Defined by the equation: \[a_{\pm }=\mathrm{e}^{(\mu _{\text{B}}- \mu _{\text{B}}^{\unicode{x29B5}})\ \nu \ R\ T}\] where \(\mu _{\text{B}}\) is the @C01032@ 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. \(\mu _{\text{B}}^{\unicode{x29B5}}\) is the @C01032@ of B in its @S05925@, usually the hypothetical ideal solution of concentration \(\pu{1 mol kg -1}\) and at the same temperature and pressure as the solution under consideration.