Phosphate
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/* Cellular function */
A '''phosphate''', in [[inorganic chemistry]], is a [[Salt (chemistry)|salt]] of [[phosphoric acid]]. Inorganic phosphates are [[mining|mined]] to obtain [[phosphorus]] for use in industry.
<ref>[http://fipr1.state.fl.us/PhosphatePrimer ''Phosphate Primer'', website of the Florida Institute of Phosphate Research]</ref>
<ref>[http://www.foodproductdesign.com/articles/661ingredient2.html "Figuring Out Phosphates," ''Food Product Design'', June 2006, Lynn A. Kuntz]</ref>
In [[organic chemistry]], a phosphate, or [[organophosphate]], is an [[ester]] of phosphoric acid. Organic phosphates are important in [[biochemistry]] and [[biogeochemistry]].
== Chemical properties ==
[[Image:Phosphate.png|thumb|right|The general chemical structure of a phosphate]]
[[Image:Phosphate Group.PNG|right|thumb|This is the [[structural formula]] of the phosphoric acid [[functional group]] as found in a weakly [[acidic]] [[aqueous solution]]. In more [[basic (chemistry)|basic]] aqueous solutions, the group donates the two [[hydrogen]] [[atom]]s and [[ion]]izes as a phosphate group with a negative charge of 2.
<ref>{{cite book | last = Campbell | first = Neil A. | authorlink = Neil Campbell | coauthors = Reece, Jane B. | title = Biology | edition = Seventh Edition | publisher = Benjamin Cummings | date = 2005 | location = [[San Francisco, California]] | pages = 65 | id = ISBN 0-8053-7171-0 }}</ref>]]
The phosphate ion is a [[polyatomic ion]] with the [[empirical formula]] [[phosphorus|P]][[oxygen|O]]<sub>4</sub><sup>3−</sup> and a [[molar mass]] of 94.973 g/mol; it consists of one central [[phosphorus]] atom surrounded by four identical oxygen atoms in a [[tetrahedron|tetrahedral]] arrangement. The phosphate ion carries a negative three formal charge and is the [[conjugate acid|conjugate base]] of the hydrogenphosphate ion, H<sub></sub>PO<sub>4</sub><sup>2−</sup>, which is the conjugate base of H<sub>2</sub>PO<sub>4</sub><sup>−</sup>, the dihydrogen phosphate ion, which in turn is the conjugate base of H<sub>3</sub>PO<sub>4</sub>, [[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)<sub>3</sub>
A phosphate salt forms when a positively-charged ion attaches to the negatively-charged oxygen atoms of the ion, forming an ionic [[chemical compound|compound]]. Many phosphates are not [[solubility|soluble]] in [[water]] at [[standard temperature and pressure]].
In dilute aqueous solution, phosphate exists in four forms. In strongly-basic conditions, the phosphate ion (PO<sub>4</sub><sup>3−</sup>) predominates, whereas in weakly-basic conditions, the hydrogen phosphate ion (HPO<sub>4</sub><sup>2−</sup>) is prevalent. In weakly-acid conditions, the dihydrogen phosphate ion (H<sub>2</sub>PO<sub>4</sub><sup>−</sup>) is most common. In strongly-acid conditions, aqueous phosphoric acid (H<sub>3</sub>PO<sub>4</sub>) is the main form.
