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.