Acid dissociation constants (25 °C)

Stepwise pKa values of common weak acids and conjugate acids of weak bases — the table behind every acid–base calculation on this site.


Nitric acid	$\ce{HNO3}$	$\ce{NO3-}$	1	−1.400	25.1
Sulfuric acid	$\ce{H2SO4}$	$\ce{HSO4-}$	1	—	—
Sulfuric acid	$\ce{HSO4-}$	$\ce{SO4^2-}$	2	1.990	0.0102
Acetic acid	$\ce{CH3COOH}$	$\ce{CH3COO-}$	1	4.756	1.75 × 10⁻⁵
Formic acid	$\ce{HCOOH}$	$\ce{HCOO-}$	1	3.745	1.80 × 10⁻⁴
Benzoic acid	$\ce{C6H5COOH}$	$\ce{C6H5COO-}$	1	4.204	6.25 × 10⁻⁵
Chloroacetic acid	$\ce{ClCH2COOH}$	$\ce{ClCH2COO-}$	1	2.865	0.00136
Hydrofluoric acid	$\ce{HF}$	$\ce{F-}$	1	3.170	6.76 × 10⁻⁴
Nitrous acid	$\ce{HNO2}$	$\ce{NO2-}$	1	3.150	7.08 × 10⁻⁴
Hypochlorous acid	$\ce{HOCl}$	$\ce{OCl-}$	1	7.530	2.95 × 10⁻⁸
Hydrocyanic acid	$\ce{HCN}$	$\ce{CN-}$	1	9.210	6.17 × 10⁻¹⁰
Boric acid	$\ce{B(OH)3}$	$\ce{B(OH)4-}$	1	9.237	5.79 × 10⁻¹⁰
Phenol	$\ce{C6H5OH}$	$\ce{C6H5O-}$	1	9.997	1.01 × 10⁻¹⁰
Ammonium / ammonia	$\ce{NH4+}$	$\ce{NH3}$	1	9.245	5.69 × 10⁻¹⁰
Methylamine	$\ce{CH3NH3+}$	$\ce{CH3NH2}$	1	10.645	2.26 × 10⁻¹¹
Pyridine	$\ce{C5H5NH+}$	$\ce{C5H5N}$	1	5.200	6.31 × 10⁻⁶
Tris buffer	$\ce{TrisH+}$	$\ce{Tris}$	1	8.072	8.47 × 10⁻⁹
Phosphoric acid	$\ce{H3PO4}$	$\ce{H2PO4-}$	1	2.148	0.00711
Phosphoric acid	$\ce{H2PO4-}$	$\ce{HPO4^2-}$	2	7.198	6.34 × 10⁻⁸
Phosphoric acid	$\ce{HPO4^2-}$	$\ce{PO4^3-}$	3	12.375	4.22 × 10⁻¹³
Carbonic acid	$\ce{H2CO3}$	$\ce{HCO3-}$	1	6.351	4.46 × 10⁻⁷
Carbonic acid	$\ce{HCO3-}$	$\ce{CO3^2-}$	2	10.329	4.69 × 10⁻¹¹
Sulfurous acid	$\ce{H2SO3}$	$\ce{HSO3-}$	1	1.857	0.0139
Sulfurous acid	$\ce{HSO3-}$	$\ce{SO3^2-}$	2	7.172	6.73 × 10⁻⁸
Hydrogen sulfide	$\ce{H2S}$	$\ce{HS-}$	1	7.020	9.55 × 10⁻⁸
Hydrogen sulfide	$\ce{HS-}$	$\ce{S^2-}$	2	14.000	1.00 × 10⁻¹⁴
Oxalic acid	$\ce{H2C2O4}$	$\ce{HC2O4-}$	1	1.250	0.0562
Oxalic acid	$\ce{HC2O4-}$	$\ce{C2O4^2-}$	2	4.266	5.42 × 10⁻⁵
Phthalic acid	$\ce{H2P}$	$\ce{HP-}$	1	2.943	0.00114
Phthalic acid	$\ce{HP-}$	$\ce{P^2-}$	2	5.432	3.70 × 10⁻⁶
Citric acid	$\ce{H3Cit}$	$\ce{H2Cit-}$	1	3.128	7.45 × 10⁻⁴
Citric acid	$\ce{H2Cit-}$	$\ce{HCit^2-}$	2	4.761	1.73 × 10⁻⁵
Citric acid	$\ce{HCit^2-}$	$\ce{Cit^3-}$	3	6.396	4.02 × 10⁻⁷
Glycine	$\ce{H3N+CH2COOH}$	$\ce{H3N+CH2COO-}$	1	2.350	0.00447
Glycine	$\ce{H3N+CH2COO-}$	$\ce{H2NCH2COO-}$	2	9.778	1.67 × 10⁻¹⁰
EDTA	$\ce{H6Y^2+}$	$\ce{H5Y+}$	1	0.000	1
EDTA	$\ce{H5Y+}$	$\ce{H4Y}$	2	1.500	0.0316
EDTA	$\ce{H4Y}$	$\ce{H3Y-}$	3	2.000	0.01
EDTA	$\ce{H3Y-}$	$\ce{H2Y^2-}$	4	2.690	0.00204
EDTA	$\ce{H2Y^2-}$	$\ce{HY^3-}$	5	6.130	7.41 × 10⁻⁷
EDTA	$\ce{HY^3-}$	$\ce{Y^4-}$	6	10.370	4.27 × 10⁻¹¹

Strong acids (HCl, HNO₃, the first step of H₂SO₄) ionize completely in water and are not listed. The pKa₂ of H₂S is poorly known; 14 is used here.

Sources

D. C. Harris, Quantitative Chemical Analysis, 9th ed., W. H. Freeman (2016), Appendix G (thermodynamic constants, I → 0).
W. M. Haynes (ed.), CRC Handbook of Chemistry and Physics, 97th ed. (2016), “Dissociation constants of inorganic and organic acids”.

Sources

Used in these lessons