KINETICS AND MECHANISM OF OXIDATION OF FORMIC-ACID BY CHROMIUM(VI) - INHIBITION BY MANGANESE(II) ION

Experiments have been done to clarify the mechanism of the chromium(VI)-formic acid reaction in aqueous perchloric acid media. both in the absence and in the presence of manganese(II) ion. Two mechanisms are proposed, one for slightly acid solutions (I) and the other for strongly acid ones (II). Both mechanisms are first order in both oxidant and reductant, but they differ in the order to hydrogen ion (1 for mechanism I and 3 for mechanism II). The activation energy for mechanism I (60.4 +/- 0.9 kJ mol-1) is considerably higher than that for mechanism II (41.2 +/- 0.4 kl mol-1). The rate-controlling steps proposed for mechanisms I and II are the H2CrO4-HCOOH and HCrO3+-HCOOH2+ reactions, respectively. Mechanism I is inhibited by manganese(II) ion, but mechanism II is not, which suggests that mechanism I involves Cr(IV) as an intermediate, whereas mechanism II does not.