Editing Phosphate (section)

==Chemical properties==
The phosphate ion has a [[molar mass]] of 94.97&nbsp;g/mol, and consists of a central [[phosphorus]] atom surrounded by four [[oxygen]] atoms in a [[tetrahedron|tetrahedral]] arrangement. It is the [[conjugate acid|conjugate base]] of the hydrogen phosphate ion {{chem2|[HPO4](2-)}}, which in turn is the conjugate base of the dihydrogen phosphate ion {{chem2|[H2PO4]-}}, which in turn is the conjugate base of [[phosphoric acid|orthophosphoric acid]], {{chem2|H3PO4}}.

Many phosphates are [[solubility|soluble]] in [[water]] at [[standard temperature and pressure]]. The sodium, potassium, [[rubidium]], [[caesium]], and [[ammonium phosphates]] are all water-soluble. Most other phosphates are only slightly soluble or are insoluble in water. As a rule, the hydrogen and dihydrogen phosphates are slightly more soluble than the corresponding phosphates.

===Equilibria in solution===
[[File:PiSpeciation.svg|alt=|thumb|265x265px|Phosphoric acid [[Ion speciation|speciation]]]]
In water solution, orthophosphoric acid and its three derived anions coexist according to the dissociation and recombination equilibria below<ref>{{cite book|last = Campbell|first = Neil A.|author-link = Neil Campbell (scientist)|author2=Reece, Jane B.|title = Biology|edition = Seventh|publisher = [[Benjamin Cummings]]|year = 2005|location = San Francisco, California|page = 65|isbn = 0-8053-7171-0 }}</ref>
{| class="wikitable"
! Equilibrium
! Dissociation constant ''K''<sub>a</sub><ref name=pow2005/>
! p''K''<sub>''a''</sub>
|-
| {{chem2|H3PO4 <-> H2PO4- + H+}}
| <math chem>K_{a1} = \frac{[\ce{H+}][\ce{H2PO4-}]}{[\ce{H3PO4}]} \approx 7.5 \times 10^{-3}</math>
| p''K''<sub>a1</sub>&nbsp;=&nbsp;2.14
|-
| {{chem2|H2PO4- <-> HPO4(2-) + H+}}
| <math chem>K_{a2} = \frac{[\ce{H+}][\ce{HPO4^2-}]}{[\ce{H2PO4-}]} \approx 6.2 \times 10^{-8}</math>
| p''K''<sub>a2</sub>&nbsp;=&nbsp;7.20
|-
| {{chem2|HPO4(2-) <-> PO4(3-) + H+}}
| <math chem>K_{a3} = \frac{[\ce{H+}][\ce{PO4^3-}]}{[\ce{HPO4^2-}]} \approx 2.14 \times 10^{-13}</math>
| p''K''<sub>a3</sub>&nbsp;=&nbsp;12.37
|}
Values are at 25{{nbsp}}°C and 0 ionic strength.

The p''K''<sub>''a''</sub> values are the [[pH]] values where the concentration of each species is equal to that of its [[conjugate base]]s.  At pH 1 or lower, the phosphoric acid is practically undissociated.  Around pH 4.7 (mid-way between the first two p''K''<sub>''a''</sub> values) the dihydrogen phosphate ion, {{chem2|[H2PO4]-}}, is practically the only species present.  Around pH 9.8 (mid-way between the second and third p''K''<sub>''a''</sub> values) the monohydrogen phosphate ion, {{chem2|[HPO4](2-)}}, is the only species present. At pH 13 or higher, the acid is completely dissociated as the phosphate ion, {{chem2|[PO4](3-)}}.

This means that salts of the mono- and di-phosphate ions can be selectively crystallised from aqueous solution by setting the pH value to either 4.7 or 9.8.

In effect, {{chem2|H3PO4}}, {{chem2|[H2PO4]−}} and {{chem2|[HPO4](2-)}} behave as separate [[weak acid]]s because the successive p''K''<sub>''a''</sub> differ by more than 4.

Phosphate can form many [[polymer]]ic ions such as [[pyrophosphate]], {{chem2|[P2O7](4-)}}, and [[Sodium triphosphate|triphosphate]], {{chem2|[P3O10](5-)}}. The various [[metaphosphate]] ions (which are usually long linear polymers) have an empirical formula of {{chem2|[PO3]−}} and are found in many compounds.

