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.