Monohydrogen phosphate

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Hydrogen phosphate

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Template:Longitem	1998
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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 Template:Chem/link reacts with water Template:Chem/link by hydrolysis, which can give hydrogenphosphate:

Template:Chem/link + H₂O ⇌ 2 Template:Chem/link

Acid-base equilibria

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

Equilibrium	Dissociation constant, pKa
H3PO4 ⇌ Template:Chem/link + H+	pKa1 = 2.14Template:Efn
Template:Chem/link ⇌ Template:Chem/link + H+	pKa2 = 7.20
Template:Chem/link ⇌ Template:Chem/link + H+	pKa3 = 12.37

Examples

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