A @[email protected] which defines the ability of an ion-selective electrode to distinguish a particular ion from others. The @[email protected] @[email protected], \(K_{\text{A,B}}^{\text{pot}}\), is evaluated by means of the emf response of the ion-selective electrode in mixed solutions of the primary ion, A, and interfering ion, B (fixed interference method) or less desirable, in separate solutions of A and B (separate solution method). The activities of the primary ion, A, and the interfering ion, B, at which \(K_{\text{A,B}}^{\text{pot}}\) is determined should always be specified, as the value of \(K_{\text{A,B}}^{\text{pot}}\) is defined by the modified Nikolsky–Eisenman equation. The smaller the value of \(K_{\text{A,B}}^{\text{pot}}\) the greater the electrode's preference for the principal ion, A. Comment: The terms @[email protected] constant and @[email protected] are frequently used instead of @[email protected] @[email protected] However, in order to standardize the terminology associated with ion-selective electrodes, use of the term @[email protected] @[email protected] is recommended, as is the fixed interference method for its evaluation. This @[email protected] @[email protected] is not identical to the similar term used in separation science.