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 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