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 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

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[a]
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

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