Phosphorus Vapour

That phosphorus, in common with other non-metals, forms complex molecules in the gaseous state was established early in the nineteenth century by determinations of vapour density. At a temperature of 500° C. the relative density (air = 1) is 4.35; at higher temperatures it falls, being 3.632 at 1484° C. and 3.226 at 1677° C. The molecule P₄ requires a density of 4.294. The molecular complexity thus revealed is confirmed by the low value of the ratio of the specific heats, namely, 1.175 at 300° C.

Many determinations at the higher temperatures have been made by measuring the pressures of a known weight of gaseous phosphorus enclosed in a fixed volume. The phosphorus was vaporised by means of an electric furnace in a completely closed vessel, the pressures being read on a quartz spiral manometer, which obviated the necessity of having any manometric fluid. From measurements of pressure, temperature and volume, the apparent molecular weight, or mean relative weight of the kinds of molecules which are present, can be calculated by the usual applications of the gas laws. Different amounts of phosphorus confined in the same volume exert different pressures, and a comparison of these with the relative densities indicates the effect of pressure upon dissociation for the particular temperature. From about 500° C. to 600° C. densities show the presence of P₄ molecules only. Between 600° C. (at which the density is 61.5 to 61.9 independently of the pressure) and 1200° C. dissociation takes place progressively, possibly in the stages

P₄ ⇔ 2P₂ ⇔ 4P

An attempt was made to calculate equilibrium constants for the separate dissociations, the heats of which could thus be determined:—

P₄ → 2P₂ - 31.5 Cals.
P₂ → 2P -45.5 Cals.

By means of the simpler assumption that each kind of dissociation is complete before the next begins, the degrees of dissociation a at each temperature and pressure can be calculated directly from the densities, as in the following table.

The mean heat of dissociation, calculated from the variations in the dissociation constant between 1100° and 1200° C., is 49.2 Cals. per mol (P₄). From this the constant, K, was calculated at 1300° C., and the percentage dissociations at this temperature. At low pressures and above this temperature, the dissociation into P₂ being now complete, it is probable that single atoms, P, will begin to appear. On the assumption that P₄, P₂ and P are all present at an early stage, it is calculated that at 1200° C. and low pressures there is already a considerable percentage of P.

The dissociation constants K⁴ (P⁴ ⇔ 2P²) and K² (P² ⇔ 2P) are calculated to be:

t° C	800	1000	1200
K4	350	40.0	2.11
K2	7.4	9.0	36.0

The atomicity of phosphorus vapour under the critical conditions may be calculated from the van der Waals coefficient b, which represents the actual volume occupied by the molecules and is given by the equation



The value of b for phosphorus in combination is obtained additively from the critical data of phosphine. For the compound b is 0.00233, and for three hydrogen atoms 3b is 0.001086, whence b for combined phosphorus (1 atom) is 0.001244. The ratio of this to b for the free element gives the atomicity, 4.33, of the latter.

The refractivity at three wavelengths was determined by means of the Jamin refractometer, a weighed quantity of phosphorus being gradually vaporised in an evacuated tube of fused silica with transparent plane parallel ends.

λ	6800 Å	5893 Å	5100 Å
(n-l)×106	1200	1212	1230

The equation giving n -1 as a function of λ is



From this the value of n-1 at λ = 5893 is about 0.0012.

When the element or some of its compounds are introduced into a flame, or when hydrogen containing a little phosphorus vapour is burned, a green colour is observed which has been resolved into bands in the orange, yellow and green. The passing of an electric discharge through a tube containing phosphorus vapour at low pressures also showed a green colour which was resolved into similar bands. Other observations summarised in the appropriate works of reference and comprehensive papers.

It has now been established that the emission spectrum of the element lies in the ultraviolet. Prominent lines in the arc and spark spectra are found at λ = 2555.7, 2554.0, 2536.4, 2534.75 Å. These are also seen in the spectra from the vapour in a Geissler tube, and in addition other lines at 2497.3 and 2484.1. A photographic record of the condensed spark spectrum showed lines at 2555.0, 2553.3, 2535.6, 2534 Å.

When phosphoric acid or its salts are introduced into a carbon arc they give a mixed line and band spectrum. There is a band at 3286 to 3246 Å and others are found at 2635, 2625, 2611, 2597, 2588 and 2571 Å. Sharp lines are seen at 2555.0 to 2553.37 Å, which represent the double line of the element at 2535.74 to 2534.12 Å. There are several faint streaks at 2477 to 2385 Å. The arc spectrum obtained by introducing phosphorus pentoxide into a copper arc contains at least 35 lines of wavelengths ranging from 2550 to 1672 Å. Many of these are connected together as systems of constant frequency difference, and some of these series are due to doubly and trebly ionised phosphorus. The low-voltage arc spectrum also shows lines and bands.

The spectrum of the electrodeless discharge shows, in the spectral region transmitted by fluorite (CaF₂), 12 lines between 1859.4 and 1671.5 Å. These also depend upon the degrees of ionisation of the phosphorus atom.

From the foregoing data it is evident that these spectra are similar to those which proceed from the slow combustion of phosphorus.

Neither phosphorus vapour nor the compounds of phosphorus with colourless elements show selective absorption in the visible region. The vapour shows general absorption in the ultraviolet from a wavelength of about 2500 Å at 150° C., 2820 Å at 190° and 2960 Å at 220°. Phosphine transmits rays down to about 2230 Å, and phosphorus trichloride down to about 2590 Å. The infra-red absorption by PH₃ is described under that compound.

When the vapour of phosphorus at 600° to 700° C. and 1 mm. pressure (therefore containing a considerable proportion of diatomic molecules) is confined in a sealed quartz tube and exposed to the spark lines 2195 and 2144, Å of cadmium, 2100 and 2062 Å of zinc or 1990 and 1935 Å of aluminium, it gives a fluorescent emission consisting of a resonance series in the region 3500 to 1900 Å.