Just as we found in the case of aqueous salt solutions that at temperatures above the cryohydric or eutectic point, two different solutions could exist, one in equilibrium with ice, the other in equilibrium with the salt (or salt hydrate), so in the case of iodine and chlorine there can be two solutions above the eutectic point B, one containing a lower proportion of chlorine in equilibrium with iodine, the other containing a higher proportion of chlorine in equilibrium with iodine monochloride. The composition of the latter solution is represented by the curve BCD. As the concentration of chlorine is increased, the temperature at which there is equilibrium between iodine monochloride and solution rises until a point is reached at which the composition of the solution is the same as that of the solid. At this point (C), iodine monochloride melts. Addition of one of the components will lower the temperature of fusion, and a continuous curve,[[243]] exhibiting a retroflex portion as in the case of CaCl2,6H2O, will be obtained. At temperatures below its melting point, therefore, iodine monochloride can be in equilibrium with two different solutions.
The upper portion of this curve, CD, can be followed downwards to a temperature of 22.7°. At this temperature iodine trichloride can separate out, and a second quadruple
point (D) is obtained. This is the eutectic point for iodine monochloride and iodine trichloride.
By addition of heat and increase in the amount of chlorine, the iodine monochloride disappears, and the system passes along the curve DE, which represents the composition of the solutions in equilibrium with solid iodine trichloride. The concentration of chlorine in the solution increases as the temperature is raised, until at the point E, where the solution has the same composition as the solid, the maximum temperature is reached; the iodine trichloride melts. On increasing still further the concentration of chlorine in the solution, the temperature of equilibrium falls, and a continuous curve, similar to that for the monochloride, is obtained. The upper branch of this curve has been followed down to a temperature of 30°, the solution at this point containing 99.6 per cent. of chlorine.[[244]] The very rounded form of the curve is due to the trichloride being largely dissociated in the liquid state.
One curve still remains to be considered. As has already been mentioned, iodine monochloride can exist in two crystalline forms, only one of which, however, is stable at temperatures below the melting point; the two forms are monotropic (p. [44]). The stable form which melts at 27.2°, is called the α-form, while the less stable variety, melting at 13.9°, is known as the β-form. If, now, the presence of α-ICl is excluded, it is possible to obtain the β-form, and to study the conditions of equilibrium between it and solutions of iodine and chlorine, from the eutectic point F to the melting point G. As the β-ICl becomes less stable in presence of excess of chlorine, it has not been possible to study the retroflex portion of the curve represented by the dotted continuation of FG.
The following table gives some of the numerical data from which Fig. 42 was constructed.[[245]]
Iodine and Chlorine.
I. Invariant systems.
| Temperature. | Pressure. | Phases present. | ||
| Solid. | Liquid. | Vapour. | ||
| 7.9° | 11 mm. | I2,α-ICl | I
| I + Cl0.92 |
| 0.9° | — | I2,β-ICl | I
| — |
| 22.7° | 42 mm. | α-ICl,ICl3 | I
| I + Cl1.75 |
| [-102° | <1 atm. | ICl3,Cl2 | I
| I + Cln] |