===Biochemistry of phosphates===
<!-- This heading is an anchor linked from other articles -->
In [[biological system]]s, phosphorus can be found as free phosphate anions in solution ('''inorganic phosphate''') or bound to organic molecules as various [[organophosphate]]s.<ref>Jastrzab Renata, Nowak Martyna, Zabiszak Michał, Odani Akira, Kaczmarek Małgorzata T. [https://www.sciencedirect.com/science/article/pii/S0010854521000448 Significance and properties of the complex formation of phosphate and polyphosphate groups in particles present in living cells] [https://doi.org/10.1016/j.ccr.2021.213810 10.1016/j.ccr.2021.213810] ''Coordination Chemistry Reviews''
Volume 435, 15 May 2021, 213810</ref>

Inorganic phosphate is generally denoted '''P<sub>i</sub>''' and at physiological ([[Homeostasis|homeostatic]]) [[pH]] primarily consists of a mixture of {{chem2|[HPO4](2−)}} and {{chem2|[H2PO4]−}} ions. At a neutral pH, as in the [[cytosol]] (pH = 7.0), the concentrations of the orthophosphoric acid and its three anions have the ratios
<math chem display=block>\begin{align}
  \frac{[\ce{H2PO4-}]}{[\ce{H3PO4}]} &\approx 7.5 \times 10^4 \\[4pt]
  \frac{[\ce{HPO4^2-}]}{[\ce{H2PO4-}]} &\approx 0.62 \\[4pt]
  \frac{[\ce{PO4^3-}]}{[\ce{HPO4^2-}]} &\approx 2.14 \times 10^{-6}
\end{align}</math>

Thus, only the {{chem2|[H2PO4]−}} and {{chem2|[HPO4](2−)}} ions are present in significant amounts in the cytosol (62% {{chem2|[H2PO4]−}}, 38% {{chem2|[HPO4](2−)}}). In extracellular fluid (pH = 7.4), this proportion is inverted (61% {{chem2|[HPO4](2−)}}, 39% {{chem2|[H2PO4]−}}).

Inorganic phosphate can also be present as [[pyrophosphate]] anions {{chem2|[P2O7](4-)}}, which give orthophosphate by [[hydrolysis]]:
:{{chem2|[P2O7](4-) + H2O <-> 2 [HPO4](2−)}}

Organic phosphates are commonly found in the form of esters as [[nucleotide]]s (e.g. [[Adenosine monophosphate|AMP]], [[Adenosine diphosphate|ADP]], and [[Adenosine triphosphate|ATP]]) and in [[DNA]] and [[RNA]]. Free orthophosphate anions can be released by the hydrolysis of the [[phosphoanhydride]] bonds in ATP or ADP. These [[phosphorylation]] and [[dephosphorylation]] reactions are the immediate storage and source of energy for many [[metabolism|metabolic]] processes. ATP and ADP are often referred to as [[high-energy phosphate]]s, as are the [[phosphagen]]s in muscle tissue.  Similar reactions exist for the other nucleoside [[nucleoside diphosphate|diphosphates]] and [[nucleoside triphosphate|triphosphates]].

===Bones and teeth===
An important occurrence of phosphates in biological systems is as the structural material of bone and teeth. These structures are made of crystalline [[calcium phosphate]] in the form of [[hydroxylapatite|hydroxyapatite]]. The hard dense enamel of [[mammalian teeth]] may contain [[fluoroapatite]], a [[hydroxyl|hydroxy]] calcium phosphate where some of the [[hydroxyl]] groups have been replaced by [[fluoride]] ions.

===Medical and biological research uses===
Phosphates are medicinal salts of phosphorus. Some phosphates, which help cure many [[urinary tract infection]]s, are used to make urine more acidic. To avoid the development of [[calcium stone]]s in the urinary tract, some phosphates are used.<ref name=":0">{{Cite web|title=Phosphate Supplement (Oral Route, Parenteral Route) Description and Brand Names - Mayo Clinic|url=https://www.mayoclinic.org/drugs-supplements/phosphate-supplement-oral-route-parenteral-route/description/drg-20070193|access-date=2020-11-20|website=www.mayoclinic.org}}</ref> For patients who are unable to get enough phosphorus in their daily diet, phosphates are used as dietary supplements, usually because of certain disorders or diseases.<ref name=":0" /> Injectable phosphates can only be handled by qualified health care providers.<ref name=":0" />

===Plant metabolism===
Plants take up phosphorus through several pathways: the [[arbuscular mycorrhizal]] pathway and the direct uptake pathway.