Phosphine

The electronegative character of an element is shown by—

1. Electrolytic dissociation of hydrogen ion in its hydrogen compounds.
2. Displacement of hydrogen from hydrogen compounds by alkali and other metals.
3. Stability of hydrides towards heat.

With respect to condition (a), phosphorus has none of the negative character possessed by its neighbour sulphur in the sixth group. With respect to (b), the phosphides and the nitrides may well be compared, and the modes of formation and stability of these compounds show that phosphorus is less electronegative than its congener nitrogen. With respect to (c), the dissociation of phosphine is more rapid and more complete than that of ammonia. There is apparently no reversible equilibrium between phosphorus and hydrogen as there is between nitrogen and hydrogen. Both hydrogen sulphide and ammonia are formed to a limited extent by direct combination at moderate temperatures and pressures, whereas phosphine is not formed in this way. [Note.—The production of phosphine has been observed when white phosphorus is heated in a sealed tube with hydrogen.]

While investigating the action of alkalies on phosphorus Gengembre in 1783 prepared a spontaneously inflammable gas. This reaction was expected to produce a " liver of phosphorus " or alkali poly phosphide similar to " liver of sulphur " or alkali poly- sulphide, which is prepared from sulphur under analogous conditions. Several other chemists about this time prepared phosphine by similar methods, or by heating phosphorous acid. The composition of the gas, as an analogue of ammonia, was demonstrated by Davy, and the difference in composition between the gaseous hydride and the liquid hydride (q.v.) to which the spontaneous inflammability is due was established by Dumas. It was demonstrated by Graham that the spontaneous inflammability could be removed by means of carbon dioxide, hydrogen chloride, nitric oxide, arsenious oxide, concentrated sulphuric, phosphoric and arsenic acids.

The pale glow which hovers over marshes, and which, under the name of " will o' the wisps," "Jack o' lanterns," etc., has been the subject of many legends, has been attributed to traces of phosphine with other gases produced by the decomposition of organic matter. This is rendered probable by the observation that phosphine is produced by the putrefaction of proteins.

1. The original method is still in general use. The alkali employed may be hydroxide of sodium, potassium, calcium or barium. The reaction is usually represented by the equation
   
   4P + 3KOH + 3H₂O = PH₃ + 3KH₂PO₂
   
   This reaction really amounts to a hydrolysis of phosphorus, which can be effected also by water at high temperatures. The equations reduced to their simplest forms and dissected are:—
   
   3P + 3H₂O = 3OPH + 3H
   P +3H = PH₃
   30PH +3H₂O =3H₃PO₂
   
   The phosphine so obtained usually inflames spontaneously on coming into contact with the air; each bubble as it escapes forms a beautiful vortex ring of smoke. It was early shown that the spontaneous inflammability is due to the presence of small quantities of a hydride, liquid at ordinary temperatures, which has the empirical composition (PH₂)_x and the molecular composition P₂H₄. The phosphorus must therefore react also according to the equation
   
   6P + 4KOH + 4H₂O = 4KH₂PO₂ + P₂H₄
   
   The presence of hydrogen in proportions up to 50 or 60 per cent, has been accounted for by the oxidation-hydrolysis of the hypophosphite:—
   
   KH₂PO₂ + 2H₂O = KH₂PO₄ + 2H₂

A tubulated retort is filled with alkali and the phosphorus is added in small pieces. The tubulure is then closed by a delivery tube, which is connected with a Kipp's apparatus or other source of hydrogen (the air may also be displaced by adding a little ether). The neck of the retort is connected with a tube which dips under water. The air is displaced by hydrogen and the retort then gently warmed so as to melt the phosphorus and give a steady evolution of gas. When sufficient has been collected the residual gas is expelled by a current of hydrogen and the phosphorus allowed to solidify before the retort is opened.

If phosphine which is not spontaneously inflammable is required, the gas is washed by passing it through a Woulfe bottle containing concentrated hydrochloric acid or alcoholic potash.

The gas prepared in this manner contains hydrogen (up to 90 per cent.), but can be prepared pure by allowing water, dilute alkali or aqueous ether to drop on to phosphonium iodide. The product may be mixed with carbon dioxide, dried by phosphorus pentoxide and condensed in liquid air. The condensate is distilled, rejecting the first and last fractions, the middle being pure dry phosphine.

