Phosphoric Acid H3PO4
Phosphoric acid H3PO4 as a polyprotic acid is defined by the following three dissociation reactions and pKa values:
|(1)||H3PO4 = H+ + H2PO4-||pKa = 2.14|
|(2)||H2PO4- = H+ + HPO4-2||pKa = 7.207|
|(3)||HPO4-2 = H+ + PO4-3||pKa = 12.346|
the corresponding anions are1
Based on these reaction data, aqion calculates the equilibrium composition and pH of a given phosphoric acid aqueous solution.2 This is done for the concentration range between 1 and 10-10 mol/L in the following table:
|PO4_total||= initial concentration of phosphoric acid in mol/L and in mM|
|species [%]||= (species in mM) / (PO4_total in mM) × 100 %|
The molar concentration of the four species H3PO4, H2PO4-, HPO4-2, and PO4-3 are taken from the output table Ions. In the calculations activity corrections are considered.
For large acid concentrations, the solution is mainly dominated by the undissociated H3PO4. At 10-2 M, the pH is close to pKa = 2.14, giving an equimolar mixture of H3PO4 and H2PO4-.
Below 10-3 M, the solution is mainly composed of H2PO4-, while HPO4-2 becoming non-negligible for very dilute solutions. The orthophosphate ion, PO4-3, is always negligible.