Term: mean activity of an electrolyte in solution
https://doi.org/10.1351/goldbook.M03774

Definition:
Defined by the equation: \[a_{\pm }=\mathrm{e}^{(\mu _{\text{B}}- \mu _{\text{B}}^{\unicode{x29B5}})\ \nu \ R\ T}\] where µ B is the chemical potential of the solute B in a solution containing B and other species. The nature of 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. Na+ + NO3− or Ba2+ + 2Cl− or 2Al3+ + 3SO42−. v is the total number of ions making up the group i.e. 2, 3 and 5 respectively in the above examples. µ B ⦵ is the chemical potential of B in its standard state, usually the hypothetical ideal solution of concentration 1 mol kg-1 and at the same temperature and pressure as the solution under consideration.

Related Terms:
1) activity (http://doi.org/10.1351/goldbook.A00113).
2) chemical potential (http://doi.org/10.1351/goldbook.C01032).
3) standard state (http://doi.org/10.1351/goldbook.S05925).

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

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. https://doi.org/10.1351/goldbook.M03774

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