Term: acidity function https://doi.org/10.1351/goldbook.A00081 Definition: Any function that measures the thermodynamic hydron-donating or -accepting ability of a solvent system, or a closely related thermodynamic property, such as the tendency of the lyate ion of the solvent system to form Lewis adduct. (The term 'basicity function' is not in common use in connection with basic solutions.) Acidity functions are not unique properties of the solvent system alone, but depend on the solute (or family of closely related solutes) with respect to which the thermodynamic tendency is measured. Commonly used acidity functions refer to concentrated acidic or basic solutions. acidity functions are usually established over a range of composition of such a system by UV/VIS spectrophotometric or NMR measurements of the degree of hydronation (protonation or Lewis adduct formation) for the members of a series of structurally similar indicator bases (or acids) of different strength: the best known of these functions is the Hammett acidity function H0 (for uncharged indicator bases that are primary aromatic amines). Related Terms: 1) hydron (http://doi.org/10.1351/goldbook.H02904). 2) lyate ion (http://doi.org/10.1351/goldbook.L03653). 3) lewis adduct (<em>of a compound, of a medium</em>) (http://doi.org/10.1351/goldbook.L03510). 4) acidity (<em>of a compound, of a medium</em>) (http://doi.org/10.1351/goldbook.A00079-1). 5) amines (<em>of a compound, of a medium</em>) (http://doi.org/10.1351/goldbook.A00274). 6) acidity (<em>of a compound, of a medium</em>) (<em>of a compound, of a medium</em>) (http://doi.org/10.1351/goldbook.A00079-2). Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1081 (https://doi.org/10.1351/pac199466051077) Citation: 'acidity function' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.A00081 License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.