Kinetic Study of Indomethacin under Isothermal Conditions

Thermal analysis is a routine method in the solution of pharmaceutical problems such as the control of raw materials, the determination of purity, the qualitative and quantitative analysis of drug formulation, tests of thermal stability and compatibility, the determination of kinetic parameters etc. Kinetic study has become a crucial point in thermal analysis, having as main purpose to determine the mechanism(s) of decomposition and the calculation of the parameters of the Arrhenius equation. The results are further used to predict the system behaviour in various circumstances. The present work reports a kinetic study regarding the indomethacin thermal decomposition under isothermal conditions and in a nitrogen atmosphere, for the temperature steps: 260; 265; 270; 275 and 280 degrees C. The TG/DTG data were processed by three methods: isothermal model-fitting, isothermal-isoconversional and Friedman isothermal-isoconversional. The model-fitting defines a single reaction step, while the model-free approach represented by isothermal-isoconversional methods gave dependences of the activation energy, of the extent of conversion. The obtained results are in good accord with the similar data which resulted in non-isothermal conditions from a previous work. The careful treatment of the kinetic parameters obtained in different thermal conditions was confirmed to be necessary, as well as a different strategy of experimental data processing.