The principles of the Phase Rule have been applied to the investigation of the equilibria between organic compounds, and Figs. 66-69 reproduce some of the results which have been obtained.[[291]]
Fig. 66, the freezing point curve (curve of equilibrium) for o-nitrophenol and p-toluidine, shows a curve of the simplest type[[292]] (type I., Fig. 63), in which two branches meet at an eutectic point. The solid phase in equilibrium with solutions represented by the left-hand branch of the curve was o-nitrophenol (m.p. 44.1°); that in equilibrium with the solutions represented by the right-hand branch, was p-toluidine (m.p. 43.3°). At the eutectic point (15.6°), these two solid phases could co-exist with the liquid phase. This equilibrium curve, therefore, shows that o-nitrophenol and p-toluidine do not combine with one another.
In connection with this curve, attention may be called to the interesting fact that although the solid produced by cooling the liquid phase at the eutectic point has a composition approximating to that of a compound of equimolecular proportions of the phenol and toluidine, and a constant melting point, it is nevertheless a mixture. Although, as a rule, the constituents of the eutectic mixture are not present in simple molecular proportions, there is no reason why they should not be so; and it is therefore necessary to beware of assuming the formation of compounds in such cases.[[293]]
Fig. 67, on the other hand, indicates with perfect certainty the formation of a compound between phenol and α-naphthylamine.[[294]] (Cf. curve I., Fig. 64.)
Phenol freezes at 40.4°, but the addition of α-naphthylamine lowers the freezing point as represented by the curve AC. At C (16.0°) the compound C6H5OH,C10H7NH2 is formed, and the system becomes invariant. On increasing the amount of the amine, the temperature of equilibrium rises, the solid phase now being the compound. At D, the curve passes through a maximum (28.8°), at which the solid and liquid phases have the same composition. This is the melting point of the compound. Further addition of the amine lowers the temperature of equilibrium, until at E solid α-naphthylamine separates out, and a second eutectic point (24.0°) is obtained. BE is the
freezing-point curve of α-naphthylamine in presence of phenol, the freezing point of the pure amine being 48.3°.
On account of the great sluggishness with which the compound of phenol and α-naphthylamine crystallizes, it was found possible to follow the freezing point curves of phenol and the amine to temperatures considerably below the eutectic points, as shown by the curves CF and EG.
