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 sulfuric 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) intermediates by oxalate ions and of Mn(IV) by molecular oxalic acid). Numerical values of the parameters for this model were optimized from the fitting of a series of independent kinetic experiments carried out under conditions designed to display the main dynamic characteristics of the evolution of permanganate and intermediates. The predictive power of this model was then tested. It was found to simulate accurately the times of half-disappearance of permanganate and the intermediates as a function of the initial concentration of oxalic and sulfuric acids. The model was also able to account for bistability in a CSTR for oxalic acid concentrations close to the stoichiometric ratio. Bistability was shown to stem from a coupling between the autocatalytic reduction of permanganate by a complex of Mn(II) and a substrate inhibition like effect, namely an attenuation of the inhibition of the reduction of Mn(III) toward the end of the reaction when the initial oxalic acid was almost completely consumed. The width of the hysteresis loop, the range of residence times, and the corresponding initial concentrations were also in quantitative agreement with the prediction of the model.