15441 IUPAC Gold Book - acid dissociation constant 10.1351/goldbook.15441 15441 current 1 Equilibrium constant for the following reaction of an acid \(\ce{HB}\): \[\begin{gathered} \ce{HB(aq) H^{+}(aq) + B^{-}(aq)} \\ K_{\rm{a}} = [\ce{H^{+}}][\ce{B^{-}}]/[\ce{HB}]c^{\rm{o}} \end{gathered}\] where \(c^{\rm{o}} = \pu{1 mol L-1}\) is the standard amount concentration and activity coefficients have been neglected. This constant, because activity coefficients are neglected, is valid at a specified ionic strength. The thermodynamic dissociation constant is found by suitable extrapolation of the conditional constant to zero ionic strength. Note that it is defined as a dimensionless quantity, but sometimes it is given dimensions by omitting the standard amount concentration. Because this constant differs for each acid and varies over many degrees of magnitude, the acidity constant is often represented by the additive inverse of its common logarithm, represented by the symbol \(\pu{p}K_{\rm{a}}\) (using the same mathematical relationship as [\(\ce{H^{+}}\)] is to \(\rm{pH}\)), viz.: \[\pu{p}K_{\rm{a}} = -\log_{10}K_{\rm{a}}\] In general, a larger value of \(K_{\rm{a}}\) (or a smaller value of \(\pu{p}K_{\rm{a}}\)) indicates a stronger acid, since the extent of dissociation is larger at the same concentration. PAC, 2007, 79, 1153. 'Glossary of terms used in toxicology, 2nd edition (IUPAC Recommendations 2007)' on page 1158 (https://doi.org/10.1351/pac200779071153) https://goldbook.iupac.org/terms/view/15441/html https://goldbook.iupac.org/terms/view/15441/json https://goldbook.iupac.org/terms/view/15441/plain Citation: 'acid dissociation constant' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.15441 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. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . 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. 2026-06-29T10:14:58+00:00