PHOSPHOROUS OXIDE (V): STRUCTURE, PROPERTIES, OBTAINING, USES, RISKS - CHEMISTRY - 2025

The phosphorus oxide (v) is an inorganic solid formed by oxygen and phosphorus (P) (O). Its empirical formula is P ₂ O ₅, while its correct molecular formula is P ₄ O ₁₀. It is a very hygroscopic white solid, that is, it can absorb water from the air very easily, reacting immediately with it. The reaction can be dangerous as it causes a rapid rise in temperature.

Its high tendency to absorb water has led to it being used as a drying agent in chemical laboratories, as well as a dehydrator of some compounds, that is, to eliminate water from its molecules.

Phosphorous oxide (v) powder, P ₄ O ₁₀. LHcheM. Source: Wikimedia Commons.

Phosphorous oxide (v) is also used to accelerate the binding reactions of various hydrocarbon molecules, a reaction called condensation. In addition, it allows converting certain organic acids into esters.

It has been used for example to refine gasoline, to prepare phosphoric acid H ₃ PO ₄, to obtain compounds that serve to retard fire, to make glass for vacuum applications, among many other uses.

Phosphorous oxide (v) must be kept in tightly closed containers to prevent it from coming into contact with moisture in the air. It is corrosive and can damage eyes, skin and mucous membranes.

Structure

Phosphorous oxide (v) is made up of phosphorus (P) and oxygen (O), where phosphorus has a valence of +5 and oxygen -2. The phosphorous oxide molecule (v) has four phosphorus and ten oxygen atoms and that is why its correct molecular formula is P ₄ O ₁₀.

Structure of the phosphorous oxide molecule (v), P ₄ O ₁₀. Author: Benjah-bmm27. Source: Wikimedia Commons.

It exists in three crystalline forms, as amorphous powder and in a vitreous form (as glass). In the hexagonal crystalline form, each of the phosphorous atoms is found at the vertices of a tetrahedron.

Nomenclature

- Phosphorous oxide (v)

- Phosphorous pentoxide

- Diphosphorous pentoxide

- Phosphoric pentoxide

- Phosphoric anhydride

- Tetraphosphorus decaoxide

Properties

Physical state

Crystalline white solid. The most common form is that of hexagonal crystals.

Molecular weight

283.89 g / mol

Melting point

562 ºC

Sublimation temperature

360 ºC at 1 atmosphere pressure. It means that at this temperature it goes from solid to gas without going through the liquid state.

Density

2.30 g / cm ³

Solubility

Very soluble in water. Soluble in sulfuric acid. Insoluble in acetone and ammonia.

Chemical properties

Phosphorous oxide (v) absorbs and reacts with water from the air extremely rapidly, forming phosphoric acid H ₃ PO ₄. This reaction is exothermic, which means that heat is produced during it.

Reaction of phosphorus oxide (v) with water to form phosphoric acid H ₃ PO ₄. Author: Marilú Stea.

The reaction of P ₄ O ₁₀ with water leads to the formation of a mixture of phosphoric acids whose composition depends on the amount of water and the conditions.

The reaction with alcohols leads to the formation of esters of phosphoric acid or polymeric acids depending on the experimental conditions.

P ₄ O ₁₀ + 6 ROH → 2 (RO) ₂ PO.OH + 2 RO.PO (OH) ₂

With basic oxides it forms solid phosphates.

It is corrosive. It can react dangerously with formic acid and with inorganic bases such as sodium hydroxide (NaOH), calcium oxide (CaO) or sodium carbonate Na ₂ CO ₃.

If a solution of perchloric acid HClO ₄ and chloroform CHCl ₃ is poured into phosphorous oxide (v) P ₄ O ₁₀, a violent explosion occurs.

Other properties

It is not flammable. It does not promote combustion. However, its reaction with water is so violent and exothermic that there may be a risk of fire.

Obtaining

It can be prepared by direct oxidation of phosphorus in a stream of dry air. When phosphorus comes into contact with excess oxygen, it oxidizes to form phosphorous oxide (v).

P ₄ + 5 O ₂ → P ₄ O ₁₀

Presence in nature

Phosphorous (v) oxide is found in minerals such as ilmenite, rutile, and zircon.

Ilmenite is a mineral that contains iron and titanium and sometimes has phosphorus oxide (v) in concentrations that vary between 0.04 and 0.33% by weight. Rutile is a titanium oxide mineral and can contain about 0.02% by weight of P ₂ O ₅.

Zircon sands (a mineral of the element zirconium) have phosphorus oxide (v) in 0.05-0.39% by weight.

