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 $\mathrm{X}$. $$E=\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}+\frac{RT}{z_{\mathrm{X}}F}\ln (a_{\mathrm{X}}+\sum _i{K}_{\mathrm{X},i}^{\mathrm{p}\mathrm{o}\mathrm{t}}{a}_i^{(z_{\mathrm{X}}/z_i)})$$ 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, $z_{\mathrm{X}}$ and $z_{\mathrm{i}}$ are the numbers of electrons transferred by the analyte ion $\mathrm{X}$ and $i$, respectively, $F$ is the Faraday constant, $a_{\mathrm{X}}$ and $a_{\mathrm{i}}$ the activity of $\mathrm{X}$ and $i$, respectively, and $K_{\mathrm{X},i}^{\mathrm{p}\mathrm{o}\mathrm{t}}$ is the potentiometric selectivity coefficient for interfering ion $i$ with respect to the primary ion $\mathrm{X}$.