species in solution is CHgCIGaCIdis confirmed by Figure 1. The lines in Figure 2 were obtained from the data given by Brown and Wallace (7). This figure shows agreement with the conclusion made by Brown and Wallace that these data indicate monomeric aluminum iodide. However, the belief expressed by Brown and Wallace that these data favor the existence of a 1:1 complex between solute and solvent is not confirmed by Figure 2. The lines for the benzenealuminum bromide system were obtained from vapor pressures at 17.7' C (8). The unsolvated dimer AlaBre, with perhaps slight dissociation, is indicated. Another interpretation of equation (1) or (2) is obtained by changing its form to 8-kI-I
(5)
and treating 9 and I as variables with k and I constant In this form all of the data for a given system lie on the same 0 versus I curve. When the solute is a single species obeying Raoult's law this curve is a straight line with a slope equal to k and a 9-intercept equal to -1. Plots of this type for the examples considered in
F i e 3.
Lines for the AiBra-CeH6 system from vapor pressures at 17.7-C.
the previous paragraph are shown in Figure 4. One disadvantage of this method as compared to the use of the family of lines is revealed by considering the case of the methyl iodide-aluminum iodide system. From Figure 4 one would probably conclude, in agreement with Brown, et al. that there is evidence for a 1:l complex. However, it is clear from Figure 2 that such a conclusion is not justified. Thus, although there is little to choose between the two methods with respect to ease of application, it would appear that the use of the family of lines is to be preferred, because they are less likely to lead one to unwarranted conclusions.
A third variation of the treatment is to use equation (5) in the form 1-M-8
(6)
with I and 6 treated as constants and 1 and k as variables This is essentially the same treatment as that described for equation (2). Although the graphs of equation (6) are easier to interpret than those of equation (2), the graphs of the latter equation as illustrated by Figures 1-3 may be a little easier to construct. These examples illustrate what might be called a 'valuable perspective" from which to view the problem of defining the solute species. Graphical methods can put colligative property data in a form which strongly suggests an appropriate formula for the solute. However, it must always be borne in mind that any treatment such as this assumes the validity of Raoult's law for the solute-solvent system. Consequently, the whole argument rests on the presupposition that an ideal solution exists. All of this may lead to strong evidence in favor of solute species postulated t o agree with the colligative property data, but it is not proof.
Figure 4. Test of &iration ( 5 ) for systems GoCli-CHKI (01, A l l r C H i l ( 0 ) and AIBrrCeHi(A1.
Literature Cited (1) DOLEZALEK, F., 2.Phgsik.Chem., 64,727 (1908). (2) HILDERRAND, J. H., AND SCOTT,R. L., "The Solubility of Nonelectrolytes," Reinhold Publishing Corporation, New York, N. Y., 1950, chap. 11. (3) SILLEN,L. G., Quart. Rev., 13, 146 (1959). (4) HOGFELDT, E., Ada. Chem. Smnd., 5, 1400 (1951). (5) EKELIN, K., AND SILLEN,L. G., ibid., 7,987 (1953). H. C., EDDY,L. P., AND WONG,R., J . Am. Chem. (6) BROWN, Sac., 75, 6275 (1953). (7) BRONN,H. C., WALLACE, I'M., 6279. (8) BROWN, H. C., WALLACE, Had., 6265.
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Chem Gem In J Am Chem. Sac, 70,3509 (1948), the late Professor W. M. Latimer discussed an unusual preparation of the compound CeNgHs,named melon He mentioned that when exposed to air, the compound forms the monohydrate. One of my students suggested that the hydrate should he named wtermelon. WILLIAM M. SPICER GEORGIA INSTITUTE OF TECHNOLOGY ATLANTA
Volume 38, Number 10, October 1 961
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