Monohydrogen 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.[1] Most hydrogenphosphate salts are colorless, water soluble, and nontoxic.

Hydrogen phosphate
Stereo skeletal formula of hydrogenphosphate
Aromatic ball and stick model of hydrogenphosphate
Aromatic ball and stick model of hydrogenphosphate
Space-filling 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
3D model (JSmol)
ChEBI
ChemSpider
1998
UNII
  • InChI=1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4)/p-2
    Key: NBIIXXVUZAFLBC-UHFFFAOYSA-L
  • OP(=O)([O-])[O-]
Properties
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

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, pKa[2]
H3PO4 ⇌ H
2PO−
4 + H+		 pKa1 = 2.14[lower-alpha 1]
H
2PO−
4 ⇌ HPO2−
4 + H+		 pKa2 = 7.20
HPO2−
4 ⇌ PO3−
4 + H+		 pKa3 = 12.37
  1. Values are at 25 °C and 0 ionic strength.

Examples

References
  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.
  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.
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