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 [CH3CO OH] on the reaction rate was determined at 30 degrees C. Formation of carbon dioxide was also confirmed. The oxidation rate increases with [ oxalic acid] and [CH3COOH] while it decreases with [H2SO4], [HClO4], a nd pH. The rate law governing the oxidation of oxalic acid over a wide range of conditions is rate = k(1) K-esl [oxalic acid](T) [Cr(VI)](T) 1 + K-esl [oxalic acid](T), where only undissociated oxalic acid is ki netically active. Kinetic evidence for the formation of a Cr(VI)-oxali c acid 1 : 1 complex has been obtained and the equilibrium constant fo r their formation has been determined. The 1:1 complex exists most lik ely in an open chain Form. The rate-limiting step of the oxidation rea ction involves the breaking of the C-C bond in the 1:2 complex. Oxidiz ing ability of Cr(VI) species have been discussed, Mechanism with the associated reaction kinetics is assigned. (C) 1998 John Wiley & Sons, Inc.