A kinetic study of the thermal decomposition process of potassium metabisulfite: Estimation of distributed reactivity model

The thermal decomposition kinetics of potassium metabisulfite was studied by thermogravimetric (TG) and differential thermogravimetric (DTG) techniques using non-isothermal experiments. The apparent activation energy (E-a) is determined using the differential (Friedman) isoconversional method. The results of the Friedman's isoconversional analysis of the TG data suggests that the investigated decomposition process follows a single-step reaction and the observed apparent activation energy was determined as 122.4 +/- 2.1 kJ mol(-1). A kinetic rate equation was derived for the decomposition process of potassium metabisulfite with contracting area model, f(alpha) = 2(1-alpha)(1/2), which is established using the Malek's kinetic procedure. The value of pre-exponential factor (A) is also evaluated and was found to be A = 1.37 x 10(12) min(-1). By applying the Miura's procedure the distributed reactivity model (DRM) for investigated decomposition process was established. From the dependence alpha versus E-a, the experimental distribution curve of apparent activation energies, f(E-a), was estimated. By applying the non-linear least-squares analysis, it was found that the Gaussian distribution model (with distribution parameters E-0 = 121.3 kJ mol(-1) and sigma = 1.5 kJ mol(-1)) represents the best reactivity model for describing the investigated process. Using the Miura's method, the A values were estimated at five different heating rates and the average A values are plotted against E-a. The linear relationship between the A and E-a values was established (compensation effect). Also, it was concluded that the E-a values calculated by the Friedman's method and estimated distribution curve, f(E-a), are correct even in the case when the investigated decomposition process occurs through the single-step reaction mechanism. (C) 2008 Elsevier Ltd. All rights reserved.