Kinetic analysis of the nonisothermal decomposition of potassium metabisulfite using the model-fitting and isoconversional (model-free) methods

The thermal decomposition kinetics of potassium metabisulfite was studied by thermogravimetry using nonisothermal experiments. The complete kinetic analysis was established by the following procedures: isoconversional methods (model-free) (including the Friedman (FR), Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO) and Vyazovkin (V) methods), master-plot method, the artificial isokinetic relationship and in addition the differential composite method. Firstly, it was established that the Friedman's and Vyazovkin's isoconversional methods are the best two methods for describing the dependence of apparent activation energy (Ea) on the degree of conversion (α) for the investigated decomposition process. Secondly, the appropriate conversion model (f(α)) of the process were selected by means of the “model-fitting” master-plot method. From the system studied, using the composite differential method we obtained the following kinetic triplet: f(α) = 2(1 − α)1/2, Ea = 121.9 kJ mol−1, A = 1.22 × 1012 min−1. Comparing both experimental and calculated thermoanalytical curves at constant heating rate assessed the adequate consistency of the kinetic triplet. It was concluded that the totally unambiguous choice of the reaction model is practically impossible based solely on the existing kinetic data, and because of this fact, the meaningful conclusions concerning the real mechanism of the investigated decomposition process should be based on additional microscopic observations.