Editing Phosphate (section)

===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.