Kinetics and mechanism of chromic acid oxidation of oxalic acid in absence and presence of different acid media. A kinetic study

Kinetics and mechanism of the reaction of Cr(VI) with oxalic acid have been studied in presence and absence of H₂SO₄, HClO₄, and CH₃COOH by monitoring the formation of Cr(III)-oxalic acid complex at 560 nm. The effect of total oxalic acid], Cr(VI)], H₂SO₄], HClO₄], and CH₃COOH] on the reaction rate was determined at 30°C. Formation of carbon dioxide was also confirmed. The oxidation rate increases with oxalic acid] and CH₃COOH] while it decreases with H₂SO₄], HClO₄], and pH. The rate law governing the oxidation of oxalic acid over a wide range of conditions is rate=k₁ K_es1 oxalic acid]_T Cr(VI)]_T 1+K_es1 oxalic acid]_T, where only undissociated oxalic acid is kinetically active. Kinetic evidence for the formation of a Cr(VI)(SINGLEBOND)oxalic acid 1:1 complex has been obtained and the equilibrium constant for their formation has been determined. The 1:1 complex exists most likely in an open chain form. The rate-limiting step of the oxidation reaction involves the breaking of the C(SINGLEBOND)C bond in the 1:2 complex. Oxidizing ability of Cr(VI) species have been discussed. Mechanism with the associated reaction kinetics is assigned. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 335–340, 1998