Monohydrogen phosphate

Hydrogen phosphate
Aromatic ball and stick model of hydrogenphosphate	Space-filling model of hydrogenphosphate
Names
	Preferred IUPAC name Hydrogenphosphate
	Systematic IUPAC name Monohydrogenphosphate; Phosphoric acid, ion(2-)
	Other names Phosphoric acid, ion(2-); Hydrophosphoric acid (2-); Biphosphate (2-)
Identifiers
CAS Number	14066-19-4;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:43474;
ChemSpider	2913859;
Gmelin Reference	1998
PubChem CID	3681305;
UNII	33UE6C4909;
CompTox Dashboard (EPA)	DTXSID10394962 ;
	InChI InChI=1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4)/p-2 Key: NBIIXXVUZAFLBC-UHFFFAOYSA-L;
	SMILES OP(=O)([O-])[O-];
Properties
Chemical formula	HPO2−; 4
Conjugate acid	Dihydrogen phosphate
Conjugate base	Phosphate
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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.^[1] 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 [P
₂O
₇]⁴⁻
reacts with water H
₂O by hydrolysis, which can give hydrogenphosphate:

[P
₂O
₇]⁴⁻
+ H₂O ⇌ 2 [HPO
₄]²⁻

Acid-base equilibria

Hydrogenphosphate is an intermediate in the multistep conversion of phosphoric acid to phosphate:

Equilibrium	Dissociation constant, pK_a^[2]
H₃PO₄ ⇌ H ₂PO⁻ ₄ + H⁺	pK_a1 = 2.14^[a]
H ₂PO⁻ ₄ ⇌ HPO²⁻ ₄ + H⁺	pK_a2 = 7.20
HPO²⁻ ₄ ⇌ PO³⁻ ₄ + H⁺	pK_a3 = 12.37

^ Values are at 25 °C and 0 ionic strength.

Examples

Diammonium phosphate, (NH₄)₂HPO₄
Disodium phosphate, Na₂HPO₄, with varying amounts of water of hydration

References

^ Schrödter, Klaus; Bettermann, Gerhard; Staffel, Thomas; Wahl, Friedrich; Klein, Thomas; Hofmann, Thomas (2008). "Phosphoric Acid and Phosphates". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_465.pub3. ISBN 978-3527306732.
^ Powell, Kipton J.; Brown, Paul L.; Byrne, Robert H.; Gajda, Tamás; Hefter, Glenn; Sjöberg, Staffan; Wanner, Hans (2005). "Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg²⁺, Cl⁻, OH⁻, CO²⁻
₃, SO²⁻
₄, and PO³⁻
₄ aqueous systems". Pure Appl. Chem. 77 (4): 739–800. doi:10.1351/pac200577040739.

[3] Values are at 25 °C and 0 ionic strength.

[Ullmann-1] Schrödter, Klaus; Bettermann, Gerhard; Staffel, Thomas; Wahl, Friedrich; Klein, Thomas; Hofmann, Thomas (2008). "Phosphoric Acid and Phosphates". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_465.pub3. ISBN 978-3527306732.

[2] Powell, Kipton J.; Brown, Paul L.; Byrne, Robert H.; Gajda, Tamás; Hefter, Glenn; Sjöberg, Staffan; Wanner, Hans (2005). "Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg²⁺, Cl⁻, OH⁻, CO²⁻
₃, SO²⁻
₄, and PO³⁻
₄ aqueous systems". Pure Appl. Chem. 77 (4): 739–800. doi:10.1351/pac200577040739.

[1]