Heating hypophosphorous or phosphorous acid or their salts (iq.v.) gives phosphine:—

2H₃PO₂ = PH₃ + H₃PO₄

The action of water or dilute acids on alkali or alkaline earth phosphides affords a method of preparation. The reactions shown by the following equations will be succeeded by others which produce hypophosphites, etc.:—

Na₃P + 3H₂O = PH₃ + 3NaOH
Na₃H₃P₂ + 3H₂O = 3NaOH + 2PH₃

Similarly calcium phosphide is readily attacked by water or dilute acids giving phosphine. A fairly pure sample of the gas may be made from calcium phosphide which has been produced in the electric furnace. The reaction is more complex than that represented by the equation

Ca₃P₂ + 6H₂O = 3Ca(OH)₂ + 2PH₃

Magnesium phosphide, aluminium phosphide and phosphides of several other metals also yield phosphine when treated with acids.

A pure phosphine may be made by the action of alkalies or even water upon phosphonium iodide or bromide. Crystalline phosphonium iodide, is mixed with broken glass in a flask fitted with a tap-funnel and delivery tube. A solution of one part of caustic potash in two parts of water is added slowly through the tap-funnel. The gas is not spontaneously inflammable if the last portions are rejected.

PH₄I + KOH = KI + PH₃ + H₂O

The gas should be washed with concentrated hydrochloric acid (to remove any P₂H₄), alkali (to remove HCl and HI), and then dried with phosphorus pentoxide.

Among many reactions by which phosphine can be prepared may be mentioned that which takes place between hydrochloric acid and diamidophosphorous acid, (NH₂)₂.POH.

Phosphine is a colourless gas which maybe condensed at low temperatures to a colourless liquid and frozen to a white solid. The boiling-point of the liquid is considerably below that of ammonia. The gas has a strong odour recalling that of decayed fish, and resembling rather the odour of the lower alkylamines than that of ammonia. It does not support ordinary combustion, and is poisonous—dilutions as large as 1 in 10,000 of air cause death in a few hours. The effects, such as embrittling of the teeth and bones, are somewhat similar to those of phosphorus. The solubility in water (about 26 volumes in 100 at 17° C.) is far lower than that of ammonia. The gas also is only sparingly soluble in alcohol and ether. It is easily decomposed by heat, depositing phosphorus in the red form and giving 3/2 of its volume of hydrogen in accordance with the equation

4PH₃ = 4P + 6H₂

The molecular weight was indicated by rough determinations of the density by the early workers; these results were, however, in all cases too low on account of admixture with hydrogen (v. supra).

Density

The density corresponds to simple molecules, PH₃, but deviations from the laws of a perfect gas are observed. The weight of a normal litre is 1.5293 to 1.5295 gram, a value which shows that under these conditions it agrees closely with Avogadro's theory. At pressures of 10 atmospheres or more, however, and at temperatures from 24.6° to 54.4° C. the compressibility is much greater than is allowed by Boyle's law. The following values refer to 24.6° C.:—

p (atm.)	1.0	10	15	20	25	30
pv	1.0	0.97	0.98	0.80	0.75	0.70

Viscosity

The relative value, based on that of air (viz. 2.191×10^-4 C.G.S. units at 15° C.), was found to be 1.129×10^-4 at 15° C. and 1.450×10^-4 C.G.S. units at 100° C. The relative collision area, πr²×10¹⁵ (in which r is the radius of the molecules in cm.), calculated from Chapman's formula, is 0.911, while for NH₃ and AsH₃ the values are 0.640 and 0.985 respectively.

Absorption in the Infra-red

The methods of determining this property are briefly described as follows. The radiation from a Nernst filament was passed through a rock-salt lens, then through either of two similar tubes, one of which was evacuated and the other filled with the gas at a known pressure. A rocking arrangement allowed either tube to be thrown quickly into the path of the rays. The beam was focussed on the collimator slit of an infra-red spectrometer furnished with a prism of rock salt, quartz or fluorite. On emerging from the second slit of the spectrometer the radiation was received by a bismuth-silver thermopile, the current from which was registered by a Paschen galvanometer.

