| Pin | Hydrogenphosphate |
|---|---|
| Systematicname | Monohydrogenphosphate Phosphoric acid, ion(2-) |
| Othernames | Phosphoric acid, ion(2-) Hydrophosphoric acid (2-) Biphosphate (2-) |
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 [P2O7]4- reacts with water H2O by hydrolysis, which can give hydrogenphosphate:
- [P2O7]4- + H2O 2 [HPO4]2−
Acid-base equilibria
Hydrogenphosphate is an intermediate in the multistep conversion of phosphoric acid to phosphate:
| Equilibrium | Dissociation constant, pKa[2] |
|---|---|
| H3PO4 H2PO4− + H+ | pKa1 = 2.14[a] |
| H2PO4− HPO42− + H+ | pKa2 = 7.20 |
| HPO42− PO43− + H+ | pKa3 = 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
- ^
- ^ 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−, CO32−, SO42−, and PO43− aqueous systems". Pure Appl. Chem.. 77 (4): 739–800. doi:10.1351/pac200577040739