autoprotolysis

Also known as: autoionization, self-ionization

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

June 18, 2026

Quick Summary

Autoprotolysis is the self-dissociation of amphoteric solvents, where one solvent molecule acts as an acid and another as a base. This process can be represented by the general equation: 2SH ⇌ SH₂⁺ + S⁻, where SH is the solvent molecule, SH₂⁺ is the lyonium ion, and S⁻ is the lyate ion.12

The equilibrium constant for this reaction is called the autoprotolysis constant, K_s, and is expressed as K_s = [SH₂⁺][S⁻]. This constant is typically small, indicating a low degree of ionization. For water, K_s is approximately 10⁻¹⁴ at room temperature. This phenomenon is fundamental to understanding acid-base equilibria in various solvents.2

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

2 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

ammonia
Physical properties
Ammonia also self-ionizes, although less so than does water. 2NH₃ ⇌ NH₄⁺ + NH₂⁻
Read More
Ammonia also self-ionizes, although less so than does water. 2NH₃ ⇌ NH₄⁺ + NH₂⁻
Read More

water
Acid-base reactions
…most clearly seen in the autoionization of water: H₂O(l) + H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq), where the (l) represents the liquid state, the (aq) indicates that the species are dissolved in water, and the double arrows indicate that the reaction can occur in either direction and an equilibrium condition exists.…
Read More
…most clearly seen in the autoionization of water: H₂O(l) + H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq), where the (l) represents the liquid state, the (aq) indicates that the species are dissolved in water, and the double arrows indicate that the reaction can occur in either direction and an equilibrium condition exists.…
Read More