Kinetics Research On The Oxidation Of Antioxidant L-Ascorbic Acid In Aqueous Solution

An evaluation method for the capacity of antioxidants to protect drug against oxidation was presented. As a new viewpoint, to determine the priority of the competitive oxidations between the antioxidant and the protected drug, and to compare the drug-protect capacity of antioxidants, it is important to determine their oxidation rate constants using chemical kinetics instead of standard oxidation (or reduction) potentials. L-ascorbic acid was used as a model for the determination of oxidation reaction rate constants in aqueous solutions. In the experiments, sufficient air or oxygen were continually inflated into the solution respectively to keep the concentration of dissolved oxygen constant. The residual concentrations of L- ascorbic acid were determined by iodimetry, and the concentration of dissolved oxygen by oxygen electrode. The data were fitted by linear regressions to obtain the reaction rate constants.When inflating air into reaction solutions, the degradation of antioxidant L-ascorbic acid obeys the pseudo-first-order kinetics at pH 6.8 and pH 9.2, and the average rate constants are (9.21 ± 0.80)×10^-3 h^-1 and (9.89 ± 0.68 ) ×10² h^-1 respectively; the degradation of L-ascorbic acid obeys 0.5-order kinetics at pH 4.0, and the average rate constant is (8.77 ± 0.23)×10¹⁵ mol^1/2·L^-1/2·h^-1.