4'-picrylamino-5'-nitrobenzo-18-crown-6 as a sensing reagent in potassium ion-selective electrode membranes

Five crown ethers, namely, 4'-picrylamino-5'-nitrobenzo-18-crown-6 (I), dibenzo-18-crown-6 (III), dibenzo-30-crown-10 (IV), dicyclohexano-18-crown-6 (V) and bis-[(benzo-15-crown-5)-15-ylmethyl pimelate] (VI) have been compared with valinomycin (II) for their role as potassium ion-sensors in PVC matrix membrane ion-selective electrodes (ISEs). Sensor I was found to be the best, but fell short of the high quality of the well established sensor II (valinomycin) in terms of selectivity towards potassium over sodium and ammonium. Nevertheless, electrodes made from membranes containing sensor I, 2-nitrophenyl octyl ether (NPOE) or 2-nitrophenyl phenyl ether and potassium tetra-p-chloro-phenylborate (anion excluder) in PVC were of long lifetimes. The loss of slope of the ISEs is linked to small falls in the electrical resistance of the ISE membranes; this being associated with leaching of sensor and solvent mediator from the membranes into test or storage solutions. No chromatographic evidence was found of anion excluder being leached.