Investigation of exchange reactions between alkaline earth metal oxides and some transition metal halides

Abstract

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.

Investigation of reactions in solids by DTA

“…DTA and simultaneous recording of electrical conductivity changes makes possible the establishment of the most essential stages in the solid-phase reactions and the role of some elementary stages involved in these complex heterogeneous processes. The use has revealed of DTA that in numerous reactions taking place in powdered mixtures of inorganic substances a decisive role is played by the formation of liquid or gaseous phases in the course of the process [1][2][3][4].…”