The model for FeCl3—HCl—H2O exhibits certain other peculiarities not found in the case of MgCl2—KCl—H2O. On examining the model more closely, it is found that the field of the ternary compound 2FeCl3,2HCl,8H2O (VII.) resembles the surface of a sugar cone, and has a projecting point, the end of which corresponds with a higher temperature than does any other point of the surface. At the point of maximum temperature the composition of the liquid phase is the same as that of the solid. This point, therefore, represents the melting point of the double salt of the above composition.
The curves representing univariant systems are of two kinds. In the one case, the two solid phases present are both binary compounds; or one is a binary compound and the other is one of the components. In the other case, either one or both solid phases are ternary compounds. Curves belonging
to the former class (so-called border curves) start from binary eutectic points, and their course is always towards lower temperatures, e.g. CL, EM, GN, IO. Curves belonging to the latter class (so-called medial curves) would, in a triangular diagram, lie entirely within the triangle. Such curves are YV, WV, VL, LM, MV, NS, ST, SO, OZ. These curves do not always run from higher to lower temperatures, but may even exhibit a point of maximum temperature. Such maxima are found, for example, at U (Fig. 116), and also on the curves ST and LV.
Finally, whereas all the other ternary univariant curves run in valleys between the adjoining surfaces, we find at the point X a similar appearance to that found in the case of carnallite, as the univariant curve here rises above the surrounding surface. The point X, therefore, does not correspond with a eutectic point, but with a transition point. At this point the ternary compound 2FeCl3,2HCl,12H2O melts with separation of 2FeCl3,12H2O, just as carnallite melts at 168° with separation of potassium chloride.
The Isothermal Curves.—A deeper insight into the behaviour of the system FeCl3—HCl—H2O is obtained from a study of the isothermal curves, the complete series of which, so far as they have been studied, is given in Fig. 117.[[369]] In this figure the lightly drawn curves represent isothermal solubility curves, the particular temperature being printed beside the curve.[[370]] The dark lines give the composition of the univariant systems at different temperatures. The point of intersection of a dark with a light curve gives the composition of the univariant solution at the temperature represented by the light curve; and the point of intersection of two dark lines gives the composition of the invariant solution in equilibrium with three solid phases. The dotted lines represent metastable systems, and the points P, Q, and R represent solutions of
the composition of the ternary salts, 2FeCl3,2HCl,4H2O; 2FeCl3,2HCl,8H2O; and 2FeCl3,2HCl,12H2O.
The farther end of the model (Fig. 116) corresponds, as already mentioned, to the temperature -30°, so that the outline evidently represents the isothermal curve for that temperature. Fig. 117 does not show this. We can, however, follow the isothermal for -20°, which is the extreme curve on the right in Fig. 117. Point A represents the solubility of 2FeCl3,12H2O in water. If hydrogen chloride is added, the concentration of ferric chloride in the solution first decreases and then increases, until at point 34 the ternary double salt 2FeCl3,2HCl,12H2O is formed. If the addition of hydrogen chloride is continued, the ferric chloride disappears ultimately, and only the ternary double salt remains. This salt can coexist with solutions of the composition represented by the curve which passes through the points 173, 174, 175. At the last-mentioned point, the ternary salt with 8H2O is formed. The composition of the solutions with which this salt is in equilibrium at -20° is represented by the curve which passes through a point of maximal concentration with respect to HCl, and cuts the curve SN at the point 112, at which the solution is in equilibrium with the two solid phases 2FeCl3,4H2O and 2FeCl3,2HCl,8H2O. The succeeding portion of the isotherm represents the solubility curve at -20° of 2FeCl3,4H2O, which cuts the dark line OS at point 113, at which the solution is in equilibrium with the two solid phases 2FeCl3,4H2O and 2FeCl3,2HCl,4H2O. Thereafter comes the solubility curve of the latter compound.
The other isothermal curves can be followed in a similar manner. If the temperature is raised, the region of existence of the ternary double salts becomes smaller and smaller, and at temperatures above 30° the ternary salts with 12H2O and 8H2O are no longer capable of existing. If the temperature is raised above 46°, only the binary compounds of ferric chloride and water and the anhydrous salt can exist as solid phases. The isothermal curve for 0° represents the solubility curve for 2FeCl3,12H2O; 2FeCl3,7H2O; 2FeCl3,5H2O; and 2FeCl3,4H2O.