Monohydrogen phosphate explained

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. 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 reacts with water by hydrolysis, which can give hydrogenphosphate:

+ H₂O 2

Acid-base equilibria

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

Equilibrium	Dissociation constant, pKa^[1]
H3PO4 + H^+	pKa1 = 2.14
+ H^+	pKa2 = 7.20
+ H^+	pKa3 = 12.37

Examples

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

References

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