Horowitz and Metzger method

For the example in which the rate constant involves concentrations expressible as mole fractions and the total number of moles is constant, the expression (88) 

For a reaction in which the order is unknown, Coats and Redfern (85) derived the following expression  

The kinetics of volatilization of polymers have been discussed in detail by Doyle (71, 84). If w is the apparent residual mass fraction calculated on the initial mass, the apparent mass fraction volatilized, v. is 

The apparent volatilization rate is found by multiplying the TG curve slope, —dwjdX by the constant heating rate, B  

For a particular volatilization step, however, the appropriate residua) mass fraction is the true one, h, calculated on the total fraction volatilized during the step, rather than on the total initial mass 

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Sestak (43) compared the kinetic results calculated by five different methods for a system corresponding to the dehydration of -CaS04 0.5H2O. The five methods evaluated mathematically were (1) Freeman and Carroll (83) (2) Doyle (84) (3) Coats and Redfern (85) (4) Horowitz and Metzger (88) and (5) Van Krevelen et al. (87). From these calculations it was found that the deviations of computed values oF E did not differ by more than 10%. Thus, all the methods appear to be satisfactory for the calculation of E within the limits of accuracy required. The errors of each method due to the inaccuracy of visual deduction of values from the TG curves were also calculated. These errors, % and e (errors in calculation of E or n, respectively), were as follows (1) Freeman and Carroll method, eE = 4% and e = 12% (2) Horowitz and Metzger method, ee = 2% (when the correct value of n is assumed) (3) Doyle method. eE = 4%. However, the magnitude of this error depends primarily on the position of the point on the TG curve on which the calculations are being performed. In the case of differential methods, me most accurate data are calculated from the medium-steep parts of the curve. For the approximation method, the accuracy depends on the determination of the curve inflection point temperature. 

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