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Phosphate



A phosphate, in inorganic chemistry, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Phosphates are important in biochemistry and biogeochemistry.

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Contents

Chemical properties

 

  The phosphate ion is a polyatomic ion with the empirical formula PO43− and a molar mass of 94.973 g/mol; it consists of one central phosphorus atom surrounded by four identical oxygen atoms in a tetrahedral arrangement. The phosphate ion carries a negative three formal charge and is the conjugate base of the hydrogenphosphate ion, HPO42−, which is the conjugate base of H2PO4, the dihydrogen phosphate ion, which in turn is the conjugate base of H3PO4, phosphoric acid. It is a hypervalent molecule (the phosphorus atom has 10 electrons in its valence shell). Phosphate is also an organophosphorus compound with the formula OP(OR)/* Chemical properties */ 3

A phosphate salt forms when a positively-charged ion attaches to the negatively-charged oxygen atoms of the ion, forming an ionic compound. Many phosphates are not soluble in water at standard temperature and pressure.

In dilute aqueous solution, phosphate exists in four forms. In strongly-basic conditions, the phosphate ion (PO43−) predominates, whereas in weakly-basic conditions, the hydrogen phosphate ion (HPO42−) is prevalent. In weakly-acid conditions, the dihydrogen phosphate ion (H2PO4) is most common. In strongly-acid conditions, aqueous phosphoric acid (H3PO4) is the main form.

More precisely, considering the following three equilibrium reactions:

H3PO4 ⇌ H+ + H2PO4
H2PO4 ⇌ H+ + HPO42−
HPO42− ⇌ H+ + PO43−

the corresponding constants at 25°C (in mol/L) are (see phosphoric acid):

K_{a1}=\frac{[\mbox{H}^+][\mbox{H}_2\mbox{PO}_4^-]}{[\mbox{H}_3\mbox{PO}_4]}\simeq 7.5\times10^{-3}
K_{a2}=\frac{[\mbox{H}^+][\mbox{HPO}_4^{2-}]}{[\mbox{H}_2\mbox{PO}_4^-]}\simeq 6.2\times10^{-8}
K_{a3}=\frac{[\mbox{H}^+][\mbox{PO}_4^{3-}]}{[\mbox{HPO}_4^{2-}]}\simeq 2.14\times10^{-13}

For a strongly-basic pH (pH=13), we find

\frac{[\mbox{H}_2\mbox{PO}_4^-]}{[\mbox{H}_3\mbox{PO}_4]}\simeq 7.5\times10^{10}  \mbox{ , }\frac{[\mbox{HPO}_4^{2-}]}{[\mbox{H}_2\mbox{PO}_4^-]}\simeq 6.2\times10^5 \mbox{ , } \frac{[\mbox{PO}_4^{3-}]}{[\mbox{HPO}_4^{2-}]}\simeq 2.14

showing that only PO43− and HPO42− are in significant amounts.

For a neutral pH (for example the cytosol pH=7.0), we find

\frac{[\mbox{H}_2\mbox{PO}_4^-]}{[\mbox{H}_3\mbox{PO}_4]}\simeq 7.5\times10^4 \mbox{ , }\frac{[\mbox{HPO}_4^{2-}]}{[\mbox{H}_2\mbox{PO}_4^-]}\simeq 0.62 \mbox{ , } \frac{[\mbox{PO}_4^{3-}]}{[\mbox{HPO}_4^{2-}]}\simeq 2.14\times10^{-6}

so that only H2PO4 and HPO42− ions are in significant amounts (62% H2PO4, 38% HPO42−). Note that in the extracellular fluid (pH=7.4), this proportion is inverted (61% HPO42−, 39% H2PO4).

For a strongly-acid pH (pH=1), we find

\frac{[\mbox{H}_2\mbox{PO}_4^-]}{[\mbox{H}_3\mbox{PO}_4]}\simeq 0.075 \mbox{ , }\frac{[\mbox{HPO}_4^{2-}]}{[\mbox{H}_2\mbox{PO}_4^-]}\simeq 6.2\times10^{-7} \mbox{ , } \frac{[\mbox{PO}_4^{3-}]}{[\mbox{HPO}_4^{2-}]}\simeq 2.14\times10^{-12}

showing that H3PO4 is dominant with respect to H2PO4. HPO42− and PO43− are practically absent.

Phosphate can form many polymeric ions, diphosphate (also pyrophosphate), P2O74−, triphosphate, P3O105−, et cetera. The various metaphosphate ions have an empirical formula of PO3 and are found in many compounds.

Phosphate deposits can contain significant amounts of naturally-occurring uranium. Subsequent uptake of such soil amendments can lead to crops containing uranium concentrations.

See also

References

  1. ^ Campbell, Neil A.; Reece, Jane B. (2005). Biology, Seventh Edition, San Francisco, California: Benjamin Cummings, 65. ISBN 0-8053-7171-0. 

3. "Figuring Out Phosphates," Food Product Design, June 2006, Lynn A. Kuntz

Further reading

Schmittner Karl-Erich and Giresse Pierre, 1999. Micro-environmental controls on biomineralization: superficial processes of apatite and calcite precipitation in Quaternary soils, Roussillon, France. Sedimentology 46/3: 463-476.

External links

  • Website of the Technische Universität Darmstadt and the CEEP about Phosphorus Recovery
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Phosphate". A list of authors is available in Wikipedia.
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