||This article's lead section may not adequately summarize key points of its contents. (October 2012)|
|Jmol-3D images||Image 1|
|Melting point||340 °C (644 °F; 613 K)|
|Boiling point||360 °C (sublimes)|
|Solubility in water||exothermic hydrolysis|
|Vapor pressure||1 mmHg @ 385 °C|
|EU classification||not listed|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Phosphorus pentoxide is a chemical compound with molecular formula P4O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrating agent.
Phosphorus pentoxide crystallizes in at least four forms or polymorphs. The most familiar one, a metastable form, shown in the figure, comprises molecules of P4O10. Weak van der Waals forces hold these molecules together in a hexagonal lattice (However, in spite of the high symmetry of the molecules, the crystal packing is not a close packing). The structure of the P4O10 cage is reminiscent of adamantane with Td symmetry point group. It is closely related to the corresponding anhydride of phosphorous acid, P4O6. The latter lacks terminal oxo groups. Its density is 2.30 g/cm3. It boils at 423 °C under atmospheric pressure; if heated more rapidly it can sublimate. This form can be made by condensing the vapor of phosphorus pentoxide rapidly, the result is an extremely hygroscopic solid.
The other polymorphs are polymeric, but in each case the phosphorus atoms are bound by a tetrahedron of oxygen atoms, one of which forms a terminal P+–O– bond (the common representation of a P=O bond as in the image above is incorrect as computational chemistry has found that an expanded d-orbital model is not a very stabilising interaction and therefore not an important contributor to bonding). The macromolecular form can be made by heating the compound in a sealed tube for several hours, and maintaining the melt at a high temperature before cooling the melt to the solid. The metastable orthorhombic, "O"-form (density 2.72 g/cm3, melting point 562 °C), adopts a layered structure consisting of interconnected P6O6 rings, not unlike the structure adopted by certain polysilicates. The stable form is a lower density phase, also orthorhombic, the so-called O' form. It consists of a 3-dimensional framework, density 3.5 g/cm3. The remaining polymorph is a glass or amorphous form; it can be made by fusing any of the others.
P4O10 is prepared by burning elemental phosphorus with sufficient supply of air:
- P4 + 5 O2 → P4O10
For most of the 20th century, phosphorus pentoxide was used to provide a supply of concentrated pure phosphoric acid. In the thermal process, the phosphorus pentoxide obtained by burning white phosphorus was dissolved in dilute phosphoric acid to produce concentrated acid. Improvements in filter technology is leading to the "wet phosphoric acid process" taking over from the thermal process, obviating the need to produce white phosphorus as a starting material. The dehydration of phosphoric acid to give phosphorus pentoxide is not practicable; on heating, metaphosphoric acid will decompose before it loses water.
Phosphorus pentoxide is a potent dehydrating agent as indicated by the exothermic nature of its hydrolysis:
- P4O10 + 6 H2O → 4 H3PO4 (–177 kJ)
However, its utility for drying is limited somewhat by its tendency to form a protective viscous coating that inhibits further dehydration by unspent material. A granular form of P4O10 is used in desiccators.
- P4O10 + RC(O)NH2 → P4O9(OH)2 + RCN
The indicated coproduct P4O9(OH)2 is an idealized formula for undefined products resulting from the hydration of P4O10.
- P4O10 + RCO2H → P4O9(OH)2 + [RC(O)]2O
The desiccating power of P4O10 is strong enough to convert many mineral acids to their anhydrides. Examples: HNO3 is converted to N2O5; H2SO4 is converted to SO3; HClO4 is converted to Cl2O7; HCF3SO3 is converted to (CF3)2S2O5.
Related phosphorus oxides
Phosphorus pentoxide is not flammable. It reacts vigorously with water and water-containing substances like wood or cotton, liberates much heat and may even cause fire. It is corrosive to metal and is very irritating – may cause severe burn to the eye, skin, mucous membrane, and respiratory tract even at concentrations as low as 1 mg/m3.
- In Anthony Burgess' The Wanting Seed, phosphorus pentoxide is a highly prized compound.[clarification needed]
- In Detective Comics #825, Batman notices that phosphorus pentoxide was at the scene of a fire, indicating that the villain Dr. Phosphorus was involved.
- In Aldous Huxley's Point Counter Point, Lord Edward bemoans societal loss of phosphorous pentoxide to his assistant Illidge.
- In Aldous Huxley's Brave New World, Henry Foster tells Lenina about the recovery of phosphorus pentoxide.
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.
- Cruickshank, D.W.J. (1964). "Refinements of Structures Containing Bonds between Si, P, S or Cl and O or N: V. P4O10". Acta Cryst. 17 (6): 677–9. doi:10.1107/S0365110X64001669.
- D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
- .Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 473. ISBN 978-0-13-175553-6.
- Thorsten Stefan, Rudolf Janoschek (2000). "How relevant are S=O and P=O Double Bonds for the Description of the Acid Molecules H2SO3, H2SO4, and H3PO4, respectively?". Molecular modeling annual 6 (2): 282–288. doi:10.1007/PL00010730.
- D. Stachel, I. Svoboda and H. Fuess (June 1995). "Phosphorus Pentoxide at 233 K". Acta Cryst. C51 (6): 1049–1050. doi:10.1107/S0108270194012126.
- Threlfall, Richard E., (1951). The story of 100 years of Phosphorus Making: 1851 - 1951. Oldbury: Albright & Wilson Ltd
- Podger, Hugh (2002). Albright & Wilson: The Last 50 Years. Studley: Brewin Books. ISBN 1-85858-223-7
- Meier, M. S. "Phosphorus(V) Oxide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.
- Joseph C. Salamone, ed. (1996). Polymeric materials encyclopedia: C, Volume 2. CRC Press. p. 1417. ISBN 0-8493-2470-X.
- Tidwell, T. T. "Dimethyl Sulfoxide–Phosphorus Pentoxide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.
- Luer, B.; Jansen, M. "Crystal Structure Refinement of Tetraphosphorus Nonaoxide, P4O9" Zeitschrift fur Kristallographie 1991, volume 197, pages 247-8.
- Phosphorus pentoxide MSDS
- Spec sheet
- Website of the Technische Universität Darmstadt and the CEEP about Phosphorus Recovery