The thermal decomposition of potassium bro- mate (KBrO3) has been studied as a function of particle size, in the range 53-150 lm, by isothermal thermogravi- metry at different temperatures, viz. 668, 673, 678, and 683 K in static air atmosphere. The theoretical and experimental mass loss data are in good agreement for the thermal decomposition of all samples of KBrO3 at all temperatures studied. The isothermal decomposition of all samples of KBrO3 was subjected to both model fitting and model-free (isoconversional) kinetic methods of analysis. Isothermal model fitting analysis shows that the thermal decomposition kinetics of all the samples of KBrO3 studied can be best described by the contracting square equation. Contrary to the expected increase in rate followed by a decrease with decrease in particle size, KBrO3 shows a regular increase in rate with reduction in particle size, which, we suggest, is an impact of melting of this solid during decomposition.

The effect of particle size on the thermal decomposition kinetics of potassium bromate

Structural defects were introduced into the potassium bromate (PB) lattice in the form of SO2− 4 and Cl− ions in the process of crystal growth. It was assumed that these doped crystals PB(Cl−) and PB(SO2− 4) are composed of a two phase system, one being the perfect PB lattice and the other distorted regions due to induced defects. Isothermal decomposition of doped and normal PB samples was carried out gasometrically between the temperature range 653–663 K. The α-t plots reveal that the process occurs through initial gas evolution, acceleratory and decay stages. It also confirmed that doping enhances the rate of the reaction, the effect being more pronounced in the case of PB(SO2− 4). The data are found to be well fitted to the Prout-Tompkins and Avrami-Erofe'ev mechanisms.

Effect of Anion Doping on the Thermal Decomposition of Potassium Bromate

Influence of trivalent ion dopants on the thermal decomposition kinetics of potassium bromate

The thermal decomposition kinetics of sodium oxalate (Na2C2O4) has been studied as a function of concentration of dopant, phosphate, at five different temperatures in the range 783–803 K under isothermal conditions by thermogravimetry (TG). The TG data were subjected to both model-fitting and model-free kinetic methods of analysis. The model-fitting analysis of the TG data of all the samples shows that no single kinetic model describes the whole α versus t curve with a single rate constant throughout the decomposition reaction. Separate kinetic analysis shows that Prout–Tompkins model best describes the acceleratory stage of the decomposition, while the decay region is best fitted with the contracting cylinder model. Activation energy values were evaluated by both model-fitting and model-free kinetic methods. The observed results favour a diffusion-controlled mechanism for the thermal decomposition of sodium oxalate.

Kinetic studies on the thermal decomposition of phosphate-doped sodium oxalate

Pure and doped samples of potassium bromate (KBrO3) were subjected to precompression and their thermal decomposition kinetics was studied by thermogravimetry at 668 K. The samples decomposed in two stages governed by the same rate law (contracting square equation), but with different rate constants, k
 1 (for a α ≤ 0.45) and k
 2 (for α ≥ 0.45), as in the case of uncompressed samples. The rate constants k
 1 and k
 2 decreased dramatically on precompression, the decrease being higher for doped samples. Cation dopants (Ba2+, Al3+) caused more desensitization effect than the anion dopants (
 $$ {\text{SO}}_{4}{}^{ 2- } $$
 , PO4
 3−) of the same magnitude of charge and concentration. The results favor ionic diffusion mechanism proposed earlier on the basis of doping studies.

Effect of precompression on isothermal decomposition kinetics of pure and doped potassium bromate

The effects of admixtures of potassium bromide (2.5% and 5%) on the thermal decomposition of potassium bromate were studied within the temperature range 653–683 K. The fraction decomposed(α) vs. time (t) relations revealed (i) initial gas evolution, (ii) acceleratory and (iii) decay steps. The data were analysed on the basis of the first-order law with two rate constantsk1 andk2,k1 being the rate constant for the initial, slow first-order process (0.02, 0.26), andk2 being the rate constant for the subsequent faster process, holding within the range 0.21 to 0.98. Upon increase of the concentration of added potassium bromide to 5%, the range for the slow and faster processes became 0.01 to 0.16 and 0.1 to 0.98, respectively. At a given temperature,k1 increased with increasing bromide concentration, whereask2 almost identical for pure potassium bromate and its mixtures (2.5% and 5% bromide).

D. Patnaik

Papers

Effects of admixtures of potassium bromide on the thermal decomposition of potassium bromate

Effect of Anion Doping on the Thermal Decomposition of Potassium Bromate