conjugateacid

Conjugate acid refers to the species formed when a base accepts a proton in Brønsted-Lowry acid-base chemistry. When a base B gains a proton, it becomes BH+, the conjugate acid of B. The corresponding conjugate base of an acid HA is A−. Conjugate acid–base pairs are related by proton transfer; the conjugate acid BH+ can donate the proton again in subsequent reactions, making BH+ an acid and B a base in the reverse direction.

In aqueous solution, the strength of BH+ as an acid is governed by its pKa. The relationship between the base B and its conjugate acid BH+ is Ka(BH+) × Kb(B) = Kw (the ionic product of water). For example, ammonia: Kb(NH3) ≈ 1.8×10−5, giving Ka(NH4+) ≈ 5.6×10−10 and pKa ≈ 9.25. This illustrates that a relatively weak base (like NH3) has a conjugate acid that is a weak acid. In contrast, a very weak base would have a stronger conjugate acid, while a very strong acid has a very weak conjugate base.

Conjugate acids are central to buffer systems, which consist of a weak acid and its conjugate base or a weak base and its conjugate acid. The equilibrium between B and BH+ helps determine the pH near the pKa of BH+. Conjugate acids, and their corresponding bases, are used in chemistry to control proton transfer in reactions and catalysis. Solvent effects can alter the relative strengths and the identity of the conjugate pair.