Potentiometric K+ responses of an ISE based on a perfluoroperhydrophenanthrene membrane doped with ionophore (1, 2.0 mmol L−1) and anionic sites at a site-to-ionophore ratio of 71 mol % (filled circles) or 33 mol % (diamonds) in the presence of 1 mmol L−1 Cs+. For comparison, the K+ response curve for a 71 mol % membrane in the absence of interfering ions is shown (open circles). The response of the 71 mol % membrane was fitted (broken line) with the model described in the text and the following parameters: Kex= 10−2.51, KKL= 1015.1, KKL2 = 106.0, KCsL = 1011.5, and KCsL2 = 108.3.