KINETICS AND MECHANISM OF THE OXIDATION OF FORMALDEHYDE BY CHROMIUM (VI)

The kinetics of the reaction between chromium(VI) and formaldehyde in aqueous perchloric acid solution has been studied spectrophotometrically. In the acidity region studied ([HClO4] = 0.073-0.731 mol l-1), the rate law.followed by the reaction is r = {(a[H+] + b[H+]2)/(1 + c[H+])] [Cr(VI)] [HCHO]. Addition of sodium perchlorate to the solution results in a decrease of the reaction rate at low ionic strengths and in an increase at high ionic strengths. The apparent activation energy decreases as the acidity of the medium increases within the range E(a) = 40.3-35.3 kJ mol-1. The experimental results are consistent with a mechanism involving both HCrO4- and H2CrO4 as active forms of the oxidant, and HCHOH+ as the active form of the reductant.