Monohydrogen phosphate
Hydrogen phosphate or monohydrogen phosphate is the inorganic ion with the formula 2-. Its formula can also be written as 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 3- and a conjugate base of dihydrogen phosphate −.
It is formed when a pyrophosphate anion reacts with water by hydrolysis, which can give hydrogenphosphate:
Acid-base equilibria
Hydrogenphosphate is an intermediate in the multistep conversion of phosphoric acid to phosphate:| Equilibrium | Dissociation constant, pKa |
| H3PO4 + H+ | pKa1 = 2.14 |
| + H+ | pKa2 = 7.20 |
| + H+ | pKa3 = 12.37 |
Examples
- Diammonium phosphate, 2HPO4
- Disodium phosphate, Na2HPO4, with varying amounts of water of hydration