Defined by the equation: $$a_{\pm }={\mathrm{e}}^{({\mu }_{\text{B}}-{\mu }_{\text{B}}^{\text{⦵}})\nu RT}$$ where ${\mu }_{\text{B}}$ 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. ${\mu }_{\text{B}}^{\text{⦵}}$ is the chemical potential of B in its standard state, usually the hypothetical ideal solution of concentration $1\mathrm{mol}\mathrm{kg}-1$ and at the same temperature and pressure as the solution under consideration.