentration was determined by 2,6-dichlorophenolindophenol titration. In the pH range investigated, the rate of cupric ion-catalyzed oxidation was found to be first order with respect to ascorbic acid. At a given temperature the specific rate constants increased with pH to about 4.50 and decreased at pH 6.00. At a given pH specific rate constants increased with temperature. Specific rate constants at pH 4.50 ranged from 48.56 M(-1)min(-1) (20(o)C) to 256.92 M(-1)min(-1) (35 degrees C). Activation energies calculated at different pH levels ranged from 9.66 to 19.03 kcal mole(-1). The enthalpy (Delta H*), entropy (Delta S*) and Gibbs free energy (Delta G*) of activation were determined at 20 degrees-35 degrees C temperature range.