Taking the good voltammetric results from F -into account, 5FSQC6 was tested for potentiometric measurements as ionophore in poly(vinyl chloride) (PVC) membranes plasticized with o-nitrophenyl octyl ether (o-NPOE) or bis(2-ethylhexyl) sebacate (DOS). The membranes also included lipophilic additives to optimize membrane selectivity. Depending on the working mechanism of the anion-selective ionophore, cationic additives might be used to stabilize the ionophore-anion complex (neutral carrier mechanism), while anionic additives stabilize the charged form of the ionophore (charged carrier mechanism). In the case of the ion-pair receptor, it is known that the cation bound by the crown ether facilitates the formation of the ionophore-anion complex. Therefore, potassium tetrakis[3,5bis(trifluoromethyl)phenyl]borate (KTFPB) was used as lipophilic additive (charged carrier mechanism). The influence of the amount of anionic additive (KTFPB) on electrode performance was tested (0, 10 and 20% relative to the ionophore). Compositions of membranes tested in this work are shown in Table . For ion-selective electrodes sensitive to monovalent ions, the theoretical slope of the characteristic is 59 mV per decade of activity change in the concentration of these ions (Nernst eq.).