Kinetic analysis of thermal decomposition for penicillin sodium salts: model-fitting and model-free methods

A kinetic study on decomposition processes of some penicillin salts was carried out. Both isothermal and dynamic thermogravimetric curves were used. As expected by their complex structures, several steps with different energies were involved in decomposition processes. Model-fitting and -free kinetic approaches were applied to nonisothermal and isothermal data. The kinetic triplet (f(alpha),A and E(a)) related to model-fitting method that defines a single step reaction resulted to be at variance with the multi-step nature of salts-decomposition. The model-free approach represented by the isothermal and nonisothermal isoconversional methods, gave different dependencies of the activation energy on the extent of conversion. The complex nature of the multi-step process of the studied compounds was more easily revealed using a broader temperature range in nonisothermal isoconversional method. The failure in the model-fitting method did not allow calculating shelf life and half-life times.