Quantification of the concentration of ionic impurities in polymeric sensing membranes with the segmented sandwich technique.
Ionic impurities in solvent polymeric membranes have been the main reason early ion-selective electrodes (ISEs) without added ion exchanger exhibited a functional potentiometric response. Today, knowledge of such impurities and their elimination becomes important in view of designing ISEs with low detection limits and voltammetric ion-selective electrodes, to increase operational lifetime, and to optimize sensing selectivity. Here, a potentiometric segmented sandwich membrane technique is used to study the amount and properties of impurities in situ directly in the membrane phase. This technique can be used with a number of ionophores and with different membrane matrixes and does not require specialized equipment. The concentration of anionic impurities in PVC-DOS (bis(2-ethylhexyl) sebacate) and PVC-NPOE (o-nitrophenyloctyl ether) was found to be on the order of 100 micromol/kg with membranes containing weakly binding potassium ionophores. The concentration of cation-exchanger sites of carboxylated PVC-NPOE was found to be higher (1.62 mmol/kg). Addition of the neutral lipophilic salt ETH 500 to both membrane segments had only a marginal effect on the results for PVC-NPOE membranes but had a large impact on the observed membrane potential for PVC-DOS membranes. Theory explains that the addition of such salt to membranes where ion pairing is predominant is essential for the accurate assessment of ionic impurities with this technique.
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Department of Chemistry, Auburn University, Alabama 36849, USA.
Bakker
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D000465:Algorithms; D007476:Ionophores / Q000737:chemistry*; D008564:Membrane Potentials; D008567:Membranes, Artificial; D011143:Polyvinyl Chloride; D012996:Solutions / Q000032:analysis
D007476:Ionophores; D008567:Membranes, Artificial; D012996:Solutions; D011143:Polyvinyl Chloride