The application of simultaneous recording of electrical conductivity and DTA in the study of binary and ternary salt systems

Abstract

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-

Thermal analysis

“…Apparatus for measurement of resistivity up to 1000°in vacuum has been constructed (144). Simultaneous electrical conductivity and DTA measurements were made on a number of sulfate systems (35) showing the electrical measurement to be effective for determination of melting and for some solid state transformations. In the Ag2SÜ4-LÍ2SO4-CS2SO4 system, successive solid state transformations were followed by steps in the conductivity curve.…”