Note 25 — Chapter XIX · PHOSPHORUS AND THE OTHER ELEMENTS OF THE FIFTH GROUP

In a liquid state the density of phosphorous chloride at 10° = 1·597, and therefore its molecular volume = 137·5/1·597 = 86·0, and that of phosphorus oxychloride is equal to 153·5/1·693 = 90·7; hence the addition of oxygen has produced considerable increase in volume, just as in the conversion of sulphur dichloride, SCl2, into sulphuryl chloride, SOCl2, the volume changes from 64 to 71. It is the same with the boiling-points; phosphorus trichloride boils at 70°, the oxychloride at 100°, sulphur dichloride at 64°, and sulphuryl chloride at 78°—that is, the addition of oxygen raises the boiling points.

The vapour density of phosphorus trichloride and oxychloride corresponds with their formulæ (Cahours, Würtz)—namely, is equal to half the molecular weight referred to hydrogen. But it is not so with phosphorus pentachloride. Cahours showed that the vapour density of phosphorus pentachloride referred to air = 3·65, to hydrogen = 52·6, whilst according to the formula PCl₅ it should be = 104·2. Hence this formula corresponds with four, and not with two, molecules. This shows that the vapour of phosphoric chloride contains two and not one molecule, that in a state of vapour it splits up, like sal-ammoniac, sulphuric acid, &c. The products of disruption must here be phosphorous chloride, PCl₃, and chlorine, Cl₂, bodies which easily re-form phosphoric chloride, PCl₅, at a lower temperature. This decomposition of phosphoric chloride in its conversion into vapour is confirmed by the fact that the vapour of this almost colourless substance shows the greenish-yellow colour proper to chlorine. This dissociation of phosphoric chloride has been considered by some chemists as a sign that phosphorus, like nitrogen, does not give volatile compounds of the type PX₅, and that such substances are only obtained as unstable molecular compounds which break up when distilled; for example, PH₃,HI, PCl₃,Cl₂, NH₃,HCl, &c. To prove that the molecule PCl₅ actually exists, Würtz in 1870 observed that when mixed with the vapour of phosphorous chloride the vapour of phosphoric chloride distils over (from 160° to 190°) perfectly colourless, and has a density which is really near to the formula—namely, to 104—and the same density was determined for the pentachloride in an atmosphere of chlorine. Hence at low temperatures and in admixture with one of the products of dissociation, there is no longer that decomposition which occurs at higher temperatures—that is, we have here a case of dissociation proceeding at moderate temperatures.

An important proof in favour of the type PX₅ is exhibited by phosphorus pentafluoride PF₅, obtained by Thorpe as a colourless gas which only corrodes glass after the lapse of time; it may be kept over mercury, and has a normal density. It is formed when liquid arsenic trifluoride, AsF₃, is added to phosphoric chloride surrounded by a freezing mixture: 3PCl₅ + 5AsF₃ = 3PF₅ + 5AsCl₃.

In general, fluorine and phosphorus give stable compounds: PF₃, POF₃, and PF₅, as would be expected from the fact that in passing from Cl to I (i.e. as the atomic weight of the halogen increases) the stability of the compounds with P and the tendency to give PX₅ (Note 24) decreases. Phosphorus trifluoride is obtained by heating a mixture of ZnF₂ and PBr₃, by the action of AsF₃ upon PCl₃, by heating phosphide of copper with PbF₂, &c. It is a strong-smelling gas, which liquefies at -10° under a pressure of 40 atmospheres, giving a colourless liquid. It dissolves easily in (is absorbed by, reacts with) water, and acts upon glass; when mixed with Cl₂ it combines with it (Poulenc, 1891), forming PCl₂F₃, a colourless gas of normal density, which is transformed into a liquid at 8°, decomposes into PF₃ + Cl₂ at 250°, and, with a small amount of water, gives oxy-fluoride of phosphorus, POF₃ (with a large amount of water it gives PH₃O₄), which Moissan (1891) obtained by the action of dry HF upon P₂O₅, and Thorpe and Tutton (1890) by heating a mixture of cryolite and P₂O₅. It is a gas of normal density, like PF₃, and was obtained by Moissan by the action of fluorine upon PF₃ (PSF₃, see Chapter XX., Note 20). Thus the forms PX₃ and PX₅ not only exist in many solid and non-volatile substances, but also as vapours.