Arrhenius concept of acids and bases

We can state the Arrhenius concept of an acid as follows: An acid is a substance that, when dissolved in water, increases the concentration of hydronium ion, H₃O⁺(aq). For simplicity, chemists frequently use the notation H⁺(aq) for the H₃O⁺(aq) ion and call it the hydrogen ion.

A base, in the Arrhenius concept, is a substance that, when dissolved in water, increases the concentration of hydroxide ion, OH⁻(aq).

An acid that releases few hydrogen ions in aqueous solution is a weak acid. The aqueous solution of a weak acid contains hydronium ions, anions, and dissolved acid molecules.

A strong base completely ionizes in aqueous solution to give OH− and a cation. Sodium hydroxide is an example of a strong base.

The principal strong bases are the hydroxides of Group 1A elements.

Despite its successes, the Arrhenius concept is limited. In addition to looking at acid–base reactions only in aqueous solutions, it singles out the OH− ion as the source of base character, when other species can play a similar role.

Brønsted-Lowry concept of acids and bases

According to the Brønsted-Lowry concept, an acid is the species donating a proton in a proton-transfer reaction. A base is the species accepting the proton in a proton-transfer reaction.

Because the emphasis in the Brønsted-Lowry concept is on proton transfer, the concept also applies to reactions that do not occur in aqueous solution. In the reaction between gas phase HCl and NH₃, for example, a proton is transferred from the acid HCl to the base NH₃:

To be a Brønsted-Lowry acid, a molecule or ion must have a hydrogen atom it can lose as an H+ ion.

Conjugate acid–base pairs

Note that NH₃ and NH₄⁺ differ by a proton. That is, NH₃ becomes the NH₄⁺ ion by gaining a proton, whereas the NH₄⁺ ion becomes the NH₃ molecule by losing a proton. The species NH₄⁺ and NH₃ are a conjugate acid-base pair. A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the loss or gain of a proton. Here NH₄⁺ is the conjugate acid of NH₃, and NH₃ is the conjugate base of NH₄⁺.

The inverse relationship between the strengths of acids and their conjugate bases is illustrated.