Abstract
The stoichiometry of the reduction of vanadium(V) by L-ascorbic acid (H2 A) has been experimentally determined to be H2A + 2V(V) -* A + 2V(IV) + 2H+. The kinetics of this reaction has been studied by stopped-flow procedures at 25° and ionic strength of 1 M (LiC10„) in acid media (0.20-1.0 M HC104). Upon mixing acidic solutions of V(V) and L-ascor- bic acid a transient appears, the uv-vis spectrum of which has a maximum at 425 nm. The observed pseudo-first-order rate constant, &obs(j, which was not acid dependent, is related to the concentration of the reagent in excess, V(V), ac- cording to the equation fcobsb = kK[V(V)]/(l + K[V(V)]). This equation is the familiar one for rapid formation of a complex followed by its rate-limiting decomposition. The experimentally determined constants are the intermediate com- plex formation constant, K = 488 ± 28 M~l, and the decomposition rate constant, k = 12.1 + 0.2 sec-1. On the basis of the measured value of kobsd and the known value for the reaction V(III) + V(V) -> 2V(IV), the two-electron reduction mechanism can be ruled out; an inner-sphere, one-electron reduction of vanadium(V) by L-ascorbic acid seems most plausible mechanism.