KINETICS OF METAL ION AND METAL CHELATE CATALYZED OXIDATION OF ASCORBIC ACID .4. URANYL ION CATALYZED OXIDATION

The kinetics of the uranyl ion catalyzed oxidation of ascorbic acid at 25 and 40° is reported. The dissociation constants of ascorbic acid at 40°, and at an ionic strength of 0.10 M (KNO3), have been obtained from potentiometric titration data and are 2.69 × 10−4 and 1.05 × 10-12. The dissociation constants of ascorbic acid in D2O at 25° and at an ionic strength of 0.10 M (KNO3) are 2.7 × 10−5 and 6.00 × 10−13. The uranyl ion catalyzed oxidation of ascorbic acid catalyzed by uranyl ion proceeds by a path first order with respect to substrate, catalyst, molecular oxygen, and hydrogen or deuterium ion concentrations, in accordance with the rate law, K[H2A][UO22+] -[H+][O2]. The uranyl ion is less active as a catalyst than is the vanadyl ion. The deuterium isotope effect, kH/kD, for the uranyl ion catalyzed reaction was found to have the low value of 1.24. Activation parameters for the uranyl ion catalyzed oxidation are reported and compared with those of vanadyl ion catalyzed and uncatalyzed oxidations of ascorbic acid. A probable mechanism is proposed for the uranyl ion catalyzed reaction. © 1969, American Chemical Society. All rights reserved.