Vapour Pressures of White and Red Phosphorus.
| Vapour pressure of liquid white phosphorus. | Vapour pressure of red phosphorus. | ||||
| Temperature. | Pressure in cm. | Temperature. | Pressure in atm. | Temperature. | Pressure in atm. |
| 165° | 12 | 360° | 3.2 | 360° | 0.1 |
| 180° | 20.4 | 440° | 7.5 | 440° | 1.75 |
| 200° | 26.6 | 494° | 18.0 | 487° | 6.8 |
| 219° | 35.9 | 503° | 21.9 | 510° | 10.8 |
| 230° | 51.4 | 511° | 26.2 | 531° | 16.0 |
| 290° | 76.0 | — | — | 550° | 31.0 |
| — | — | — | — | 577° | 56.0 |
These values are also represented graphically in Fig. 10.
At all temperatures above about 260°, transformation of the white into the red modification takes place with appreciable velocity, and this velocity increases as the temperature is raised. Even at lower temperatures, e.g. at the ordinary temperature, the velocity of transformation is increased under the influence
of light,[[69]] or by the presence of certain substances, e.g. iodine,[[70]] just as the velocity of transformation of white tin into the grey modification was increased by the presence of a solution of tin ammonium chloride (p. [40]). At the ordinary temperature, therefore, white phosphorus must be considered as the less stable (metastable) form, for although it can exist in contact with red phosphorus for a long period, its vapour pressure, as we have seen, is greater than that of the red modification, and also, its solubility in different solvents is greater[[71]] than that of the red modification; as we shall find later, the solubility of the metastable form is always greater than that of the stable.
The relationships which are met with in the case of phosphorus can be best represented by the diagram, Fig. 11.[[72]]
In this figure, BO1 represents the conditions of equilibrium of the univariant system red phosphorus and vapour, which ends at O1, the melting point of red phosphorus. By heating in capillary tubes of hard glass, Chapman[[73]] found that red phosphorus melts at the melting point of potassium iodide, i.e. about 630°,[[74]] but the pressure at this temperature is unknown.
At O1, then, we have the triple point, red phosphorus, liquid, and vapour, and starting from it, we should have the
vaporization curve of liquid phosphorus, O1A, and the fusion curve of red phosphorus, O1F. Although these have not been determined, the latter curve must, from theoretical considerations (v. p. [58]), slope slightly to the right; i.e. increase of pressure raises the melting point of red phosphorus.