At first, potentiometric experiments were performed for all electrodes in MES buffer at pH 5.5 and 4.5. The slope of the calibration curve ( for F -and SO4 2-) for the tested electrodes does not correspond to the Nernst slope (Table ). Unfortunately, the slopes of the curves correspond to cation exchange, which means that the binding of cations by the crown ether is more favoured than the complexation of anions. A slight anionic response in the MgSO4 solution (measurements in pH 5.5) is seen for each membrane, which may be due to the fact that B18C6 is a selective ionophore for potassium ions. When measured in buffer at pH 4.5, we see a slight anionic response for cations other than potassium (Na + , Mg 2+ ). For each membrane, the determination of F -and SO4 2-ions in the presence of potassium ions failed. To see if it would be possible to block cation exchange by B18C6 with potassium ions, electrodes were conditioned in 0.1M KCl. Unfortunately, this did not improve the selectivity of the electrodes to anions.