# 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

 H2PO4- dihydrogen phosphate HPO4-2 hydrogen phosphate PO4-3 phosphate (orthophosphate)

Results

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 PO4_total pH H3PO4 H2PO4- HPO4-2 PO4-3
mol/L mM % % % %
1E+00 1000 1.08 89.91 10.09 0.0000158 2.94E-16
1E-01 100 1.63 73.54 26.46 0.000112 4.72E-15
1E-02 10 2.26 41.32 58.68 0.000852 1.06E-13
1E-03 1 3.06 10.51 89.48 0.00709 4.36E-12
1E-04 0.1 4.01 1.33 98.6 0.0649 3.18E-10
1E-05 0.01 5.00 0.139 99.2 0.662 2.85E-08
1E-06 0.001 5.97 0.0141 94.5 5.54 2.36E-06
1E-07 0.0001 6.74 0.00189 74.4 25.6 6.39E-05
1E-08 0.00001 6.97 0.000951 63.3 36.7 0.000155
1E-09 0.000001 7.00 0.000874 61.8 38.2 0.000171
1E-10 0.0000001 7.00 0.000866 61.7 38.3 0.000173

Notation:

 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.

Discussion

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.

Footnotes

1. The thdyn database wateq4f (used by aqion) contains only the two species defined by 2 and 3. The data for the missing species “H2PO4”, i.e. 1, were taken from the thdyn database minteq

2. Examples how to calculate pH values are given here