Inorganic Acids and Bases - pKa Values

A comprehensive reference table of pKa values for inorganic acids and bases measured at 25°C. Includes definitions of acid dissociation constants and pKa values. The table covers monoprotic acids, diprotic acids, triprotic acids, tetraprotic acids, and hydrated metal ions. For bases, pKa values are given for their conjugate acids and are marked with an asterisk (*).

Acid nameFormulapKa1pKa2pKa3pKa4
AmmoniaNH39.24*
Arsenious acidH3AsO39.29
Boric acidH3BO39.24
Chlorous acidHClO21.94
Cyanic acidHOCN3.46
Hydrazoic acidHN34.72
HydrazineN2H48.1*
Hydrobromic acidHBr-9
Hydrochloric acidHCl-7
Hydrocyanic acidHCN9.21
Hydrofluoric acidHF3.17
Hydroiodic acidHI-10
Hydrogen peroxideH2O211.65
HydroxylamineH2NOH5.94
Hypobromous acidHOBr8.62
Hypochlorous acidHOCl7.54
Hypoiodous acidHOI10.64
Hypophosphorous acidH3PO21.23
Iodic acidHIO30.78
Nitrous acidHNO23.25
Nitric acidHNO3-1.37
Perchloric acidHClO4-1.620
Periodic acidHIO41.64
Sulfamic acidH2NSO3H1.05
Tetrafluoroboric acidHBF40.5
Thiocyanic acidHSCN-1.8
WaterH2O13.995
Boric acidH3BO39.27201420
Carbonic acidH2CO36.3510.33
Chromic acidH2CrO4-0.986.49
Germanic acidH2GeO39.0112.3
Hydrogen selenideH2Se3.8911.0
Hydrogen sulfideH2S7.0219
Hydrogen tellurideH2Te2.61811
Phosphorous acidH3PO31.3206.720
Selenous acidH2SeO32.628.32
Selenic acidH2SeO4<01.7
Silicic acidH2SiO39.9111.81
Sulfurous acidH2SO31.857.20
Sulfuric acidH2SO4-31.99
Tellur(VI)ic acidH6TeO67.68181118
Tellurous acidH2TeO36.278.43
Arsenic acidH3AsO42.266.7611.29
Phosphoric acidH3PO42.167.2112.32
Pyrophosphoric acidH4P2O70.912.106.709.32
Orthosilicic acidH4SiO49.93011.83012301230
Aluminium (III) ionAl3+4.85
Barium (II) ionBa2+13.4
Beryllium(II) ionBe2+5.7
Calcium(II) ionCa2+12.6
Chromium (III) ionCr3+3.95
Copper (II) ionBa2+7.34
Iron (III) ionFe3+2.17
Lead (II) ionBa2+7.8
Lithium (I) ionLi+13.8
Magnesium (II) ionMg2+11.4
Manganese (II) ionMn2+10.59
Nickel (II) ionNi2+9.86
Sodium (I) ionNa2+14.8
Scandium (III) ionSc3+4.61
Strontium (II) ionSr2+13.2
Uranium (IV) ionU4+0.68
Vanadium (III) ionV3+2.9
Zinc (II) ionZn2+8.96

Definitions

Acid Dissociation Constant (Ka)

An acid dissociation constant, Ka, 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 as:

HA + H₂O ⇌ A⁻ + H₃O⁺

where HA is an acid that dissociates into A⁻ (the conjugate base) and a hydrogen ion which combines with water to form a hydronium ion.

The dissociation constant is defined as:

Ka = [A⁻][H⁺] / [HA]

pKa Value

For many practical purposes, the logarithmic constant is more convenient:

pKa = -log₁₀(Ka)

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

Acid Strength Classification:

  • Strong acids: pKa < -2 (dissociation is effectively complete)
  • Weak acids: pKa approximately -2 to 12 in water

Henderson-Hasselbalch Equation

After rearranging and using pH = -log₁₀[H⁺]:

pH = pKa + log([A⁻]/[HA])

A solution with 50% dissociation has pH equal to the pKa of the acid.

Polyprotic Acids

Polyprotic acids can lose more than one proton, with multiple dissociation constants (Ka₁, Ka₂, etc.) and corresponding pKa₁, pKa₂, etc.

Bases

For bases, the pKa value is given for the conjugate acid BH⁺:

BH⁺ ⇌ B + H⁺

The pKb for a base can be calculated from the pKa of its conjugate acid:

pKb = 14 - pKa (at 25°C, where pKw = 14)

Notes

  • pKa values in the table are measured at 25°C unless otherwise indicated with a superscript temperature.
  • For bases, pKa values are marked with an asterisk (*) and represent the pKa of their conjugate acid.
  • All data applies to dilute aqueous solutions at ambient temperature.