Formation of Compounds.—In the case just discussed, the components crystallized out from solution in the pure state. If, however, combination can take place between two of the components, the relationships will be somewhat different; the curves which are obtained in such a case being represented in Fig. 97. From the figure, we see that the two components B
and C form a compound, and the freezing point curve of the binary system has therefore the form shown in Fig. 64 (p. [209]). Further, there are two ternary eutectic points, K1 and K2, the solid phases present being A, B, and compound, and A, C, and compound respectively.
The particular point, now, to which it is desired to draw attention is this. Suppose the ternary eutectic curves projected on a plane parallel to the face of the prism containing B and C, i.e. suppose the concentrations of the two components B and C, between which interaction can occur, expressed in terms of a constant amount of the third component A,[[332]] curves will then be obtained which are in every respect analogous to the freezing point curves of binary systems. Thus, suppose the eutectic curves k1K and k2K in Fig. 95 projected on the face BC of the prism, then evidently a curve will be obtained consisting of two branches meeting in an eutectic point. On the other hand, the projection of the ternary eutectic curves in Fig. 97 on the face BC of the prism, will give a curve consisting of three portions, as shown by the outline k1K1K2k2 in Fig. 97.
Various examples of this have been studied, and the following table contains some of the data for the system ethylene bromide (A), picric acid (B), and β-naphthol (C), obtained by Bruni.[[333]]
| Temperature. | Solid phases present. | |
| Point k1 | 9.41° | Ethylene bromide, picric acid. |
| Curve k1K1 | — | ,, ,, |
| Point K1 | 9.32° | Ethylene bromide, picric acid, and β-naphthol picrate. |
| Curve K1D′K2 | — | Ethylene bromide, β-naphthol picrate. |
| Point D′ | 9.75° | ,, ,, ,, ,, |
| Point K2 | 8.89° | ,, ,, β-naphthol, and picrate. |
| Curve K2k2 | — | ,, ,, β-naphthol. |
| Point k2 | 9.04° | ,, ,, ,, |
From what has been said, it will be apparent that if the ternary eutectic curve of a three-component system (in which one of the components is present in constant amount) is determined, it will be possible to state, from the form of curve obtained, whether or not the two components present in varying amount crystallize out pure or combine with one another to form a compound. It may be left to the reader to work out the curves for the other possible systems; but it will be apparent, that the projections of the ternary eutectic curves in the manner given will yield a series of curves alike in all points to the binary curves given in Figs. 63-65, pp. [208]-210.
Since, from the method of investigation, the temperatures of the eutectic curves will depend on the melting point of the third component (A), it is possible, by employing substances with widely differing melting points, to investigate the interaction of the two components (e.g. two optical antipodes) B and C over a range of temperature; and thus determine the range of stability of the compound, if one is formed. Since, in some cases, two substances which at one temperature form mixed crystals combine at another temperature to form a definite compound, the relationships which have just been described can be employed, and indeed, have been employed, to determine the temperature at which this change occurs.[[334]] By means of this method, Adriani found that below 103° i-camphoroxime exists as a racemic compound, while above
that temperature it occurs as a racemic mixed crystal[[335]] (cf. p. [219]).