A kinetic study of electron transfer from L‐Ascorbic acid to sodium perborate and potassium peroxy disulphate in aqueous acid and micellar media

Abstract

Kinetics of oxidation of L-ascorbic acid (H2A) by sodium perborate (SPB) and peroxy disulphate (PDS) have been investigated in aqueous acid and micellar media. Reaction kinetics indicated first-order dependence on both |oxidant| and |H2A|. Increase in ionic strength (μ) increased reaction rate only in H2SO4 media. Rate of SPB oxidation of H2A has been accelerated by acidity in HNO3 and HCl media while a decreasing trend is observed in HClO4 and H2SO4 media. The results are interpreted by various theories of acidity functions. Reaction rate is enhanced by the addition of added |H2O2| indicating a H2O2 coordinated boron species to be active in the present system. In the absence of micelle, increase in |acid| altered the PDS(SINGLEBOND)H2A reaction rate marginally (a very small positive effect with HClO4 and negative effect with H2SO4). Most plausible mechanisms have been proposed on the basis of experimental results. Activation parameters evaluated for specific kinetic constants are in accord with outer sphere electron transfer mechanism. In SPB(SINGLEBOND)H2A system, addition of anionic micelle (Sodium lauryl sulfate) increased the rate, stabilizing the cationic species in the transition state in all the acid media. Although rate of PDS oxidation of H2A was catalyzed by TX and inhibited by SDS at critical micellar concentration (CMC) increase in |acid| (both HClO4 and H2SO4) beyond 9.6 × 10−4 M decreased the rate of oxidation. This trend was explained due to the repulsive interaction of coanion, HA−, and negatively charged micellar species. © 1996 John Wiley & Sons, Inc.