Part 4 (1/2)
VAPOUR PRESSURES OF WHITE AND RED PHOSPHORUS.
------------------------------------------------------------------------- Vapour pressure of liquid white phosphorus.
Vapour pressure of red
phosphorus.
-------------------------------------------------+----------------------- Temperature.
Pressure
Temperature.
Pressure
Temperature.
Pressure
in cm.
in atm.
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.
[Ill.u.s.tration: 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 {47} 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 relations.h.i.+ps which are met with in the case of phosphorus can be best represented by the diagram, Fig. 11.[72]
In this figure, BO_{1} represents the conditions of equilibrium of the univariant system red phosphorus and vapour, which ends at O_{1}, the melting point of red phosphorus. By heating in capillary tubes of hard gla.s.s, Chapman[73] found that red phosphorus melts at the melting point of pota.s.sium iodide, _i.e._ about 630,[74] but the pressure at this temperature is unknown.
At O_{1}, then, we have the triple point, red phosphorus, liquid, and vapour, and starting from it, we should have the {48} vaporization curve of liquid phosphorus, O_{1}A, and the fusion curve of red phosphorus, O_{1}F.
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.
[Ill.u.s.tration: FIG. 11.]
When white phosphorus is heated to 44, it melts. At this point, therefore, we shall have another triple point, white phosphorus--liquid--vapour; the pressure at this point has been calculated to be 3 mm.[75] This point is the intersection of three curves, viz. sublimation curve, vaporization curve, and the fusion curve of white phosphorus. The fusion curve, O_{2}E, has been determined by Tammann[76] and by G. A. Hulett,[77] and it was found that increase of pressure by 1 atm. raises the melting point by 0.029. The sublimation curve of white phosphorus has not yet been determined.
As can be seen from the table of vapour pressures (p. 46), the vapour pressure of white phosphorus has been determined up to 500; at temperatures above this, however, the velocity with which transformation into red phosphorus takes place is so great as to render the determination of the vapour pressure {49} at higher temperatures impossible. Since, however, the difference between white phosphorus and red phosphorus disappears in the liquid state, the vapour pressure curve of white phosphorus must pa.s.s through the point O_{1}, the melting point of red phosphorus, and must be continuous with the curve O_{1}A, the vapour pressure curve of liquid phosphorus (_vide infra_). Since, as Fig. 10 shows, the vapour pressure curve of white phosphorus ascends very rapidly at higher temperatures, the ”break” between BO_{1} and O_{1}A must be very slight.
As compared with monotropic substances like benzophenone, phosphorus exhibits the peculiarity that transformation of the metastable into the stable modification takes place with great slowness; and further, the time required for the production of equilibrium between red phosphorus and phosphorus vapour is great compared with that required for establis.h.i.+ng the same equilibrium in the case of white phosphorus. This behaviour can be best explained by the a.s.sumption that change in the molecular complexity (polymerization) occurs in the conversion of white into red phosphorus, and when red phosphorus pa.s.ses into vapour (depolymerization).[78]