Phosphorus Oxides

the compounds of phosphorus and oxygen. The known oxides are the suboxide P₄O, the oxide P₄O ₂ (P₂O), the peroxide P₂O₆ (PO₃), the trioxide (phosphorous anhydride) P₄O₆ (P₂O₃), the pentoxide (phosphoric anhydride) P₄O ₁₀ (P₂O₅), and the tetroxide (PO₂)_n. The most important are phosphoric anhydride, phosphorous anhydride, and phosphorus tetroxide.

Phosphoric anhydride, P₄O₁₀ (P₂O₅), is an unusually hygroscopic white powder prone to polymorphism. (The exact number of modifications has not been established.) In P₄O₁₀, the phosphorus atom is surrounded by four oxygen atoms (tetrahedral structure), three of which serve as the vertices of three adjoining PO₄ tetrahedra, forming P—O—P bonds. The commercial product is a white, snowy mass with a density of 2.28–2.31 g/cm³, a sublimation point of 358°–362°C, and a melting point of 420°C. It contains mainly the crystalline hexagonal modification (H form) with an admixture of the amorphous modification. The composition of the H form is P₄O₁₀; the two remaining crystalline modifications, whose structures are polymeric, have not been thoroughly studied.

Phosphoric anhydride exhibits a strong dehydrating effect, which makes it possible to remove not only adsorbed water but also water of crystallization and even water of constitution. The compound dissolves in water with the liberation of heat, forming polymeric phosphoric acids (cyclic and linear); with a sufficiently large amount of water, orthophosphoric acid is eventually formed. Phosphoric anhydride reacts with basic oxides to yield phosphates, with halides to yield oxyhalides, and with metals to yield a mixture of phosphates and phosphides; it reacts freely with all organic substances that are basic in character. Phosphoric anhydride reacts with dry and moist ammonia to form ammonium phosphates, which contain P—NH—P bonds in addition to P—O—P bonds. Light causes P₄O₁₀ to luminesce.

In industry, P₄O₁₀ is produced by burning elemental phosphorus in an excess of dry air with subsequent condensation of the solid product from the vapor. Sublimation is used to remove impurities (phosphoric acids). Phosphoric anhydride in vapor or smoke form dries the mucosa and induces coughing, asthma, pulmonary edema, and skin burns; safety regulations must therefore be observed.

Phosphoric anhydride is used to remove water from gases and liquids (those not reacting with P₄O₁₀). In organic and inorganic synthesis, it serves as a condensing agent, and it is sometimes used as a component of phosphate glasses and as a catalyst.

Phosphorous anhydride, P₄O₆ (P₂O₃), is a colorless, flocculent substance with a monoclinic crystal structure, a density of 2.135 g/cm³, a melting point of 23.8°C, and a boiling point of 175.4°C; it is soluble in carbon disulfide and benzene. The dissolution of P₄O₆ in cold water yields phosphorous acid, H₃PO₃; in hot water, elemental phosphorus, phosphine, phosphoric acid, and other compounds are formed. When heated above 210°C, phosphorus trioxide dissociates into PO₂ and red phosphorus. The compound is readily oxidized by air to the pentoxide. The trioxide is obtained by the oxidation of phosphorus under conditions where the amount of air is limited. Phosphorus trioxide is widely used in organic synthesis.

Phosphorus tetroxide, (PO₂)_n, is a flocculent white powder that forms lustrous crystals after sublimation; its density is 2.54 g/cm³ at 22.6°C. Data have been obtained on the polymeric structure of the tetroxide. Phosphorus tetroxide is freely soluble in water, with which it interacts to form mainly H₃PO₃ and condensed polyphosphoric acids, as well as a small quantity of PH₃. Like the trioxide, it can be obtained by burning phosphorus at a low temperature with a limited amount of air. It can also be obtained by heating P₄O₆ in a sealed tube at 250°C with subsequent purification.