autoprotolys

Autoprotolysis, also called self-proton transfer or autoionization, is the process by which molecules of a solvent transfer a proton to another molecule of the same substance, generating ions. This behavior requires an amphoteric solvent, one that can act as both a proton donor and a proton acceptor. Autoprotolysis underpins the existence of a pH-like scale for solvents and determines the concentration of ions in pure solvent and in solutions.

In water, autoprotolysis is best known raised by the equilibrium 2 H2O ⇌ H3O+ + OH−. At 25 °C the equilibrium constant Kw equals [H3O+][OH−] ≈ 1.0 × 10−14, so in neutral water both ions are present at about 1 × 10−7 M. Kw is temperature dependent and increases with rising temperature. This self-ionization is fundamental to aqueous acid-base chemistry, where pH is defined in terms of the activities of hydronium and hydroxide ions.

Other solvents exhibit autoprotolysis as well, but to different extents. In liquid ammonia, for example, 2 NH3 ⇌ NH4+ + NH2− occurs, but with a far smaller autoprotolysis constant than water, indicating much lower ionization at standard conditions. Alcohols such as ethanol can also autoprotolyze, with reactions like 2 C2H5OH ⇌ C2H5OH2+ + C2H5O−, again typically to a much lesser extent than in water. The magnitude of autoprotolysis constants varies widely among solvents and depends on temperature and solvent structure.

Autoprotolysis is central to solvent chemistry, influencing pH-like measures, acid-base equilibria, buffer behavior, and electrochemical processes in non-aqueous media.