The paper discusses three fast methods for determination of the reaction order, as follows: the single-point method proposed by Kissinger, Horowitz and Metzger, an original two-point method, and the three-point method suggested by a Gorbachev's paper. These methods cannot elucidate the reaction mechanism, but they can help in the rapid derivation of the apparent kinetic parametersn andE.
The complexation of uranyl ion with acetate ions was investigated in 20% ethanolic solution by using cyclic voltammetry. The
uranium formed 1:1 and 1:2 complexes with acetate ions. The values of log β1 and log β2 for uranyl acetate complexes were 2.05 ± 0.08 and 5.25 ± 0.06 respectively. The diffusion coefficient and heterogeneous rate
constants for the reduction of uranyl ion at hanging mercury drop electrode in 20% ethanolic solution of acetate ions were
0.43 × 10−5 cm2 s−1 and 2.26 × 10−3 cm s−1, respectively. Thermodynamic parameters were also evaluated by finding the effect of temperature on the heterogeneous rate
constants. The values of ΔH*, ΔS* and
The equation for calculation of the activation energy of the diffusion of the evolved products through the matrix (E) from a single TG curve were proposed by solving Fick's laws. The solution is based on the similarly theory by utilizing
a Fourier number.
The proposed method was examined by using mass loss data for the dehydroxylation of some micas with and without FeO (muscovite
and its varieties and lepidolite) as determined from their TG curves. TheE values for the first stage of the dehydroxylation of these micas areE1,=85±10 kJ mol−1; for the final stageE2=380±40 kJ mol−1 and for the mass loss connected with fluorineEF=85±10 kJ mol−1.
A differential method is proposed which uses local heating rates to evaluate non-isothermal kinetic parameters. The method
allows to study the influence of the deviation of the true heating rate with respect to the programmed one on the values of
the kinetic parameters. For application, the kinetic parameters of the following solid-gas decomposition reaction were evaluated:
[Ni(NH3)6]Br2(s)→[Ni(NH3)2]Br2(s)+4NH3(g). The results obtained revealed significant differences between the values of the non-isothermal kinetic parameters obtained
by using local heating rates and those obtained by using the programmed heating rate. It was also demonstrated that the kinetic
equation which makes use of the local heating rates permits a better description of the experimental (α, t) data than the
kinetic equation which uses the programmed constant heating rate.
An improved version of the Coats-Redfern method of evaluating non-isothermal kinetic parameters is presented. The Coats-Redfern approximation of the temperature integral is replaced by a third-degree rational approximation, which is much more accurate. The kinetic parameters are evaluated iteratively by linear regression and, besides the correlation coefficient, the F test is suggested as a supplementary statistical criterion for selecting the most probable mechanism function. For applications, both non-isothermal data obtained by theoretical simulation and experimental data taken from the literature for the non-isothermal dehydration of Mg(OH)2 have been processed.
Authors:T. Wanjun, L. Yuwen, Z. Hen, W. Zhiyong, and W. Cunxin
A new approximate formula for temperature integral is proposed. The linear dependence of the new fomula on x has been established. Combining this linear dependence and integration-by-parts, new equation for the evaluation of kinetic
parameters has been obtained from the above dependence. The validity of this equation has been tested with data from numerical
calculating. And its deviation from the values calculated by Simpson's numerical integrating was discussed. Compared with
several published approximate formulae, this new one is much superior to all other approximations and is the most suitable
solution for the evaluation of kinetic parameters from TG experiments.
The authors present data concerning the evaluation of kinetic parameters of the decomposition of a Mannich compound by using
the classical method of constant heating rate thermal analysis and the new one of controlled rate thermal analysis (CRTA).
The data processed using the CRTA method allow to obtain more reliable kinetic parameters according to the proposed reaction