Title: mean activity of an electrolyte in solution
Long Title: IUPAC Gold Book - mean activity of an electrolyte in solution
DOI: 10.1351/goldbook.M03774
Status: current

Definition
Defined by the equation: \[a_{\pm }=\mathrm{e}^{(\mu _{\rm{B}}- \mu _{\rm{B}}^{\unicode{x29B5}})\ \nu \ R\ T}\] where \(\mu _{\rm{B}}\) is the chemical potential of the solute $\ce{B}$ in a solution containing B and other species. The nature of $\ce{B}$ must be clearly stated: it is taken as a group of ions of two kinds carrying an equal number of positive and negative charges, e.g. $\ce{Na^{+}} + \ce{NO3^{−}}$ or $\ce{Ba^{2+}} + \ce{2Cl^{−}}$ or $\ce{2Al^{3+}} + \ce{3SO4^{2−}}$. \(v\) is the total number of ions making up the group i.e. 2, 3 and 5 respectively in the above examples. \(\mu _{\rm{B}}^{\unicode{x29B5}}\) is the chemical potential of $\ce{B}$ in its standard state, usually the hypothetical ideal solution of concentration \(\pu{1 mol kg -1}\) and at the same temperature and pressure as the solution under consideration.

Related Terms
- activity: https://goldbook.iupac.org//terms/view/A00113
- chemical potential: https://goldbook.iupac.org//terms/view/C01032
- standard state: https://goldbook.iupac.org//terms/view/S05925

Source
- PAC, 1974, 37, 499. 'Electrochemical nomenclature' on page 510 (https://doi.org/10.1351/pac197437040499)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/M03774/html
- json: https://goldbook.iupac.org/terms/view/M03774/json
- xml: https://goldbook.iupac.org/terms/view/M03774/xml

Citation: Citation: 'mean activity of an electrolyte in solution' 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.M03774
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Accessed: 2026-07-21T16:16:44+00:00