The ion pair receptor 5FSQC6 (2 mg), PVC (66 mg), plasticizer (o-NPOE or DOS) (133 mg) and different amounts of lipophilic additives (0-20 mol% relative to the ionophore amount) were dissolved in 2 mL of THF. The resulting cocktail was poured into a glass ring and was left to dry freely in the air for 24 h. In the next step, small discs were cut out from the membrane and were mounted in commercial IS-561 bodies with Ag/AgCl inner electrodes (Sigma-Aldrich). Membranes were conditioned in 0.05 M MES buffer (pH 5.5 or 4.5) or 0.01 M KCl.