A KINETIC-STUDY OF THE OXIDATION OF L-ASCORBIC-ACID BY CHROMIUM(VI)

The reaction between chromium(VI) and L-ascorbic acid has been studied by spectrophotometry in the presence of aqueous citrate buffers in the pH range 5.69 - 7.21. The reaction is slowed down by an increase of the ionic strength. At constant ionic strength, manganese(II) ion does not exert any appreciable inhibition effect on the reaction rate. The rate law found is r = K(p)k(r)[Cr(VI)][L-ascorbic acid][H+]/(1 + K(p)[H+]) where K(p) is the equilibrium constant for protonation of chromate ion and k(r) is the rate constant for the redox reaction between the active forms of the oxidant (hydrogenchromate ion) and the reductant (L-hydrogenascorbate ion). The activation parameters associated with rate constant k(r) are E(a) = 20.4 +/- 0.9 kJ mol-1, DELTAH(not-equal) = 17.9 +/- 0.9 kJ mol-1, and DELTAS(not-equal) = - 152 +/- 3 J K-1 mol-1. The reaction thermodynamic magnitudes associated with equilibrium constant K(p) are DELTAH-0 = 16.5 +/- 1.1 kJ mol-1 and DELTAS0 = 167 +/- 4 J K-1 mol-1. A mechanism in accordance with the experimental data is proposed for the reaction.