Equation describing the contribution of interfering ions, \(i\), to the electrode potential of an ion-selective electrode that is mainly dominated by the primary ion \(\ce{X}\). \[E = {\rm{const}} + \frac{RT}{z_{\rm{X}}F} \ln \left(a_{\ce{X}} + \sum\limits_i K_{{\ce{X}},i}^{\rm{pot}}a_{i}^{(z_{\ce{X}}/z_i)} \right)\] where \(E\) is the measured electrode potential of an ISE when the only variables are activities in the test solution. \(R\) is the gas constant, \(T\) the thermodynamic temperature, \(z_{\rm{X}}\) and \(z_{\rm{i}}\) are the numbers of electrons transferred by the analyte ion \(\ce{X}\) and \(i\), respectively, \(F\) is the Faraday constant, \(a_{\rm{X}}\) and \(a_{\rm{i}}\) the activity of \(\ce{X}\) and \(i\), respectively, and \(K_{{\ce{X}},i}^{\rm{pot}}\) is the potentiometric selectivity coefficient for interfering ion \(i\) with respect to the primary ion \(\ce{X}\).