The application of DTA with the simultaneous recording of electrical conductivity variation in the study of the thermal behaviour of a-oxyalkylphosphonates and their analogues is considered. The investigation of electrical conductivity variation during a reaction helps clarify certain problems of the phosphonate-phosphate rearrangement and the decomposition of a-oxyalkylphosphonates to their components.Depending on the substituents near the c~-carbon atom and on the electronic environment of the phosphorus atom, in the presence of an alkaline catalyst c~-oxyalkylphosphonates undergo either phosphonate-phosphate rearrangement or decomposition to the initial components on being heated [1].

DTA and the simultaneous recording of electrical conductivity were applied for the investigation of exchange reactions between alkaline earth metal oxides and some lead, copper and nickel halides. Although the possibility of the gaseous phase is not excluded for the majority of the reactions investigated, it is either the appearance of a liquid phase or the polymorphous transformation of CaO that has the decisive effect on the interaction mechanism.Reactions of the alkaline earth metal oxides have been repeatedly studied, but at present there is no common view as to their mechanisms. This phenomenon of the constancy of the temperature at which one reagent begins to interact with others is referred to as the Hedvall effect. Since the alkaline earth metal oxides are strongly hygroscopic, this constancy of the temperature at which interaction starts led some investigators to suggest that Hedvall and Tamman had used hydroxide-contaminated oxides whose melting points were rather low, and the interaction had been brought about by the presence of a liquid phase [4,5]. A number of attempts were made [6][7][8] to reproduce the Hedvall effect. Some of these ended in failure, as was the case with calcium and magnesium oxides for instance [8]. In the case of barium and strontium oxides the Hedvall effect was reproduced only when the experiment was carried out in air. However, none of the above works made the point of demonstrating the appearance of a liquid phase at the moment of interaction.

The advantages of the simultaneous recording of electrical conductivity curve and DTA curves in the investigation of salt mixtures is demonstrated on a number of examples. With sufficiently slow recording, at a heating rate of 5--10~ all the changes occurring in the mixture investigated, and connected with the differing electrical conductivities of the different phases which appear and disappear in the process of heating or cooling, are well reflected on the electrical conductivity curve.The application of DTA with the simultaneous recording of electrical conductivity variations was considered in the investigation of the phase diagrams of binary and ternary anhydrous salt systems. The electrical conductivity variation curve helps in the understanding of the nature of the phase transformations occurring during the heating and cooling of the substances investigated.The electrical conductivity curves of certain substances have singularities by which these substances can be distinguished from similar compounds.Thus, in the case of lithium, caesium and silver sulphates a large jump in the electrical conductivity (sharp rise of the electrical conductivity curve) is observed at the temperatures of the polymorphous transformations of these salts [1][2][3], in contrast to all other sulphates, where no such phenomenon is observed.In the case of double salts, the electrical conductivity curves exhibit a great sensitivity to the presence of the least traces of the melt phase. Thus the recorded effects corresponding to the melting of the eutectic mixture or the solid solution, or compound formation can be distinguished from those of other phase transformations occurring in the solid state. Results and discussionLet us consider the curves of solidified sodium and silver sulphate melt mixtures. The phase diagram of a given system is characterized by the formation of a continuous series of solid solutions without a minimum for high-temperature a-modifications [4,5]. The low-temperature fl-modifications also form a continuous series of solid solutions with a minimum AgzSO4 content of 40 mole ~ at 170 ~ High-temperature a-solid solutions have high electrical conductivity (partic-

Reactions in mixtures of solid alkali metal sulphates with barium chloride were investigated by DTA. The decisive role of an intermediate liquid was demonstrated.It was established that the application of DTA with simultaneous recording of electrical conductivity in the investigation of the mechanism of interaction of mechanical non-equilibrium mixtures of anhydrous salts leads to data on the transitory existence of a labile eutectic, which to a certain extent affects the course of the reactions of the mixtures of solids.

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