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It is proved that under the conditions of dynamic thermoanalytical examinations the course of a decomposition reaction is determined primarily by the gas-and heattransfer partial processes, i.e. indirectly by the experimental conditions. This raises the question of whether it is justified to make kinetic calculations based on the shapes of similar curves. A new measuring technique is reported which uses a slightly modified Derivatograph. With this new method decomposition reactions can be examined under "quasi-isothermal" and "quasi-isobaric" conditions. The pernicious effects of the gasand heat-transfer processes are thus eliminated, and the shapes of the curves obtained provide the possibility of studying the decomposition reactions in a new and more thorough way.It seems evident that the results of thermoanalytical examinations can be used to draw conclusions on the kinetics and possibly the mechanism of reactions leading to equilibrium. Accordingly, many approximate calculation methods have been elaborated for the determination of reaction order and activation energy. Despite the perfect logic upon which these calculations were generally based, they did not always fulfil expectations. This is due in part to the fact that the thermal curves used for the calculations very often gave a distorted picture of the transformation processes.Thermal decomposition reactions leading to equilibrium consist of a number of chemical and physical partial processes. These partial processes are usually interdependent on each other in a complex way. Table 1 gives a schematic picture of the correlations. The course of the reaction is determined by the rate of the slowest partial process. If this is commensurable with the heating rate applied, then the transformation with increasing temperature will take place with delay over a more or less broad temperature interval.Of the many partial processes, only two may be regarded as really chemical. One takes place at the interface as demonstrated in Fig. 1. This process is the release of gas molecules from the lattice and their rebuilding into it. The other is the chemisorption and desorption of gas molecules at the interface. Since both of these processes leading to equilibrium take place extremely rapidly, they do not usually influence the course of the transformation.Nucleus formation and nucleus growth may be regarded as a transition between chemical and physical processes. Simultaneously with the departure of the gas L Thermal Anal. 5, 1973

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In the course of thermal decomposition of the magnesium sulphate hydrates MgSO4.nH20 (n = 7; 6; 5; 4; 3; 2; 5/4; 1) the intermediate steps MgSO4.3H20, MgSO4.2H20, MgSO4.H20 and fl -MgSO4 are observed under quasi-isothermal and quasi-isobaric conditions at p ~ 0.1 MPa dependent on the water contents. The structure of the obtained monohydrate phase is identical with that of kieserite. Thermal decomposition of the magnesium sulphate hydrates is essentially influenced by the water vapour partial pressure.The system MgSO4.H20 is characterized by the occurrence of a great number of hydrates; however, above 25 ~ only MgSO4.7H20 (epsomite), MgSO4.6H20 (hexahydrite) and MgSO4.H20 (kieserite) are contained in the form of stable solid phases in aqueous solution [