Journal of Thermal Analysis, Vol. 36 (1990) 733-742 KINETIC INVESTIGATION OF SOME STEROIDS BY THERMOGRAVIMETRY H. Giiler and D. Saraydzn DEPARTMENT OF CHEMISTRY, CUMHURIYET UNIVERSITY,58140 SIVAS,TURKEY (ReceivedApril 20, 1989) The kinetic values of thermal degradation of some steroids were calculated by using TG and DTG curves and the Freeman-Carroll and the Jeres methods. The n and Ea values calcu- lated by the Jeres method are more reasonable. The kinetic thermal stabilities of the simple functional groups of the steroids were compared by using the Ea values of Jeres, and the fol- lowing sequences were found: 17r-OH > 17.octv I > 17-Ac-CHO > 17-keto; 3r-OH > 3-keto > 3a-OH; and 5a-H > 5 fl-H > A 5(6) > A ~. The k, Z, A/f*, AS* and AG* values were calculated at the maximum decomposition rate temperatures by using the Jeres values. The AS * values are negative and suggest a high ordering of the transition state. The AH* and AG* values are positive, as expected. The thermal degradation of steroids is a nonisothermal process involving mass loss: As "~ Bs + Cg (1) where As = original solid reactant; Bs = solid residue, which may or may not be present, and Cg = gaseous product evolved. Thus, the activation energy of the activated complex and the order of the degradation reaction can be found by the Freeman-CarroU [1] and the Jeres [2] methods. The Freeman-Carroll method is the most widely used method for the kinetic analysis of nonisothermal decomposition processes despite the great number of other methods proposed. The advantages of this method are that considerably fewer experimental thermogravimetric data are required than in the isothermal method, and that the kinetics can be obtained in a con- tinuous manner over an entire temperature range without any missing regions [3]. In general, the Freeman-Carroll method postulates a kinetic equation of the type John Wiley& Sons, Limited, Chichester Akaddmiai IOad6, Budapest