Phosphorus Pentoxide, P₂O₅

Vigorous combustion of phosphorus in air produces a voluminous white powder which is very deliquescent and hisses when dropped into water, evolving much heat and giving a liquid of acid reaction. These salient facts were observed by the early workers on phosphorus, e.g. by Robert Boyle in 1681. The compound was analysed by many of the leading chemists at the beginning of the nineteenth century, and the empirical formula P₂O₅ was established.

This oxide is formed by combustion, in a full supply of air or oxygen, of white phosphorus (ignition temperature about 60° c.), of phosphorous oxide (50°-70° c.), of red phosphorus (about 260° c.), and of the phosphines and other combustible compounds. The white powder prepared in the laboratory or technically by these methods is always impure, containing a little phosphorous oxide, metaphosphoric acid, etc., while a part of the phosphorus usually escapes combustion and remains as red phosphorus. The preparation of a pure product requires further treatment. Thus the pentoxide may be thrown into a red-hot porcelain basin and stirred in a current of oxygen. The crude oxide may also be resublimed in a current of oxygen and passed over platinised asbestos or platinum sponge. The product used by Baker in experiments on intensive drying was also distilled, without the aid of a catalyst, in a current of oxygen at 180° to 210° c. The yield was about 10 per cent. A convenient apparatus for this preparation was described by Finch and Peto. The ordinary pentoxide was pushed continually through a glass tube down the vertical limb of a heated iron T-piece which was traversed by a current of oxygen. The product, which was collected in a wide-mouthed glass bottle, was partly crystalline and partly amorphous.

The purified oxide should be devoid of any alliaceous odour or odour of phosphine, and should not reduce a boiling solution of mercuric chloride. When dissolved in water and the solution neutralised with alkali (to methyl orange) it should give a white precipitate with silver nitrate which should not darken after boiling for five minutes.

The pentoxide exists in crystalline and vitreous forms, the transformation temperature of which has been given as 440° c. Sublimation proceeds with moderate speed between 180° and 250° c. and the vapour pressure may reach 760 mm. at 360° c. When it is sublimed at 360° c. in a current of oxygen the oxide forms brilliant crystals together with some of the amorphous material which is considered to be a product of polymerisation. The crystalline form, by x-ray examination, is that of the rhombohedral system with 12P₂O₅ in a unit cell; the lengths of the axes are a = 11.12, b = 1.12 Å.

The melting-point was found to vary between 560° and 570° C. according to the time of heating.

The vapour density indicated a molecular weight of 336 at a red heat and 300 at 1400° C. At the higher temperature, therefore, the molecule approximates to P₄O₁₀ (M = 284).

The inconsistent behaviour on sublimation suggests that phosphorus pentoxide, like sulphur trioxide and phosphorus itself (q.v.), contains at least two crystalline forms, a metastable form with a higher and a stable form with a lower vapour pressure. These are present as a mixture below 300° C., at which temperature the vapour pressure of the metastable form becomes appreciable and then increases rapidly, reaching 3.5 atm. at 400° C. From this temperature upwards the pressure of the stable form becomes appreciable and equilibrium conditions are more easily obtained. The more volatile form may be sublimed away; consequently an abrupt fall of pressure above 400° C. has been observed. A stable vapour pressure curve has been obtained up to and beyond the triple point. Pure dry oxygen was passed over English P₂O₅ which was heated to 270° C. The sublimate was received directly in the tensimeter, which was contained in an electric furnace wound with nichrome wire which gave temperatures above 400° C., which were controlled within ±2° C. The pressures were measured by a glass manometer with quartz thread indicator. The following typical values are taken from the tabulated results:—

Vapour pressures of phosphorus pentoxide

t° C	Solid.			Liquid.
	449	522.5	557.5	596.5	613.5	656	700
p cm. mercury	2.3	12.6	32.7	68.7	83.9	135.6	216

The triple point is found at 55.5 cm. of mercury and 580° C.

The affinity of formation of P₂O₅ evidently is very great, since no dissociation was observed at the highest temperatures mentioned above. When heated in the oxyhydrogen flame the oxide gives a continuous spectrum. In respect of this great stability P₂O₅ differs markedly from its congeners N₂O₅ and As₂O₅.

The heat of formation is also by far the highest in the Group and is higher than the heat of oxidation per atom of any other non-metal. The value per mol of solid P₂O₅ from solid white phosphorus and gaseous oxygen is given as 369.9 Cals., 369.4 Cals. The heats of formation from red phosphorus are rather lower.

Phosphoric oxide has also an exceptionally high affinity of hydration, on which account it is universally used, where its chemical properties permit, as the most powerful drying agent for neutral or acid gases and liquids and also in desiccators. The heat of hydration of crystalline P₂O₅ is given as 44.6 Cals., 40.8 Cals. The amorphous variety when dissolved in much water evolved 33.8 Cals. and the vitreous variety 29.1 Cals. Hence heat is evolved when the crystalline variety is transformed into the amorphous variety.

Many possible structures may be assigned to the molecule. Whichever of these is adopted the phosphorus must be quinquevalent according to the old representation, or quadrivalent with a semipolar bond according to the "octet " theory. Thus

or

Phosphorus pentoxide was reduced by hydrogen at a red heat in the presence of nickel, and by carbon at high temperatures, but not by silicon.

The alkali and alkaline earth metals react with great energy when heated with the oxide, giving oxides and phosphides.

When heated with halides of phosphorus the two molecules give oxyhalides, the change being represented by the equation

P₄O₁₀ + 6PCl₅ = 10POCl₃

or P₄O₁₀ + 6PBr₅ = 10POBr₃

The pentoxide also gives the oxychloride when heated with the halides of other non-metals, although the other non-metal does not necessarily give oxyhalide also. Thus with boron trichloride

P₂O₅ + 2BCl₃ = POCl₃.BCl₃ + BPO₄

and with carbon tetrachloride in a sealed tube between 200° and 300°

P₂O₅ + 2CCl₄ = COCl₂ + CO₂ + 2POCl₃

With an excess of P₂O₅

2P₂O₅ + 3CCl₄ = 3CO₂ + 4POCl₃

Oxyhalides of phosphorus are also produced when the pentoxide is heated with fluorides or chlorides of the alkali and alkaline earth metals; thus POF₃ is produced with calcium fluoride and POCl₃ with sodium chloride.

When brought into contact with water under various conditions phosphorus pentoxide forms one or more of differently hydrated acids. Metaphosphoric acid (q.v.) probably is the first product.

The pentoxide has a powerful dehydrating effect upon oxyacids and is therefore used in preparing anhydrides from these. It also removes halogen from halogen hydracids under some conditions, giving oxyhalides, e.g. POF₃ from HF. Hydrogen chloride is completely absorbed by the oxide and gives a liquid from which POCl₃ can be distilled.

Phosphorus pentoxide cannot be oxidised further by ordinary oxidising agents, except to compounds of the " peracid " type. The oxygen is displaced by metathesis as just shown, but not by anhydrous halogens, except fluorine, which at a dull red heat gives PF₅ and POF₃.

Dry ammonia gives amido- or imido-phosphoric acids (q.v.), and also salts such as diammonium amidopyrophosphate.