Conjugate Acid Definition

Conjugate Acid-Base Pairs

A conjugate acid is formed when a base gains a hydrogen or proton.
A conjugate acid is formed when a base gains a hydrogen or proton. Jutta Klee / Getty Images

Conjugate Acid Definition

Conjugate acids and bases are Bronsted-Lowry acid and base pairs, determined by which species gains or loses a proton. When a base dissolves in water, the species that gains a hydrogen (proton) is the base's conjugate acid.

Acid + Base → Conjugate Base + Conjugate Acid

In other words, a conjugate acid is the acid member, HX, of a pair of compounds that differ from each other by gain or loss of a proton. A conjugate acid can release or donate a proton.

Conjugate Acid Example

When the base ammonia reacts with water, the ammonium cation is the conjugate acid that forms:

NH3(g) + H2O(l) → NH+4(aq) + OH(aq)