Appendix I: Ionization Constants of Selected Weak Acids and Bases

Ionization Constants for Select Acids

Ka determined at 25 °C.

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Acid

Formula and Ionization Equation

Ka

pKa
Acetic CH3COOH + H2O ⇌ H3O+ + CH3COO- 1.8 × 10-5 4.74
Arsenic H3AsO4 + H2O ⇌ H3O+ + H3AsO4- K1 = 6.17 × 10-3 2.210
H2AsO4- + H2O ⇌ H3O+ + HAsO42- K2 = 1.17 × 10-7 6.932
HAsO42- + H2O ⇌ H3O+ + AsO43- K3 = 3.09 × 10-12 11.523
Benzoic C6H5COOH + H2O ⇌ H3O+ + C6H5COO- 1.2 × 10-4 3.92
Boric B(OH)3(H2O) + H2O ⇌ H3O+ + B(OH)4- 5.8 × 10-10 9.24
Carbonic H2CO3 + H2O ⇌ H3O+ + HCO3- K1 = 4.3 × 10-7 6.37
HCO3- + H2O ⇌ H3O+ + CO32- K2 = 4.7 × 10-11 10.33
Citric H3C6H5O7 + H2O ⇌ H3O+ + H2C6H5O7- K1 = 1.4 × 10-3 2.85
H2C6H5O7- + H2O ⇌ H3O+ + HC6H5O72- K2 = 4.5 × 10-5 4.35
HC6H5O72- + H2O ⇌ H3O+ + C6H5O73- K3 = 1.5 × 10-6 5.82
Chlorous HClO2 + H2O ⇌ H3O+ + ClO2- 1.1 × 10-2 1.96
Formic HCOOH + H2O ⇌ H3O+ + HCOO- 3.0 × 10-4

3.52
Hydrazoic HN3 + H2O ⇌ H3O+ + N3- 1.0 × 10-5 5.00
Hydrocyanic HCN + H2O ⇌ H3O+ + CN- 3.3 × 10-10 9.48
Hydrofluoric HF + H2O ⇌ H3O+ + F- 6.8 × 10-4 3.17
Hydrogen peroxide H2O2 + H2O ⇌ H3O+ + HO2- 2.1 × 10-12 11.68
Hydrosulfuric† H2S + H2O ⇌ H3O+ + HS- K1 = 1 × 10-7 7.0
HS- + H2O ⇌ H3O+ + S2- K2 = 1 × 10-19 19.0
Hypochlorous HOCl + H2O ⇌ H3O+ + OCl- 6.8 × 10-8 7.17
Nitrous HNO2 + H2O ⇌ H3O+ + NO2- 7.41 × 10-4 3.130
Oxalic H2C2O4 + H2O ⇌ H3O+ + HC2O4- K1 = 5.5 × 10−2 1.26
HC2O4- + H2O ⇌ H3O+ + C2O42- K2 = 1.4 × 10−4 3.85
Phenol HC6H5O + H2O ⇌ H3O+ + C6H5O- 1.7 × 10-10 9.77
Phosphoric H3PO4 + H2O ⇌ H3O+ + H2PO4- K1 = 7.2 × 10-3 2.14
H2PO4- + H2O ⇌ H3O+ + HPO42- K2 = 6.3 × 10-8 7.20
HPO42- + H2O ⇌ H3O+ + PO43 K3 = 4.6 × 10-13 12.34
Phosphorous H3PO3 + H2O ⇌ H3O+ + H2PO3- K1 = 2.4 × 10-2 1.62
H2PO3- + H2O ⇌ H3O+ + HPO32- K2 = 2.9 × 10-7 6.54
Propanoic CH3CH2COOH + H2O ⇌ H3O+ + CH3CH2COO- 1.33 × 10-5 4.85
Selenic H2SeO4 + H2O ⇌ H3O+ + HSeO4- K1 = very large ---
HSeO4- + H2O ⇌ H3O+ + SeO42- K2 = 2.2 × 10-2 1.66
Selenous H2SeO3 + H2O ⇌ H3O+ + HSeO3- K1 = 2.5 × 10-3 2.60
HSeO3- + H2O ⇌ H3O+ + SeO32- K2 = 1.6 × 10-9 8.80
Sulfuric H2SO4 + H2O ⇌ H3O+ + HSO4- K1 = very large ---
HSO4- + H2O ⇌ H3O+ + SO42- K2 = 1.1 × 10-2 1.96
Sulfurous H2SO3 + H2O ⇌ H3O+ + HSO3- K1 = 1.7 × 10-2 1.77
HSO3- + H2O ⇌ H3O+ + SO32- K2 = 6.3 × 10-8 7.2
Tellurous H2TeO3 + H2O ⇌ H3O+ + HTeO3- K1 = 7.1 × 10-7 6.15
HTeO3- + H2O ⇌ H3O+ + TeO32- K2 = 4.0 × 10-9 8.40
Taken from Hogfeldt, E. and Perrin, D. D., Stability Constants of Metal-Ion complexes, 1st ed. Oxford: New York: Pergamon, 1979-1982. International Union of Pure and Applied Chemistry Commission on Equilibrium Data.
From Myers, R., Journal of Chemical Education, Vol. 63, 1986, pp. 687-690.

Ionization Constants for Select Bases

Kb determined at 25 °C.


Base Formula and Ionization Equation Kb pKb
Ammonia ‡ NH3 + H2O ⇌ NH4+ + OH 1.77 × 10−5 4.752
Aniline§ C6H5NH2 + H2O ⇌ C6H5NH3+ + OH 3.9 × 10−10 9.41
Dimethylamine§ (CH3)2NH + H2O ⇌ (CH3)2NH2+ + OH 5.8 × 10−4 3.24
Ethylenediamine (CH2)2(NH2)2 + H2O ⇌ (CH2)2(NH2)2H+ + OH K1 = 7.8 × 10−5 4.11
(CH2)2(NH2)2H+ + H2O ⇌ (CH2)2(NH2)2H22+ + OH K2 = 2.1 × 10−8 7.68
Hydrazine N2H4 + H2O ⇌ N2H5+ + OH K1 = 1.2 × 10−6 5.92
N2H5+ + H2O ⇌ N2H62+ + OH K2 = 1.3 × 10−15 14.89
Hydroxylamine NH2OH + H2O ⇌ NH3OH+ + OH 9.3 × 10−9 8.03
Methylamine CH3NH2 + H2O ⇌ CH3NH3+ + OH 5.0 × 10−4 3.30
Pyridine C5H5N + H2O ⇌ C5H5NH+ + OH 1.6 × 10−9 8.80
Trimethylamine§ (CH3)3N + H2O ⇌ (CH3)3NH+ + OH 6.2 × 10−5 4.21
Taken from Hogfeldt, E. and Perrin, D. D., Stability Constants of Metal-Ion complexes, 1st ed. Oxford: New York: Pergamon, 1979-1982. International Union of Pure and Applied Chemistry Commission on Equilibrium Data.
From Read, A. J., Journal of Solution Chemistry, Vol. 11, No. 9, 1982, pp. 649-664.
§From Meites, L., Ed. Handbook of Analytical Chemistry, 1st ed. New York: McGraw-Hill, 1963.
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