6.12: Acid-Base Ionization Constant

Acid-Base Ionization Constant

John Moore, Jia Zhou, and Etienne Garand

Ionization Constants for Select Acids (a table for bases is below)

K_a determined at 25 °C.

Acid	Formula and Ionization Equation	Ka	pKa
Acetic	CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻	1.8 × 10⁻⁵	4.74
Arsenic	H₃AsO₄ + H₂O ⇌ H₃O⁺ + H₂AsO₄⁻	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
Butyric	CH₃CH₂CH₂COOH + H₂O ⇌ H₃O⁺ + CH₃CH₂CH₂COO^-	1.5 × 10⁻⁵	4.82
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
Chloroacetic	CH₂ClCOOH + H₂O ⇌ H₃O⁺ + CH₂ClCOO^-	1.4 × 10^-3	2.85
4-chlorobutyric	CH₂ClCH₂CH₂COOH + H₂O ⇌ H₃O⁺ + CH₂ClCH₂CH₂COO^-	3.0 × 10^-5	4.53
3-chlorobutyric acid	CH₃CHClCH₂COOH + H₂O ⇌ H₃O⁺ + CH₃CHClCH₂COO^-	8.9 × 10^-5	4.05
2-chlorobutyric acid	CH₃CH₂CHClCOOH + H₂O ⇌ H₃O⁺ + CH₃CH₂CHClCOO^-	1.3 × 10^-3	2.89
Chlorous	HClO₂ + H₂O ⇌ H₃O⁺ + ClO₂⁻	1.1 × 10⁻²	1.96
Dichloroacetic	CHCl2COOH + H₂O ⇌ H₃O⁺ + CHCl2COO^-	4.5 × 10^-2	1.35
Formic	HCOOH + H₂O ⇌ H₃O⁺ + HCOO⁻	1.8 × 10⁻⁴	3.75
Hydrazoic	HN₃ + H₂O ⇌ H₃O⁺ + N₃⁻	1.0 × 10⁻⁵	5.00
Hydrochloric	HCl + H₂O ⇌ H₃O⁺ + Cl^-	1.2 × 10^6	-6.1
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
Hypobromous	HOBr + H₂O ⇌ H₃O⁺ + OBr^-	3 × 10^-9	8.5
Hypochlorous	HOCl + H₂O ⇌ H₃O⁺ + OCl⁻	6.8 × 10⁻⁸	7.17
Hypoiodous	HOI + H₂O ⇌ H₃O⁺ + OI^-	3 × 10^-11	10.5
Nitric	HNO₃ + H₂O ⇌ H₃O⁺ + NO₃^-	27	-1.43
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 = 4.0 × 10^3	-3.6
	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
Trichloroacetic	CCl3COOH + H₂O ⇌ H₃O⁺ + CCl3COO^-	0.17	0.77
Trifluoroacetic	CF3COOH + H₂O ⇌ H₃O⁺ + CF3COO^-	0.30	0.52

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.
Also from Serjeant, E. P. and Dempsey, B. (eds.), “Ionization Constants of Organic Acids in Aqueous Solution,” IUPAC Chemical Data Series No. 23, Pergamon Press, Oxford, 1979.
^†From Myers, R., Journal of Chemical Education, Vol. 63, 1986, pp. 687-690.

Ionization Constants for Select Bases

K_b determined at 25 °C.

Base	Formula and Ionization Equation	Kb	pKb
Acetylide	C₂H^- + H₂O ⇌ OH^- + C₂H₂	1 × 10^11	-11
Amide	NH₂^- + H₂O ⇌ OH^- + NH₃	1 × 10^20	-20
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
Ethoxide	CH₃CH₂O^- + H₂O ⇌ OH^- + CH₃CH₂OH	1 × 10^2	-2
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)₂H₂^²⁺ + OH-	K₂ = 2.1 × 10⁻⁸	7.68
Hydrazine	N₂H₄ + H₂O ⇌ N₂H₅^⁺ + OH-	K₁ = 1.2 × 10⁻⁶	5.92
	N₂H₅^⁺ + H₂O ⇌ N₂H₆^²⁺ + OH-	K₂ = 1.3 × 10^-15	14.89
Hydride	H^- + H₂O ⇌ OH^- + H₂	1 × 10^21	-21
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.
Also from Serjeant, E. P. and Dempsey, B. (eds.), “Ionization Constants of Organic Acids in Aqueous Solution,” IUPAC Chemical Data Series No. 23, Pergamon Press, Oxford, 1979.
^‡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.