KINETIC MODELING OF THE KMNO4 H2C2O4/H2SO4 REACTION - ORIGIN OF THE BISTABILITY IN A CSTR/

The mechanism of the reduction of permanganate by oxalic acid in sulfu ric acid medium was completely described by a model incorporating the specific reactivities of permanganate and of various Mn(III) and Mn(IV ) reaction intermediates. It involved 14 steps including 8 equilibria (dissociation of the two diacids and complexation of Mn(III) intermedi ates by oxalate ions and of Mn(IV) by molecular oxalic acid). Numerica l values of the parameters for this model were optimized from the fitt ing of a series of independent kinetic experiments carried out under c onditions designed to display the main dynamic characteristics of the evolution of permanganate and intermediates. The predictive power of t his model was then tested. It was found to simulate accurately the tim es of half-disappearance of permanganate and the intermediates as a fu nction of the initial concentration of oxalic and sulfuric acids. The model was also able to account for bistability in a CSTR for oxalic ac id concentrations close to the stoichiometric ratio. Bistability was s hown to stem from a coupling between the autocatalytic reduction of pe rmanganate by a complex of Mn(II) and a substrate inhibition like effe ct, namely an attenuation of the inhibition of the reduction of Mn(III ) toward the end of the reaction when the initial oxalic acid was almo st completely consumed. The width of the hysteresis loop, the range of residence times, and the corresponding initial concentrations were al so in quantitative agreement with the prediction of the model.