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.