The Liquid Range of High-Melting Salts

The many molten salts that behave in a similar manner to sodium chloride are discussed in the present chapter. Their liquid range thus extends fi-om the melting point, r i, to the critical temperature, T, but at ambient pressures the boiling point. 

is relevant as the upper limit of this range. The melting points of alkali metal salts, and when known also their boiling points, are shown in Table 3.1, mainly taken from several compilations [1-3], converted from °C to the K-scale. Differences between the values reported in the literature may exceed 10 °C in some cases, although better agreement is seen in most cases. 

The alkali metal salts generally melt without decomposition, and where there are no entries in Table 3.1 there appear to be no available data. On the other hand for the univalent copper, silver and thallium salts the empty spaces in this table denote that the salts decompose on heating or are unstable in the first place (in particular Cu (I) salts). In the case of the divalent metal salts, the oxides and sulfides melt at very high temperatures, but most of the salts with polyatomic anions decompose on heating. Univalent molten salts with monatomic anions boU at ambient pressures without decomposition, hence many of their boUing points could be reported in Table 3.1. This is rarely the case for molten salts with polyatomic anions, exceptions being some of the hydroxides and cyanides. 

The charge-unsymmetrical salts with the univalent halide anions generally melt without decomposition, but some of them (e.g., HgU and AICI3) have low melting points ( 300 °C) because they are not ionic liquids, in the sense that the bonding between the atoms is mainly covalent. Such salts are not dealt with here. The melting points, T, and boUing points at ambient pressures, T, when known, of the ionic salts are shown in Table 3.2, taken mainly from the compilation in [1] with some additional entries from [2] or as annotated, converted from C to the K-scale. 

Temperatures above ca. 1500 °C have not been obtained with accuracies better than 2 °C, and those where 3 is the last digit reported in Table 3.2 (because of the conversion from °C to the K-scale) are generally reported to the nearest 10 °C, those ending in 73 are generally reported to the nearest 100 C. Salts with no entries either are low melting, i.e., not characteristically ionic, or decompose on heating, for instance because the cation is reduced by bromide and iodide ions to lower valence states (e.g., EuBra). 

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