Mechanistic aspects associated with the oxidation of L-ascorbic acid at the 1,2-dichloroethane vertical bar water interface

The biologically relevant oxidation of L-ascorbic acid (AH(2)) was studied at an externally polarised 1,2-dichloroethane / water interface by a combination of electrochemical and spectroscopic techniques. In the presence of an electron acceptor (chloranil) in 1,2-dichloroethane, the basic features of the voltammetric signal are dependent on the concentration ratio of the redox species. In the presence of an excess of chloranil, a reversible signal is readily observed. A similar voltammetric response is developed when the semiquinone radical anion of chloranil (Q(.-)) is generated by homogeneous reduction in the presence or decamethylferrocene (DMFc) in the organic phase, These results suggest that Q(.-) is formed homogeneously rather than by heterogeneous electron transfer from AH(2) to Q under these conditions. However, in the presence of an excess of ascorbate, an irreversible voltammetric response is the dominant feature. Replacing chloroanil by tetracyanoquinodimethane also provides irreversible features indicating that heterogeneous electron transfer can also take place. The occurrence of both homogeneous and heterogeneous electron transfer pathways is also suggested by potential modulated reflectance (PMR) spectroscopy. (C) 2001 Elsevier Science B.V. All rights reserved.