<gallery>
Image:3-phosphoric-acid-3D-balls.png|<center>H<sub>3</sub>PO<sub>4</sub></center>
Image:2-dihydrogenphosphate-3D-balls.png|<center>H<sub>2</sub>PO<sub>4</sub><sup>−</sup></center>
Image:1-hydrogenphosphate-3D-balls.png|<center>HPO<sub>4</sub><sup>2−</sup></center>
Image:0-phosphate-3D-balls.png|<center>PO<sub>4</sub><sup>3−</sup></center>
</gallery>
More precisely, considering the following three equilibrium reactions:
:H<sub>3</sub>PO<sub>4</sub> {{Unicode|⇌}} H<sup>+</sub> + H<sub>2</sub>PO<sub>4</sub><sup>−</sup>
:H<sub>2</sub>PO<sub>4</sub><sup>−</sup> {{Unicode|⇌}} H<sup>+</sub> + HPO<sub>4</sub><sup>2−</sup>
:HPO<sub>4</sub><sup>2−</sup> {{Unicode|⇌}} H<sup>+</sub> + PO<sub>4</sub><sup>3−</sup>
the corresponding constants at 25°C (in mol/L) are (see [[phosphoric acid]]):
: <math> K_{a1}=\frac{[\mbox{H}^+][\mbox{H}_2\mbox{PO}_4^-]}{[\mbox{H}_3\mbox{PO}_4]}\simeq 6.92\times10^{-3}</math>
: <math>K_{a2}=\frac{[\mbox{H}^+][\mbox{HPO}_4^{2-}]}{[\mbox{H}_2\mbox{PO}_4^-]}\simeq 6.17\times10^{-8}</math>
: <math> K_{a3}=\frac{[\mbox{H}^+][\mbox{PO}_4^{3-}]}{[\mbox{HPO}_4^{2-}]}\simeq 4.79\times10^{-13}</math>
For a strongly-basic pH (pH=13), we find
: <math>\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</math>
showing that only PO<sub>4</sub><sup>3−</sup> and HPO<sub>4</sub><sup>2−</sup> are in significant amounts.
For a neutral pH (for example the cytosol pH=7.0), we find
: <math> \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}</math>
so that only H<sub>2</sub>PO<sub>4</sub><sup>−</sup> and HPO<sub>4</sub><sup>2−</sup> ions are in significant amounts (62% H<sub>2</sub>PO<sub>4</sub><sup>−</sup>, 38% HPO<sub>4</sub><sup>2−</sup>). Note that in the extracellular fluid (pH=7.4), this proportion is inverted (61% HPO<sub>4</sub><sup>2−</sup>, 39% H<sub>2</sub>PO<sub>4</sub><sup>−</sup>).
For a strongly-acid pH (pH=1), we find
: <math>\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}</math>
showing that H<sub>3</sub>PO<sub>4</sub> is dominant with respect to H<sub>2</sub>PO<sub>4</sub><sup>−</sup>. HPO<sub>4</sub><sup>2−</sup> and PO<sub>4</sub><sup>3−</sup> are practically absent.
Phosphate can form many polymeric ions such as [[diphosphate]] (also [[pyrophosphate]]), P<sub>2</sub>O<sub>7</sub><sup>4−</sup>, and [[triphosphate]], P<sub>3</sub>O<sub>10</sub><sup>5−</sup>. The various [[metaphosphate]] ions have an empirical formula of PO<sub>3</sub><sup>−</sup> and are found in many compounds.
Phosphate deposits can contain significant amounts of naturally occurring [[uranium]]. Uptake of these substances by plants can lead to high uranium concentrations in crops.
== Cellular function ==
Phosphate is useful in animal [[cell]]s as a [[buffering agent]]. The kinds of phosphate that are useful as buffers include NaH<sub>2</sub>PO<sub>4</sub> and H<sub>2</sub>PO<sub>4</sub><sup>-</sup>.
== See also ==
<div style="-moz-column-count:2; column-count:2;">
*[[organophosphorus]] compounds
*[[Phosphine]] - PR<sub>3</sub>
*[[Phosphine oxide]] - OPR<sub>3</sub>
*[[Phosphinite]] - P(OR)R<sub>2</sub>
*[[Phosphonite]] - P(OR)<sub>2</sub>R
*[[Phosphite]] - P(OR)<sub>3</sub>
*[[Phosphinate]] - OP(OR)R<sub>2</sub>
*[[Phosphonate]] - OP(OR)<sub>2</sub>R
*Phosphate - OP(OR)<sub>3</sub>, such as [[triphenyl phosphate]]
*[[Polyphosphate]] - P<sub>n</sub>
</div>
== Further reading ==
{{Expert-subject|Chemicals}}
* 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.
* http://www.fluoridealert.org/phosphate/overview.htm#9, discusses environmental hazards of the phosphate fertilizer industry
== References ==
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== External links ==
* [http://www.phosphorus-recovery.tu-darmstadt.de Website of the Technische Universität Darmstadt and the CEEP about Phosphorus Recovery]
[[Category:Functional groups]]
[[Category:Phosphorus compounds]]
[[Category:Organophosphorus]]
[[Category:Organophosphates]]
[[Category:Phosphates]]
[[Category:Oxoanions]]
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