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Phosphoric Acid H3PO4

Phosphoric acid H₃PO₄ as a polyprotic acid is defined by the following three dissociation reactions and pK_a 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 pK_a = 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