The equilibrium @C01124@ obtained by application of the law of mass action to @I03167@ and characterizing quantitatively the ability of an @I03171@ to select one of two ions present in the same solution. The ions involved in the exchange should be specified as subscripts. Examples: Exchange: \[k_{\text{Mg}/\text{Ca}}=\frac{\text{[Mg]}_{\text{S}}/\text{[Ca]}_{\text{S}}}{\text{[Mg]}_{\text{M}}/\text{[Ca]}_{\text{M}}}\] Mg²⁺ – Ca²⁺ Exchange: SO₄²⁻ – Cl⁻ \[k_{\text{SO}_{4}/\text{Cl}} = \frac{\text{[}\text{SO}_{4}\text{]}_{\text{S}}/\text{[Cl]}_{\text{S}}^{2}}{\text{[}\text{SO}_{4}\text{]}_{\text{M}}/\text{[Cl]}_{\text{M}}^{2}}\] In the above equations subscript S refers to the @I03171@ ('stationary phase') and M to the external solution ('mobile phase'). For exchanges involving @C01371@ differing in their charges, the numerical value of \(k_{\text{A}/\text{B}}\) depends on the choice of the concentration scales in the @I03171@ and the external solution (@M03969@ scale, molar scale, @A00296@ scale, etc.). Concentration units must be clearly stated for an exchange of ions of differing charges. The corrected @S05563@ @C01124@ (\(k_{\text{A}/\text{B}}^{a}\)) is calculated in a way identical to the @S05563@ @C01124@ except that the concentrations in the external solutions are replaced by activities. This term should not be used as a synonym for @S05614@.