Thermal decomposition studies: Part X. Thermal decomposition kinetics of calcium oxalate monohydrate — correlations with heating rate and samples mass

Abstract

Quantitative correlations between kinetic parameters (energy of activation E, pre-exponential factor A) and procedural factors (heating rate, sample mass) in non-isothermal thermogravimetry have been made for the first time. The effect of heating rate and sample mass on the first two stages of thermal decomposition of calcium oxalate monohydrate in a nitrogen atmosphere ▪ has been evaluated in detail using non-isothermal thermogravimetry. Kinetic parameters are calculated from the TG curves using three integral methods (two “exact” and one “approximate”). The values obtained by the “approximate” method are higher than the corresponding values from the “exact” integral methods. For the decomposition of calcium oxalate to carbonate, the kinetic parameters are not much affected by heating rates or sample masses in the ranges studied. For the dehydration of CaC 2O 4 · H 2O, the parameters show a systematic decrease with increase in either heating rate or sample mass. The values of E and log A obtained by all three equations are best correlated to heating rate as rectangular hyperbole of the type: ▪ The best fits for correlation to sample mass are parabolae of the type E (or log A) = constant × (mass) 2 − constant × mass ÷ constant SYMBOLS USED A = pre-exponential factor α = fraction decomposed φ = heating rate in deg min −1 E = energy of activation n = order parameter R = gas constant T i = temperature of inception of reaction T f = temperature of completion of reaction T s = DTG peak temperature Δ T = T − T s