Chemical Education Today
Letters The Purification of Water by Zone Melting: A Phase Diagram Interpretation Oughton, Xu, and Battino published a paper in this Journal on the application of zone melting to the purification of water from NaCl aqueous solutions (1). They reported conductivity results showing the dependence of the solution concentration on the percentage of ice. I would like to comment on this interesting paper by giving an interpretation of some of their results taking into account the binary phase diagram.
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B' L
C
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A
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The authors noticed that as the concentration of the salt increases, the so-called melting point of the solution decreases. This could be shown on the schematic diagram (Figure 1). Let us consider two starting solutions A and A´, having different initial salt concentrations. Suppose the final mixtures have 50% liquid and 50% ice, on cooling, the representative points of the systems run along ABC and A´B´C´ (with SC = CL and S´C´ = C´L´). According to the diagram, the temperature T´corresponding to the line S´C´L´ should be lower than the temperature T corresponding to the line SCL. Consequently, if the systems, which have nearly the same mass, were placed in the same conditions of cooling (both in the center or around the perimeter of the freezer), it “takes longer for solutions of higher concentration to attain the same percentage frozen”. Table 1 in the paper indicates that for solutions having the same initial salt concentration, the higher the percentage frozen, the richer the solution. This is also shown on the phase diagram: on cooling solution A, the representative point of the system runs along ABCD. If C corresponds to 50% ice and D to 75%, the solution in equilibrium with ice in D is richer in salt than that in C. Literature Cited 1. Oughton, J.; Xu, S.; Battino, R. J. Chem. Educ. 2001, 78, 1373.
D
Mohamed Jemal
Fraction NaCl Figure 1. A schematic H2O/NaCl phase diagram.
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Journal of Chemical Education
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Faculty of Science Chemistry Department 1060 Tunis, Tunisia
[email protected] Vol. 81 No. 7 July 2004
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www.JCE.DivCHED.org