Phosphine, arsine and stibine showed a number of deep bands, also fine structure and smaller bands. The intensity of the bands decreased as a rule in passing towards the visible end of the infra-red spectrum. Bands numbered I to VI formed a nearly harmonic sequence, in which the wavelengths of corresponding bands increased with increase in the atomic weight of the Fifth Group element. In the following table the wavelengths A in microns (μ =0.001 mm.) and the wave-numbers (per cm.) are given for corresponding bands. The percentage absorption a refers to the gas in the tube under a pressure of one atmosphere. The ratios of the wave-numbers of the corresponding bands for each pair of gases are nearly constant.

Solubility

Phosphine dissolves in 5 to 10 times its volume of water at ordinary temperatures. The solubility, expressed as "Bunsen's absorption coefficient," is 0.26 volume at N.T.P. in 1 volume of water at 17° C.

When the liquefied gas was brought into contact with water and solidified by releasing the pressure, the resulting crystalline solid was found to be a hydrate, perhaps PH₃.H₂O.

Phosphine is only slightly soluble in alcohol or ether. It is readily absorbed by wood charcoal, to the extent of about 500 volumes by 1 volume of the charcoal. Coconut charcoal absorbs 69 volumes of the gas at 0° C.

The gas may be condensed under pressure at a temperature which is attainable by the use of solid carbon dioxide. It can be liquefied at the ordinary temperature under a pressure of 30 atmospheres. The principal thermal constants are as follows:—

Boiling-point, -85° C.,
Melting-point, -132.5° C.
Critical temperature, 54° C.,
Critical pressure, 70.5, atm.
Critical volume, 4.6 c.c./gram.

The vapour pressures and isobaric densities of liquid (D_L) and vapour (D_V) for liquid phosphine are as follows:—

0.760

t° C	-105.9	-101.2	-97.7	-93.1	-86.6	-86.2
p (mm)	237	319	393	498	719	760
DL	0.756	0.753	0.746	0.745
DV	0.0379	0.02101	0.02122	0.02151

t° C	+49.4	44.4	39.4	29.4	24.6	18.4	8.4	2.4
p (atm)	62.4	56.1	50.8	41.3	37.1	32.6	27.2	23.4
DL	0.417	0.469	0.502	0.536	0.545	0.559	0.595	0.618

From the first set of results the density of the liquid at its boiling-point is 0.744, and increases with falling temperature according to the equation

D_T = 0.744[1 + 0.0008(Tb-T)]

The vapour pressure has also been expressed by the formula

log p = -845.57/T + 1.75 log T – 0.0₂61931T + 4.61480

The surface tension of liquid phosphine indicates a certain degree of association, since the coefficient of decrease of molar surface energy with increase of temperature is about 1.7 instead of 2.0:—

t° C	-105.9	-101.2	-97.6	-93.1
σ	22.783	22.095	21.533	20.798
σ (VM)2/3	287.2	279.6	273.4	265.4

The refractivity of the liquid is 1.323 for white light at 11.0° C. and 1.317 for the D line at 17.5° C.

The dielectric constant is 2.55 at -60° C. and 2.71 at -25° C.

Chemical Properties.—Decomposition.—Phosphine is an unstable gas which can be decomposed by heat alone, and is easily oxidised by oxygen and by oxidising agents such as the halogens.

The velocity of decomposition at constant volume and at temperatures between 300° and 500° C. was studied by van't Hoff and his coworkers. Concentrations of the undecomposed phosphine at any time can be calculated from the pressure, which, of course, rises during the reaction. The way in which the velocity constant can be calculated from the pressure is as follows.

The equation is arrived at in the following way:—A fraction x of 1 original mol of PH₃ at a pressure p₀ is decomposed after a time t giving 3x/2 mols of hydrogen and (1 + x/2) mols of the mixed gas, which will therefore have a pressure p = (1 + x/2)p₀.

