Pulsradiolytische Untersuchung zur Oxidation der Ascorbinsäure durch OH-Radikale und Halogen-Radikal-Komplexe in wäßriger Lösung / Pulse Radiolysis Studies of the Oxidation of Ascorbic Acid by OH-radicals and Halide Radical Anion Complexes in Aqueous Solution

Abstract

Ascorbic acid is oxidized by the OH radical and by radical anion complexes X2⁻ (X: Cl, Br, J, SCN) to yield the “ascorbic acid radical” which finally disappears by second order. The pK of this radical was found to be about 3. It has several precursor radicals formed by the addition of OH or X to ascorbic acid. They lose H2O or HX, respectively, with different rates or are desactivated in radical-radical reactions. The precursor radicals are acids of different pK values. Since ascorbic acid exists in two tautomeric forms in aqueous solution, two precursor radicals are postulated for the reaction of OH with ascorbic acid: They lose water or OH⁻ with different rates to form the ascorbic acid radical. The absorption spectra of these species were measured. All radicals including the ascorbic acid radical have strong absorption at 3600 A, although somewhat different extinction coefficients. The decay of the 3600 A absorption in the 100 μs and millisecond range, which in earlier work of BIELSKI et al. 3 has been attributed solely to the disappearance of the ascorbic acid radical is partly due to the loss of water of long-lived precursors and to radical-radical desactivation of precursors and the ascorbic acid radical. Earlier conclusions of BIELSKI et al. about the pK of the ascorbic acid radical and about its protonization cannot be confirmed taking into consideration the complex precursor reactions. The kinetics is strongly dependent on the dose below about 400 rad. The precursor radicals formed by the addition of X to ascorbic acid are generally shorter lived than the OH addition precursors.