Molten salts thermodynamic data

Thermodynamic data show that the stabilities of the caesium chloride-metal chloride complexes are greater than the conesponding sodium and potassium compounds, and tire fluorides form complexes more readily tlrair the chlorides, in the solid state. It would seem that tire stabilities of these compounds would transfer into tire liquid state. In fact, it has been possible to account for the heats of formation of molten salt mixtures by the assumption that molten complex salts contain complex as well as simple anions, so tlrat tire heat of formation of the liquid mixtures is tire mole fraction weighted product of the pure components and the complex. For example, in the CsCl-ZrCU system the heat of formation is given on each side of tire complex compound composition, the mole fraction of the compound... 

A series of experiments have been undertaken to evaluate the relevant thermodynamic properties of a number of binary lithium alloy systems. The early work was directed towards determination of their behavior at about 400 °C because of interest in their potential use as components in molten salt batteries operating in that general temperature range. Data for a number of binary lithium alloy systems at about 400 °C are presented in Table 1. These were mostly obtained by the use of an experimental arrangement employing the LiCl-KCl eutectic molten salt as a lithiumconducting electrolyte. 

G. J. Janz, J. Phys. Chem. Ref Data 17, Supplement (1988) Thermodynamic and Transport Properties for Molten Salts Correlation Equations for Critically Evaluated Density, Surface Tension, Eleetrieal Conduetance and Viseosity Data, American Chemical Society-American Institute of Physics-National Bureau of Standards, Washington, DC, 1988. 

The thermodynamic data for Cu2S-FeS (Krivsky and Schuhmann, 1957) show that these sulphides mix to form approximately ideal ionic liquids. These are molten salts in which the heat of mixing is essentially zero, and the entropy of mixing is related to the ionic fractions of the cations and anions. In the present case the ionic fractions yield values for the activities of the two sulphides... 

Reversible cell potentials have been the source of much thermodynamic data on aqueous electrolytes. In recent years, this technique has been extended to nonaqueous solutions and to molten salt systems. Its use for aqueous solutions, relative to other techniques, has decreased. Various ion specific electrodes have been developed in recent years. These are used primarily in analytical chemistry and have not produced much thermodynamic data. 

So from electrical data, it is possible to get information on partial thermodynamic functions of the salt and then develop thermodynamic models for quantitative interpretation of the conductivity variation with composition. These models are not very different from those already developed for molten salt mixtures or metallic alloys. 

In aqueous solution, thorium exists as Th(IV), and no definitive data have been presented for the presence of lower-valent thorium ions in this medium. The standard potential for the Th(IV)/Th(0) couple has not been determined from experimental electrochemical data. The values presented thus far for the standard reduction potential have been calculated from thermodynamic data or estimated from spectroscopic measurements. The standard potential for the four-electron reduction of Th(IV) ions has been estimated as —1.9 V in two separate references 12. The reduction of Th(OH)4 to Th metal was estimated at —2.48 V in the same two publications. Nugent et al. calculated the standard potential for the oxidation ofTh(III) to Th(IV) as +3.7 V versus SHE, while Miles provides a value of +2.4 V [13]. The standard potential measurements from studies in molten-salt media have been the subject of some controversy. The interested reader is encouraged to look at the summary from Martinot [10] and the original references for additional information [14]. 

There are some density data for solid salts above ambient temperature which are given in the form of thermal expansion coefficients. These have been listed when they seemed reliable. Above the melting point, density data are scarce. Most are available for alkali halides but those available for salts are taken from the critically evaluated compilation Janz, G.J., Thermodynamics and transport properties for molten salts, correlation equations for critically evaluated density, surface tension, electrical conductance, and viscosity data,./. Phys. Chem. Reference Data, 17, Suppl. 2, 1988. 

Databases with assessed thermodynamic data for hundreds of substances are available, including alloys, semiconductors, geochemical compounds (silicates and other main-group oxides), aqueous solutions, and molten salts. The bulk of the commercially available databases are on metallurgical systems since the CALPHAD method finds ready applicability in the fields of metals processing and alloy development. 

A large number of investigations have been reported on spectroscopic, thermodynamic, and other equilibrium properties of chalcogen tetrahalides (e.g., 158-162). They include vibrational spectroscopic analyses of SeCU and TeCU in the solid on the basis of the known structures 89, 373) and in the gas phase (37), equilibrium measurements of SeCU and TeCU in molten salts (f12,376,422), determination of enthalpies of formation 335, 339, 433), other equilibrium studies, and determination of thermodynamic data from vapor pressure measurements, mass spectrometric investigations, conductivity experiments, and thermal phase analysis in the solid (37, 39,203,275, 333, 337, 339, 340, 341, 342,379, 402, 403). 

Raman spectra of (M, M )F-A1F3 (M, M = Li, Na, K) molten salts at 1293 K have been obtained by Robert and Gilbert (2000). The intensity ratios between the bands that are characteristic of the different complexes, are strongly affected by the M/M ratio, especially when one of the alkaline metal cations is Li+. Its presence, together with another cation seems to produce an increase in the acidity of the melt. The result of the de-convolution of the spectra compares well with vapor pressure data, showing the same kind of deviation. Quantitative modeling has not been possible because there is a lack of thermodynamic data allowing a comparison with spectroscopic results. 

A preliminary examination of themodynamic data on silicates indicates that correlations developed for molten salts may be useful in understanding and ultimately in predicting magnitudes of the thermodynamic solution properties of silicates. 

Thermodynamic data have also been calculated for carbon—oxygen reactions in fused salts [7, 8], The oxidation of solid carbon principally yields carbon dioxide at low temperature and carbon monoxide at high temperature. In this case, at constant temperature, the CO/CO2 concentration ratio at solid carbon depends on pressure. The carbon—oxygen electrode is used as reference to investigate cryolite—alumina melts at c. 1000°C [9] and molten slags at higher temperatures. 

Recent experimental work has indicated that the existing thermodynamic data on fluoride and chloride salts of zirconium and hafnium in a molten salt environment are unreliable. These data are required for the evaluation of non-aqueous processes for the separation of the two metals. ... 

---