Brønsted-Löwry acid-base theory

What is the Brønsted-Löwry acid-base theory?

In 1938, Brønsted and Löwry improved the Arrhenius acid-base theory and classified acidic and basic compounds as follows:

  • Acid: substance (molecular or ionic) capable of donate protons (H+) to another substance acting as a base.
  • Base: substance (molecular or ionic) capable of accepting protons (H+) from an acid.

According to these definitions, when an acid (AH) gives up a proton, it becomes a base (A), because the reverse reaction can occur where A accepts a proton:

AH(acid) -> A + H+

A(conj. base) + H+ -> AH

B(base) + H+ -> BH+

BH+(conj. acid) -> B + H+

The acid and base pairs AH and A are called conjugate acid and base.

Therefore, the protons given up must be accepted by another substance and vice versa, i.e. conjugated acids and bases go in pairs.


HCl(acid1) + H2O(base2) -> Cl(base1) + H3O+(acid2)

NH3(base1) + H2O(acid2) -> NH4+(acid1) + OH(base2)

As we can see in this theory the solvent in which the dissociation has taken place is taken into account. As we will see later, if an acid is strong its conjugate base will be weak and vice versa.

Amphoteric substances

Se puede observar que el agua puede actuar como ácido o como base. These substances that can behave in many cases as acids or as bases are called amphoteric, amphiprotic or ampholytic.


This theory ultimately extends Arrhenius' theory, and three types of acids can be distinguished:

  • Neutral acids: such as hydrochloric acid (HCl).
  • Anionic acids: e.g. bisulfate ion (HSO4)
  • Cationic acids: e.g. ammonium ion (NH4+)