Salt effect on metallic ion-catalysed autoxidation of l-ascorbic acid

Abstract

Kinetics of Cu(II)-catalysed autoxidation of ascorbic acid in aqueous solutions has been studied in the presence of halides, thiocyanate and cyanide ions. The effect of these additives on the rate can be explained in terms of “ligand effect” which includes coordination ability (the order of spectrochemical series 2), trans effect 3 and instability effect (or leaving ability 3) of the anion of additive. Fast consumption of ascorbic acid even in the absence of Cu(II) occurs on addition of KNO 2 or KF. Virtually no effect is observed with KNO 3 probably because NO 3 − is a good leaving ligand and its ionic strength does not affect the autoxidation. The acceleration with KCl is maximum, when [Cl −] is nearly equal to initial concentration of ascorbic acid. Effect of oxygen pressure ( p o) on the rate in the presence of Cl − suggests the intermediary metallic complex composed of O 2, Cl − and ascorbate ion. Complex formation and ligand effect are further confirmed both in the autoxidation of ascorbic acid catalysed by Ni(II), Co(II) and Fe(III) and also in the autoxidations of catechol and phenol catalysed by these metallic ions.