Phosphorus pentoxide

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Phosphorus pentoxide
IUPAC name
Phosphorus pentoxide
Other names
Diphosphorus pentoxide
Phosphorus(V) oxide
Phosphoric anhydride
Tetraphosphorus decaoxide
Tetraphosphorus decoxide
Molar mass 283.886 g/mol
Appearance White powder
Odor Odorless
Density 2.39 g/cm3
Melting point 360 °C (680 °F; 633 K) (sublimes)
Boiling point Sublimes
Reacts exothermically
Solubility Reacts with ethanol, methanol, isopropanol
Soluble in sulfuric acid
Insoluble in acetic acid, acetone, ammonia, benzene, chloroform, dichloromethane, hexane, toluene, xylene
Vapor pressure 1 mmHg at 385 °C
Safety data sheet FisherScientific
Flash point Non-flammable
Related compounds
Related compounds
Dinitrogen pentoxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Phosphorus pentoxide or diphosphorus pentoxide is an inorganic chemical compound with molecular formula P4O10, more often written as P2O5.



Phosphorus pentoxide is the anhydride of phosphoric acid. It will react exothermically with water to yield the said acid.

P2O5 + 3 H2O → 2 H3PO4

Phosphorus pentoxide is also a very potent dehydrating agent. It is even capable of robbing water from sulfuric acid, allowing one to distill sulfur trioxide from it.

P2O5 + H2SO4 → H3PO4 + SO3

This can be a quick method of preparing SO3 without the use for contact process.


Phosphorous pentoxide is a white solid, with a pungent odor and strong deliquescent. Its density is 2.30 g/cm3. It boils at 423 °C under atmospheric pressure; if heated more rapidly it can sublimate.


Phosphorus pentoxide is sold by chemical suppliers, but is somewhat difficult to acquire. Storing it is also tricky due to its strong hygroscopy, though this can be limited by keeping the compound in a sealed bag.


Phosphorus pentoxide can be made by burning elemental phosphorus, in a moisture-free environment. One way is to burn phosphorus in a large stainless steel pot with a lid, which has been kept in a dry box. The resulting phosphorus pentoxide fumes from the fire are allowed to settle and cool, then they're scraped off the pot in a dry environment. If you don't need high purity pentoxide, you can do this in open air, but you must move quickly.




Phosphorus pentoxide is an extremely powerful desiccant, and contact with bare hand must be avoided. Contact with eyes will result in severe eye damage, even blindness.


Phosphorus pentoxide must be kept in closed and sealed bottles, away from any moisture. Never store it in aluminium, and galvanized or tin-plated containers! Stainless steel or glass containers are ideal.[1]


Phosphorus pentoxide can be neutralized by very slowly adding it to crushed ice, followed by addition of a cooled carbonate/bicarbonate solution.



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