protolyse

Protolyse, or protolysis, is a chemical reaction in which a proton is transferred from a donor species (an acid) to an acceptor species (a base). This definition follows the Brønsted–Lowry concept of acids and bases. In a protolytic reaction, an acid HA donates a proton to a base B, yielding the conjugate base A− and the conjugate acid BH+. The general representation is HA + B ⇌ A− + BH+. Protolysis is a fundamental step in most acid–base processes and can occur in any solvent, including gas phase and liquid solutions.

In aqueous solution, water often participates as both participant and solvent. For example, an acid HA may transfer a proton to water, forming A− and H3O+. Conversely, water can donate a proton to a base, forming OH− and H3O+. The position of the protolysis equilibrium is governed by the relative acidity of the species involved, usually expressed via pKa values. A larger difference in pKa between conjugate acid BH+ and conjugate base A− shifts the equilibrium toward the weaker acid and conjugate base.

Protolysis also underpins many processes beyond simple acid–base neutralization, including hydrolysis, enzyme-catalyzed proton transfers, and acid-catalyzed reactions in organic synthesis. It is distinct from bond cleavage that does not involve proton transfer, and is influenced by solvent effects and the broader framework of acid–base theory. In water itself, autoprotolysis 2 H2O ⇌ H3O+ + OH− illustrates protolysis as a fundamental solvent property.