Applications

As a dehydrating and drying agent

Due to its great greed for water, it is one of the best known dehydrating agents and very effective at temperatures below 100 ° C.

It can extract water from substances that are themselves considered dehydrating agents. For example, you can remove water from sulfuric acid H ₂ SO _{4 by} converting it to SO ₃ and from nitric acid HNO _{3 by} converting it to N ₂ O ₅.

Dehydration of sulfuric acid by phosphorus oxide (v). Author: Marilú Stea.

Basically it can dry all those liquids and gases with which it does not react, so it allows to remove traces of humidity from vacuum systems.

In organic chemistry reactions

Phosphorous oxide (v) serves to close rings of organic compounds and other condensation reactions.

It allows the esterification of organic acids with the possibility of distinguishing between primary aliphatic carboxylic acids (carbon chain without rings with the –COOH group at one end) and aromatic acids (–COOH group attached to the benzene ring), since the latter do not react.

It also serves to remove a molecule of H ₂ O from the amides R (C = O) NH ₂ and converts them into nitriles R-CN. In addition, it catalyzes or accelerates oxygenation, dehydrogenation and polymerization reactions of bitumen.

P ₄ O ₁₀ is widely used in organic chemistry laboratories. Author: jdn2001cn0. Source: Pixabay.

In fuel refining

Since the 1930s, certain studies indicated that phosphorus (v) oxide exerted a refining action on gasoline, increasing its octane number.

The refining action of P ₄ O ₁₀ is mainly due to condensation reactions (union of different molecules) and not polymerization (union of equal molecules).

P ₄ O ₁₀ accelerates the direct alkylation of aromatic hydrocarbons with olefins, the conversion of olefins to naphthenes and their partial polymerization. The alkylation reaction increases the octane number of gasoline.

In this way a high quality refined gasoline is obtained.

Some petroleum derivatives can be improved by the action of P ₄ O ₁₀ on their molecules. Author: drpepperscott230. Source: Pixabay.

In various applications

Phosphorous oxide (v) is used to:

- Prepare phosphoric acid H ₃ PO ₄

- Obtain acrylate esters and surfactants

- Prepare phosphate esters that are used as flame retardants, solvents and diluents

- Conversion of phosphorous trichloride to phosphorus oxychloride

- Laboratory reagent

- Manufacture special glasses for vacuum tubes

- Increase the melting point of the asphalt

- Serve as a standard molecule in the determination of phosphorus or phosphates in phosphate rock, fertilizers and Portland cement, in the form of P ₂ O ₅

- Improve the bonds between certain polymers and the ivory layer that teeth have

Some special glasses such as those for vacuum tubes require the use of P ₄ O ₁₀ during their manufacture. Tvezymer. Source: Wikimedia Commons.

Risks

Phosphorous (v) oxide should be kept in sealed containers and in cool, dry, well-ventilated places.

This serves to prevent it from coming into contact with water, as it can react violently with it, generating a lot of heat, to the point of burning nearby materials that are combustible.

Phosphorous (v) oxide dust is irritating to the eyes and respiratory tract and corrosive to the skin. May cause eye burns. If swallowed, it causes fatal internal burns.

References

1. US National Library of Medicine. (2019). Phosphoric anhydride. Recovered from pubchem.ncbi.nlm.nih.gov.
2. Nayler, P. (2001). Bitumens: Modified. Chemical Modification. In Encyclopedia of Materials: Science and Technology. Recovered from sciencedirect.com.
3. Malishev, BW (1936). Phosphorus Pentoxide as a Refining Agent for Gasoline. Industrial & Engineering Chemistry 1936, 28, 2, 190-193. Recovered from pubs.acs.org.
4. Epps, Jr. EA (1950). Photometric Determination of Available Phosphorus Pentoxide in Fertlizers. Analytical Chemistry 1950, 22, 8, 1062-1063. Recovered from pubs.acs.org.
5. Banerjee, A. et al. (1983). Use of Phosphorus Pentoxide: Esterification of Organic Acids. J. Org. Chem. 1983, 48, 3108-3109. Recovered from pubs.acs.org.
6. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
7. Kirk-Othmer (1994). Encyclopedia of Chemical Technology. Fourth Edition. John Wiley & Sons.
8. Ogliari, FA et al. (2008). Synthesis of phosphate monomers and bonding to dentin: Esterification methods and use of phosphorus pentoxide. Journal of Dentistry, Volume 36, Issue 3, March 2008, pages 171-177. Recovered from sciencedirect.com.