Inorganic Acids and Bases - pKa Values

Values for the negative logarithm of the acid dissociation constant, pKa, of inorganic acids and bases, as well as hydrated metal ions.

Definitions of the acid dissociation constant and pK_a are given below the table.

pK_avalues given in the table are measuered at 25°C, unless other temperature(°C) is indicated with superscript at the pKa value.

Below the table, figures showing the fractions of the different acid forms in aqueous solutions at varying pH are given for some common acids (values calculated from the tabulated pK_as).
NB! For bases, the dissosiation constant is given in terms of the pK_a value of their conjugated acid, BH⁺. These values are marked with an *.

Inorganic Acids and Bases - pKa Values
Acid name	Formula	pK_a1	pK_a2	pK_a3	pK_a4
Monoprotic acids
Ammonia	NH₃	9.24*
Arsenious acid	H₃AsO₃	9.29
Boric acid	H₃BO₃	9.24
Chlorous acid	HClO₂	1.94
Cyanic acid	HOCN	3.46
Hydrazoic acid	HN₃	4.72
Hydrazine	N₂H₄	8.1*
Hydrobromic acid	HBr	-9
Hydrochloric acid	HCl	-7
Hydrocyanic acid	HCN	9.21
Hydrofluoric acid	HF	3.17
Hydroiodic acid	HI	-10
Hydrogen peroxide	H₂O₂	11.65
Hydroxylamine	H₂NOH	5.94
Hypobromous acid	HOBr	8.62
Hypochlorous acid	HOCl	7.54
Hypoiodous acid	HOI	10.64
Hypophosphorous acid	H₃PO₂	1.23
Iodic acid	HIO₃	0.78
Nitrous acid	HNO₂	3.25
Nitric acid	HNO₃	-1.37
Perchloric acid	HClO₄	-1.6²⁰
Periodic acid	HIO₄	1.64
Sulfamic acid	H₂NSO₃H	1.05
Tetrafluoroboric acid	HBF₄	0.5
Thiocyanic acid	HSCN	-1.8
Water	H₂O	13.995
Diprotic acids
Boric acid	H₃BO₃	9.27²⁰	14²⁰
Carbonic acid	H₂CO₃	6.35	10.33
Chromic acid	H₂CrO₄	-0.98	6.49
Germanic acid	H₂GeO₃	9.01	12.3
Hydrogen selenide	H₂Se	3.89	11.0
Hydrogen sulfide	H₂S	7.02	19
Hydrogen telluride	H₂Te	2.6¹⁸	11
Phosphorous acid	H₃PO₃	1.3²⁰	6.7²⁰
Selenous acid	H₂SeO₃	2.62	8.32
Selenic acid	H₂SeO₄	<0	1.7
Silicic acid	H₂SiO₃	9.91	11.81
Sulfurous acid	H₂SO₃	1.85	7.20
Sulfuric acid	H₂SO₄	-3	1.99
Tellur(VI)ic acid	H₆TeO₆	7.68¹⁸	11¹⁸
Tellurous acid	H₂TeO₃	6.27	8.43
Triprotic acids
Arsenic acid	H₃AsO₄	2.26	6.76	11.29
Phosphoric acid	H₃PO₄	2.16	7.21	12.32
Tetraprotic acids
Pyrophosphoric acid	H₄P₂O₇	0.91	2.10	6.70	9.32
Orthosilicic acid	H₄SiO₄	9.9³⁰	11.8³⁰	12³⁰	12³⁰
Hydrated metal ions
Aluminium (III) ion	Al³⁺	4.85
Barium (II) ion	Ba²⁺	13.4
Beryllium(II) ion	Be²⁺	5.7
Calcium(II) ion	Ca²⁺	12.6
Chromium (III) ion	Cr³⁺	3.95
Copper (II) ion	Ba²⁺	7.34
Iron (III) ion	Fe³⁺	2.17
Lead (II) ion	Ba²⁺	7.8
Lithium (I) ion	Li⁺	13.8
Magnesium (II) ion	Mg²⁺	11.4
Manganese (II) ion	Mn²⁺	10.59
Nickel (II) ion	Ni²⁺	9.86
Sodium (I) ion	Na²⁺	14.8
Scandium (III) ion	Sc³⁺	4.61
Strontium (II) ion	Sr²⁺	13.2
Uranium (IV) ion	U⁴⁺	0.68
Vanadium (III) ion	V³⁺	2.9
Zinc (II) ion	Zn²⁺	8.96

An acid dissociation constant, K_a, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation of acid–base reactions. In aqueous solution, the equilibrium of acid dissociation can be written symbolically as:

HA + H₂O = A^- +H₃O⁺

where HA is an acid that dissociates into A^-, (known as the conjugate base of the acid) and a hydrogen ion which combines with a water molecule to make a hydronium ion.

The chemical species HA, A^- and H₃O⁺ are said to be in equilibrium when their concentrations do not change with the passing of time. The dissociation constant is usually written as a quotient of the equilibrium concentrations (in mol/L), denoted by [HA], [A^-] and [H₃O⁺]

K_a = [A^-]*[H₃O⁺] / [HA]*[H₂O]

In all, but the most concentrated, aqueous solutions of an acid the concentration of water can be taken as constant and can be ignored. The definition can then be written more simply

HA = A^- + H⁺and K_a = [A^-]*[H⁺] / [HA]

This is the definition in common usage. For many practical purposes it is more convenient to discuss the logarithmic constant, pK_a

pK_a = -log₁₀ K_a

The larger the value of pK_a, the smaller the extent of dissociation at any given pH - that is, the weaker the acid.

A weak acid has a pK_a value in the approximate range -2 to 12 in water.

Strong acids has pK_a values of less than about -2; the dissociation of a strong acid is effectively complete such that concentration of the undissociated acid is too small to be measured. pK_a values for strong acids can, however, be estimated by theoretical means.

After rearranging the expression defining K_a, and putting pH = -log₁₀[H⁺], one obtains

pH = pK_a + [A^-] / [HA] and further

pH - pK_a = log [A^-] / [HA]

Then, a solution with 50% dissociation has pH equal to the pK_a of the acid.

Polyprotic acids are acids that can lose more than one proton. Then we have more than one dissiciation constant; K_a1, K_a2, etc.. and similar pK_a1, pK_a2, etc.

All data given in the figures apply to dilute aqueous solutions at ambient temperature.

For bases, the pk_a value is given for the conjugate bases BH⁺ and BH₂²⁺.

BH⁺ = B + H⁺

The pK_b for a base may be calculated from the pK_a value of its conjugate acid:

pK_w = pK_a + pK_b

At 25°C the pK_w is 14 and

pK_b = 14 - pK_a

The figure below shows the fractions of the different acid forms (H₂A, HA^- and A^2-for twoprotic acids) of di- and triprotic acids in aqueous solutions at varying pH:

Fractions of acid ions as function of pH