Dihydrogen phosphate

Dihydrogen phosphate or dihydrogenphosphate ion is an inorganic ion with the formula [H2PO4]-. Its formula can also be written as [PO2(OH)2]-, which shows the presence of two O-H bonds. Together with hydrogen phosphate, dihydrogen phosphate occurs widely in natural systems. Their salts are used in fertilizers and in cooking.[1] Most dihydrogen phosphate salts are colorless, water soluble, and nontoxic.

Dihydrogen Phosphate
Names
IUPAC name
Dihydrogen Phosphate
Systematic IUPAC name
Phosphoric acid, ion(1-)
Other names
Phosphoric acid, ion(1-) Dehydrophosphoric acid (1-)
Identifiers
3D model (JSmol)
Properties
[H2PO4]-
Molar mass 96.987 g/mol
Conjugate acid Phosphoric Acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Structure

The dihydrogen phosphate anion consists of a central phosphorus atom surrounded by 2 equivalent oxygen atoms and 2 hydroxy groups in a tetrahedral arrangement. The phosphorus atom is in the 5+ oxidation state. The dihydrogen phosphate ion carries an overall charge of −1 and it is the conjugate base of phosphoric acid.

Acid-base equilibria

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

Equilibrium Disassociation constant, pKa[2]
H3PO4 ⇌ H
2PO−
4 + H+		 pKa1 = 2.14[lower-alpha 1]
H
2PO−
4 ⇌ HPO2−
4 + H+		 pKa2 = 7.20
HPO2−
4 ⇌ PO3−
4 + H+		 pKa3 = 12.37

Examples

Notes
  1. Values are at 25 °C and 0 ionic strength.

References
  1. Schrödter, Klaus; Bettermann, Gerhard; Staffel, Thomas; Wahl, Friedrich; Klein, Thomas; Hofmann, Thomas (2008). "Phosphoric Acid and Phosphates". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_465.pub3.
  2. 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−, CO2−
    3    , SO2−
    4    , and PO3−
    4     aqueous systems". Pure Appl. Chem. 77 (4): 739–800. doi:10.1351/pac200577040739.
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