Illustration.—In order to illustrate the application of the principles of the Phase Rule to the study of systems formed by a volatile and a non-volatile component, a brief description may be given of the behaviour of sulphur dioxide and potassium iodide, which has formed the subject of a recent investigation. After it had been found[[236]] that liquid sulphur dioxide has the property of dissolving potassium iodide, and that the solutions thus obtained present certain peculiarities of behaviour, the question arose as to whether or not compounds are formed between the sulphur dioxide and the potassium iodide, and if so, what these compounds are. To find an answer to this question, Walden and Centnerszwer[[237]] made a complete investigation of the solubility curves (equilibrium curves) of these two components, the investigation extending from the freezing point to the critical point of sulphur dioxide. For convenience of reference, the results which they obtained are represented diagrammatically in Fig. 41. The freezing point (A) of pure sulphur dioxide was found to be -72.7°. Addition of potassium iodide lowered the freezing point, but the maximum depression obtained was very small, and was reached when the concentration of the potassium iodide in the solution was only 0.336 mols. per cent. Beyond this point, an increase in the concentration of the iodide was accompanied by an elevation of the freezing point, the change of the freezing point with the concentration being represented by the curve BC. The solid
which separated from the solutions represented by BC was a bright yellow crystalline substance. At the point C (-23.4°) a temperature-maximum was reached; and as the concentration of the potassium iodide was continuously increased, the temperature of equilibrium first fell and then slowly rose, until at +0.26° (E) a second temperature-maximum was registered. On passing the point D, the solid which was deposited from the solution was a red crystalline substance. On withdrawing sulphur dioxide from the system, the solution became turbid, and the temperature remained constant. The investigation was not pursued farther at this point, the attention being then directed to the equilibria at higher temperatures.
When a solution of potassium iodide in liquid sulphur dioxide containing 1.49 per cent. of potassium iodide was heated, solid (potassium iodide) was deposited at a temperature of 96.4°. Solutions containing more than about 3 per cent. of the iodide separated, on being heated, into two layers, and the temperature at which the liquid became heterogeneous fell as the concentration was increased; a temperature-minimum being obtained with solutions containing 12 per cent. of potassium iodide. On the other hand, solutions containing 30.9 per cent. of the iodide, on being heated, deposited potassium iodide; while a solution containing 24.5 per cent. of the salt first separated into two layers at 89.3°, and then, on cooling, solid was deposited and one of the liquid layers disappeared.
Such are, in brief, the results of experiment; their interpretation in the light of the Phase Rule is the following:—
The curve AB is the freezing-point curve of solid sulphur dioxide in contact with solutions of potassium iodide. BCD is the solubility curve of the yellow crystalline solid which is deposited from the solutions. C, the temperature-maximum, is the melting point of this yellow solid, and the composition of the latter must be the same as that of the solution at this point (p. [145]), which was found to be that represented by the formula KI,14SO2. B is therefore the eutectic point, at which solid sulphur dioxide and the compound KI,14SO2 can exist together in equilibrium with solution and vapour. The curve DE is the solubility curve of the red crystalline solid, and the
point E, at which the composition of solution and solid is the same, is the melting point of the solid. The composition of this substance was found to be KI,4SO2.[[238]] D is, therefore, the eutectic point at which the compounds KI,14SO2 and KI,4SO2 can coexist in equilibrium with solution and vapour. The curve DE does not exhibit a retroflex portion; on the contrary, on attempting to obtain more concentrated solutions in equilibrium with the compound KI,4SO2, a new solid phase (probably potassium iodide) was formed. Since at this point there are four phases in equilibrium, viz. the compound KI,4SO2, potassium iodide, solution, and vapour, the system is invariant. E is, therefore, the transition point for KI,4SO2 and KI.
Passing to higher temperatures, FG is the solubility curve of potassium iodide in sulphur dioxide; at G two liquid phases are formed, and the system therefore becomes invariant (cf. p. [121]). The curve GHK is the solubility curve for two partially miscible liquids; and since complete miscibility occurs on lowering the temperature, the curve is similar to that obtained with triethylamine and water (p. [101]). K is also an invariant point at which potassium iodide is in equilibrium with two liquid phases and vapour.
The complete investigation of the equilibria between sulphur dioxide and potassium iodide, therefore, shows that these two components form the compounds KI,14SO2 and KI,4SO2; and that when solutions having a concentration between those represented by the points G and K are heated, separation into two layers occurs. The temperatures and concentrations of the different characteristic points are as follows:—
| Point. | Temperature. | Composition of the solution per cent. KI. |
| A (m.p. of SO2) | -72.7° | — |
| B (eutectic point) | — | 0.86 |
| C (m.p. of KI,14SO2) | -23.4° | 17.63 |
| E (m.p. of KI,4SO2) | +0.26° | 39.33 |
| G (KI + two liquid phases) | (about) 88° | 24.0 |
| H (critical solution point) | 77.3° | 12 |
| K (KI + two liquid phases) | (about) 88° | 2.7 |