monohydrogen phosphate
{{Chembox
|Name = Hydrogen phosphate
|ImageFile1 =Hydrogen phosphate.svg
|ImageFile1_Ref = {{chemboximage|correct|??}}
|ImageName1 = Stereo skeletal formula of hydrogenphosphate
|ImageFileL1 = 1-hydrogenphosphate-3D-balls.png
|ImageFileL1_Ref = {{chemboximage|correct|??}}
|ImageNameL1 = Aromatic ball and stick model of hydrogenphosphate
|ImageFileR1 = 1-hydrogenphosphate-wdV-spheres.png
|ImageFileR1_Ref = {{chemboximage|correct|??}}
|ImageNameR1 = Space-filling model of hydrogenphosphate
|PIN = Hydrogenphosphate
|SystematicName = Monohydrogenphosphate
Phosphoric acid, ion(2-)
|OtherNames = Phosphoric acid, ion(2-)
Hydrophosphoric acid (2-)
Biphosphate (2-)
|Section1={{Chembox Identifiers
|CASNo = 14066-19-4
|ChEBI = 43474
|ChemSpiderID = 2913859
|Gmelin = 1998
|PubChem = 3681305
|UNII = 33UE6C4909
|StdInChI=1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4)/p-2
|StdInChIKey = NBIIXXVUZAFLBC-UHFFFAOYSA-L
|SMILES = OP(=O)([O-])[O-]
}}
|Section2={{Chembox Properties
|Formula = {{chem|HPO|4|2−}}
|ConjugateAcid = Dihydrogen phosphate
|ConjugateBase = Phosphate
}}
}}
Hydrogen phosphate or monohydrogen phosphate (systematic name) is the inorganic ion with the formula [HPO4]2-. Its formula can also be written as [PO3(OH)]2-. Together with dihydrogen phosphate, hydrogenphosphate occurs widely in natural systems. Their salts are used in fertilizers and in cooking.{{Ullmann|first1=Klaus|last1=Schrödter|first2=Gerhard|last2=Bettermann|first3=Thomas |last3=Staffel|first4=Friedrich|last4=Wahl|first5=Thomas|last5=Klein||first6=Thomas|last6=Hofmann|title=Phosphoric Acid and Phosphates|year=2008|doi=10.1002/14356007.a19_465.pub3}} Most hydrogenphosphate salts are colorless, water soluble, and nontoxic.
It is a conjugate acid of phosphate [PO4]3- and a conjugate base of dihydrogen phosphate [H2PO4]−.
It is formed when a pyrophosphate anion {{chem|[P|2|O|7|]|4-}} reacts with water {{chem|H|2|O|}} by hydrolysis, which can give hydrogenphosphate:
:{{chem|[P|2|O|7|]|4-}} + H2O {{eqm}} 2 {{chem|[HPO|4|]|2−}}
Acid-base equilibria
Hydrogenphosphate is an intermediate in the multistep conversion of phosphoric acid to phosphate:
| class="wikitable"
! Equilibrium ! Dissociation constant, pKa{{cite journal | last1=Powell | first1=Kipton J. | last2=Brown | first2=Paul L. | last3=Byrne | first3=Robert H. | last4=Gajda | first4=Tamás | last5=Hefter | first5=Glenn | last6=Sjöberg | first6=Staffan | last7=Wanner | first7=Hans | title=Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg2+, Cl−, OH−, {{chem|CO|3|2−}}, {{chem|SO|4|2−}}, and {{chem|PO|4|3−}} aqueous systems | journal=Pure Appl. Chem. | date=2005 | volume=77 | issue=4 | pages=739–800 | doi=10.1351/pac200577040739 | doi-access=free }} |
| H3PO4 {{eqm}} {{chem|H|2|PO|4|−}} + H+
| pKa1 = 2.14{{efn|Values are at 25{{nbsp}}°C and 0 ionic strength.}} |
| {{chem|H|2|PO|4|−}} {{eqm}} {{chem|HPO|4|2−}} + H+
| pKa2 = 7.20 |
| {{chem|HPO|4|2−}} {{eqm}} {{chem|PO|4|3−}} + H+
| pKa3 = 12.37 |
{{notelist}}
Examples
- Diammonium phosphate, (NH4)2HPO4
- Disodium phosphate, Na2HPO4, with varying amounts of water of hydration