Thermal behaviour of Belousov-Zhabotinskii reactions followed by heat exchange calorimetry involving simultaneous estimation of potentiometric oscillations

Abstract

The chemical oscillations of the Belousov-Zhabotinskii reactions which occur in a solution composed of KBrO 3, H 2SO 4, CH 2(COOH) 2 and Ce(NH 4) 4(SO 4) 4 · 2H 2O, have been followed simultaneously by heat exchange calorimetry of a batch type and by potentiometry in order to study the relation between the thermal behaviour and the reaction mechanism. In the assembled calorimeter, the sample and reference vessels were fixed differentially in a water bath. The heat evolved in the sample vessel was exchanged freely with the ambient water whose temperature was controlled precisely. Because this reaction does not change the effective heat capacity, it was a very suitable subject for the calorimetric study. A normal potentiometric cell was also assembled in the bath to estimate the potential difference of the same reaction. The rate of heat evolved q, the total heat effect Q and the potential difference of the bromide ions, E, were measured against time t. After the start of the reaction, large Q values were measured in the induction period, in contrast to only a small change in E which increased linearly with initial concentrations of H 2SO 4 and KBrO 3. In the oscillation period, the amount of Q evolved in each cycle decreased exponentially with t. The measured q value depended on the initial concentrations of the components, and was compared with the values of E estimated simultaneously. The threshold value of [H 2SO 4] init obtained from the relation between q and E was compared with that calculated from the rate constants obtained from the literature; fairly good agreement was obtained.