Acid and Base Ionization Constants


Related Posts:

This table has seven rows and two columns. The first row is a header row, and it labels each column, “6 Strong Acids,” and, “6 Strong Bases.” Under the “6 Strong Acids” column are the following: H C l O subscript 4 perchloric acid; H C l hydrochloric acid; H B r hydrobromic acid; H I hydroiodic acid; H N O subscript 3 nitric acid; H subscript 2 S O subscript 4 sulfuric acid. Under the “6 Strong Bases” column are the following: L i O H lithium hydroxide; N a O H sodium hydroxide; K O H potassium hydroxide; C a ( O H ) subscript 2 calcium hydroxide; S r ( O H ) subscript 2 strontium hydroxide; B a ( O H ) subscript 2 barium hydroxide.
Figure 1. Some of the common strong acids and bases are listed here. Source: OpenStax Chemistry 2e

Acid and Base Ionization Constants (OpenStax Chemistry 2e)

The relative strength of an acid or base is the extent to which it ionizes when dissolved in water. If the ionization reaction is essentially complete, the acid or base is termed strong; if relatively little ionization occurs, the acid or base is weak. As will be evident throughout the remainder of this chapter, there are many more weak acids and bases than strong ones. The most common strong acids and bases are listed in Figure 1.

The relative strengths of acids may be quantified by measuring their equilibrium constants in aqueous solutions. In solutions of the same concentration, stronger acids ionize to a greater extent, and so yield higher concentrations of hydronium ions than do weaker acids. The equilibrium constant for an acid is called the acid-ionization constant, Ka. For the reaction of an acid HA:

the acid ionization constant is written

where the concentrations are those at equilibrium. Although water is a reactant in the reaction, it is the solvent as well, so we do not include [H2O] in the equation. The larger the Ka of an acid, the larger the concentration of H3O+H3O+ and A relative to the concentration of the nonionized acid, HA, in an equilibrium mixture, and the stronger the acid. An acid is classified as “strong” when it undergoes complete ionization, in which case the concentration of HA is zero and the acid ionization constant is immeasurably large (Ka ≈ ∞). Acids that are partially ionized are called “weak,” and their acid ionization constants may be experimentally measured. A

To illustrate this idea, three acid ionization equations and Ka values are shown below. The ionization constants increase from first to last of the listed equations, indicating the relative acid strength increases in the order CH3CO2H < HNO2 < HSO4− :

Another measure of the strength of an acid is its percent ionization. The percent ionization of a weak acid is defined in terms of the composition of an equilibrium mixture:

where the numerator is equivalent to the concentration of the acid’s conjugate base (per stoichiometry, [A] = [H3O+]). Unlike the Ka value, the percent ionization of a weak acid varies with the initial concentration of acid, typically decreasing as concentration increases. Equilibrium calculations of the sort described later in this chapter can be used to confirm this behavior.

Just as for acids, the relative strength of a base is reflected in the magnitude of its base-ionization constant (Kb) in aqueous solutions. In solutions of the same concentration, stronger bases ionize to a greater extent, and so yield higher hydroxide ion concentrations than do weaker bases. A stronger base has a larger ionization constant than does a weaker base. For the reaction of a base, B:

the ionization constant is written as

Inspection of the data for three weak bases presented below shows the base strength increases in the order  NO2 − < CH2CO2−<NH3.

For acids, the relative strength of a base is also reflected in its percent ionization, computed as

but will vary depending on the base ionization constant and the initial concentration of the solution.


Flowers, P., Theopold, K., Langley, R., & Robinson, W. R. (2019, February 14). Chemistry 2e. Houston, Texas: OpenStax. Access for free at:


Related Research

Research Article: Ionization Constants pKa of Cardiolipin

Date Published: September 13, 2013 Publisher: Public Library of Science Author(s): Gerd Olofsson, Emma Sparr, Maria Gasset. Abstract: Cardiolipin is a phospholipid found in the inner mitochondrial membrane and in bacteria, and it is associated with many physiological functions. Cardiolipin has a dimeric structure consisting of two phosphatidyl residues connected by a glycerol bridge … Continue reading

Research Article: Determination of true ratios of different N-glycan structures in electrospray ionization mass spectrometry

Date Published: March 7, 2017 Publisher: Springer Berlin Heidelberg Author(s): Clemens Grünwald-Gruber, Andreas Thader, Daniel Maresch, Thomas Dalik, Friedrich Altmann. Abstract: An ideal method for the analysis of N-glycans would both identify the isomeric structure and deliver a true picture of the relative, if not absolute, amounts of the various structures in one sample. … Continue reading

Research Article: Evaluation of an electrostatic particle ionization technology for decreasing airborne pathogens in pigs

Date Published: December 8, 2015 Publisher: Springer Netherlands Author(s): Carmen Alonso, Peter C. Raynor, Peter R. Davies, Robert B. Morrison, Montserrat Torremorell. Abstract: Influenza A virus (IAV), porcine reproductive and respiratory syndrome virus (PRRSV), porcine epidemic diarrhea virus (PEDV) and Staphylococcus aureus are important swine pathogens capable of being transmitted via aerosols. The electrostatic … Continue reading

Research Article: Determination of porphyrins in oral bacteria by liquid chromatography electrospray ionization tandem mass spectrometry

Date Published: July 14, 2015 Publisher: Springer Berlin Heidelberg Author(s): Jonas Fyrestam, Nadja Bjurshammar, Elin Paulsson, Annsofi Johannsen, Conny Östman. Abstract: Biofilms in the oral cavity can be visualized by fluorescence and a common assumption is that the endogenously produced porphyrins in certain bacteria give rise to this fluorescence. Porphyrin content in oral bacteria … Continue reading