If the equation of a unimolecular reaction is referred to the concentration c₀ of PH₃ at the beginning and c after time t, then



or

Now

Therefore

or

Also, at the end of the reaction p_∞ = 3/2p₀. The final form of the equation



is equivalent to



The reaction appears to be unimolecular in any one vessel, but the constant K is not the same in different vessels. Hence it was supposed that the decomposition took place on the walls of the vessel. The constant rises with rise of temperature, and above 660° C. there is no constant:—

t° C	572	645	650	656	660	683
103 K	0.54	3.6	4.4	5.6	12	11 to 22

The effect of surface on the reaction has been further studied by adding powdered fused silica, which caused a great increase in the velocity of decomposition. The heat of activation of the PH₃ molecules calculated from the rate of increase in the velocity constant with temperature was 40,000 to 50,000 calories.

The effect of the electric discharge was investigated by the early workers; red phosphorus is deposited and other hydrophosphors are formed.

The effect of light has been studied, but the results are not consistent. Some decomposition may take place with the production of a reddish deposit, and the spontaneously inflammable fraction may be destroyed. The amounts and intensities of the short-wave radiations as well as other conditions were not known or controlled by the earlier investigators.

Oxidation

Although pure phosphine does not itself in the ordinary way inflame spontaneously with air, yet it does so when the pressure is reduced. This observation was made almost simultaneously by Davy and de la Billardiere. In the words of Davy—" I found the phosphoretted hydrogen produced a flash of light when admitted into the best vacuum that could be made by an excellent pump of Nairn's construction."

The oxidation under reduced pressure was found to take place at a fairly uniform speed at 50° C. and pressures of 760 down to about 400 mm. If mixtures of air and PH₃ were used corresponding to about 2 volumes of PH₃ to 1 volume of oxygen, the diminution of pressure per hour was uniformly 0.5 to 3.0 per cent, of the initial value. There was in only one case a slight increase in the velocity immediately before explosion occurred. Moisture retarded the reaction. Moist mixtures of phosphine and oxygen were not explosive with air at low pressures, but dry mixtures of phosphine and oxygen exploded when the oxygen pressure was reduced to 0.1 atmosphere or any lower value.

Slow oxidation at low pressures proceeds according to the equation

PH₃ + O₂ = H₂ + HPO₂

At higher pressures both metaphosphorous and phosphorous acids are formed, thus:—

4PH₃ + 5O₂ = 2HPO₂ + 2H₃PO₃ + 2H₂

If the gas was very thoroughly dried, as over phosphorus pentoxide, soda-lime or crystallised glycerine, it took fire spontaneously in air. The explosion which takes place after some hours of slow oxidation of phosphine with moist air has been attributed to the accumulation of hygroscopic compounds such as H₃PO₂ and H₃PO₃, which dry the remaining gases.

The explosion of phosphine with oxygen takes place at higher pressures when the phosphine is in considerable excess. Thus 0.5 c.c. of oxygen was mixed with 0.5 and up to 6.0 c.c. of phosphine and the expansion was determined at which the mixture would explode (at 50° C.). A total expansion of 1 volume to 5 volumes was required for the first mixture, but only 6.5 to 9.6 volumes for the second.

The vigorous combustion of phosphine produces orthophosphoric acid. A combustion with the production of 85 per cent, of this acid gave +311 calories at constant pressure. Values calculated for the heat of formation of gaseous phosphine are -11.6 Cals., -9.1 Cals., and -5.8 Cals. The free energy of formation from solid phosphorus and hydrogen at 25° C. is -3.3 Cals.

Halogens attack phosphine vigorously giving halides of hydrogen and usually of phosphorus as well. These reactions were early investigated by Thomson, Balard and Hofmann. The action of sulphur was also investigated by Davy and Dalton.

Phosphine is absorbed by acid cuprous chloride giving an addition product CuCl.PH₃. It precipitates phosphides from some metallic salts.

Phosphine reacts with the lower halides of phosphorus giving halogen hydracids and free phosphorus or a compound containing more phosphorus. With the higher halides it gives the lower halides and halogen hydracids, thus:—

3PCl₅ + PH₃ = 4PCl₃ + 3HCl

In relation, to halides of boron, phosphine resembles ammonia, giving addition compounds of similar but not identical type such as 2BF₃.PH₃ and BCl₃.PH₃, which are, however, more easily dissociated than the corresponding ammines.