NaCl-water eutectic system

Fig. 2. The system sodium chloride—water, showing the eutectic temperature, —21.12°C, and composition, 23.31 wt % NaCl.

A somewhat surprising result was that 1460 bars of pressure had little effect on the eutectic temperature (238.65 K vs. 237.45 K) (Fig. 5.22). A pressure of 1460 bars, per se, would decrease the freezing point of pure water by about —14.8K (Fig. 3.3). Dropping the temperature at which ice first formed by 12K had only a minor effect on the eutectic (AT = 1.2K) (Fig. 5.22). This is not, however, always the case. For example, for the simpler NaCl-H20 system, the calculated eutectic temperature at lbar is —21.3°C at 1460bars of pressure, the calculated eutectic temperature is —31.3°C (AT = 10.OK). As we point out repeatedly, chemical systems and their response to temperature and pressure depend, ultimately, on thermal and volumetric properties of individual constituents, which makes every system response highly individualistic. 

Many binary systems, both ideal and nonideal, have phase diagrams of the simple eutectic type. The phase diagram, water-salt, is the simple eutectic type if the salt does not form a stable hydrate. The diagram for H20-NaCl is shown in Fig. 15.10. The curve ae is the freezing-point curve for water, while efis the solubility curve, or the freezing-point curve, for sodium chloride. 

For the preparation of the LiCl-KCl eutectic, reasonable purity anhydrous LiCl is readily available, although in the United States, the fully purified eutectic can be obtained. Otherwise, treatment with dry HCl followed by dry CI2 prior to preelectrolysis at 2.7 V, and, finally, filtration, is recommended. Similar procedures, with suitable modifications to the gas purges and electrolysis voltages, may be adopted for MgCl2-KCl, NaCl-KCl, CaC -KCl, ZnC -KCl, and for the bromides and iodides. Advantageously, vitreous carbon crucibles can be used for halide melts they are reusable and can form the counterelectrode. Purity of these systems may again be assessed voltammetri-cally. Surprisingly, water is reversibly soluble at low partial pressures, but hydrolysis occurs at a 18 mm ... 

As with azeotropes, eutectics maybe ternary, quaternary, and so on, but their phase diagrams get very complex very quickly. A few important eutectics have an impact on ordinary life. Ordinary solder is a eutectic of tin and lead (63% and 37%, respectively) that melts at 183 C, whereas the melting points of tin and lead are 232 C and 327 C. Wood s metal is an alloy of bismuth, lead, tin, and cadmium (50 25 12.5 12.5) that melts at 70 C (lower than the boiling point of water ) that can be used in overhead fire sprinkler systems. NaCl and H2O make a eutectic that melts at — 21 C, which should be of some interest to communities that use salt on icy roads in the winter. (The composition of this eutectic is about 23 weight percent NaCl.) An unusual eutectic exists for cesium and potassium. In a 77 23 ratio, this eutectic melts at —48 C This eutectic would be a liquid metal at most terrestrial temperatures (and be very reactive toward water). 

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