CHAPTER IX
EQUILIBRIA BETWEEN TWO VOLATILE COMPONENTS
General.—In the two preceding chapters certain restrictions were imposed on the discussion of the equilibria between two components; but in the present chapter the restriction that only one of the components is volatile will be allowed to fall, and the general behaviour of two volatile[[239]] components, each of which is capable of forming a liquid solution with the other, will be studied. As we shall see, however, the removal of the previous restriction produces no alteration in the general aspect of the equilibrium curves for concentration and temperature, but changes to some extent the appearance of the pressure-temperature diagram. The latter would become still more complicated if account were taken not only of the total pressure but also of the partial pressures of the two components in the vapour phase; this complication, however, will not be introduced in the present discussion.[[240]] In this chapter we shall consider the systems formed by the two components iodine and chlorine, and sulphur dioxide and water.
Iodine and Chlorine.—The different systems furnished by iodine and chlorine, rendered classical by the studies of Stortenbeker,[[241]] form a very complete example of equilibria in a two-component system. We shall first of all consider the
relations between concentration and temperature, with the help of the accompanying diagram, Fig. 42.
Concentration-Temperature Diagram.—In this diagram the temperatures are taken as the abscissæ, and the composition of the solution, expressed in atoms of chlorine to one atom of iodine,[[242]] is represented by the ordinates. In the diagram, A represents the melting point of pure iodine, 114°. If chlorine is added to the system, a solution of chlorine in liquid iodine is obtained, and the temperature at which solid iodine is in equilibrium with the liquid solution will be all the lower the greater the concentration of the chlorine. We therefore obtain the curve ABF, which represents the composition of the solution
with which solid iodine is in equilibrium at different temperatures. This curve can be followed down to 0°, but at temperatures below 7.9° (B) it represents metastable equilibria. At B iodine monochloride can be formed, and if present the system becomes invariant; B is therefore a quadruple point at which the four phases, iodine, iodine monochloride, solution, and vapour, can coexist. Continued withdrawal of heat at this point will therefore lead to the complete solidification of the solution to a mixture or conglomerate of iodine and iodine monochloride, while the temperature remains constant during the process. B is the eutectic point for iodine and iodine monochloride.