Fresenius' Journal of Fresenius J Anal Chem (1993) 346:569-571 @ Spdnger-Verlag 1993 Ion exchange at neutral carrier ion-selective electrode membranes* V. Horvfith, G. Horvai, and E. Pungor Technical University of Budapest, Institute for General and Analytical Chemistry, Gell6rt t6r 4, H-1111 Budapest, Hungary Received January 18, 1993; revised March 16, 1993 Summary. Radiotracer experiments proved that neutral car- rier (valinomycin) containing plasticized PVC membranes behave as cation exchangers. The exchange capacity may be due to lipophilic salt additives, but there is also a native exchange capacity that may be due to hydrophilic salt con- taminations trapped in the membrane. Introduction The sensing part of most ion-selective electrodes (ISE) used today is either a crystalline solid, like lanthanum fluoride or silver halide, or a glass, or a gelled liquid, like plasticized PVC. Membranes made of plasticized PVC can be sensitized by incorporating into this matrix some soluble ion exchanger, like Aliquat 336, or a neutral, selective complex- ing agent, like valinomycin. These neutral complexing mole- cules are often called neutral carrieres or ionophores to remind that under certain conditions (but not necessarily in potentiometry) they can assist ion transport across mem- branes. The widespread use of the neutral carrier type elec- trode membranes is the result of extensive work in synthetiz- ing neutral ligands with a variety of selectivity profiles [1]. The neutral carrier containing the membrane typically sepa- rates two aqueous solutions and stable e.m.f, develops across the membrane in virtually any bathing solutions. Activity changes of the ions to which the electrode is selective (the primary ion) are followed by an appropriate change in the e.m.fwith a response time in the order of milliseconds [2]. In the absence of interfering ions and physical disturbances the Nernst equation is valid,whereas in the presence of interfer- ing ions the Nikolsky equation (which reduces to the Nernst equation for zero interference) is usually a good approxima- tion to describe the e.m.f. Besides these generally good behaviour, liquid membrane electrodes are prone to interfer- ence from lipophilic counter ions of the primary ion, like thiocyanate. It has been a problem from the very beginning of the use of neutral carrier type electrodes that it was difficult to find an operational model for ISE membranes prepared without the addition of any ionic species. An early model [3] stressed the similarity to bilayer membranes loaded with neutral carriers. This model regards, roughly speaking, the whole thickness of the membrane as a space charge region *Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday Correspondence to: E. Pungor containing only carrier-cation complexes but no anions. Typical ISE membranes are, however, about 0.1 mm thick, and therefore the charge density and thus also the conductiv- ity of the middle portion of the membrane would be unreal- istically low [4]. Wuhrmann et al.[5] assumed that anions are extracted into a new membrane together with cations, but there they become virtually immobile. The same research group noticed later that in radiotracer experiments cations were exchanged with the cation-selective membranes to a much larger extent than anions [6]. This experiment gave a first evidence that the membranes made from pure plasti- cized PVC exhibit cation exchanger character, but the origin of the anionic sites had to be explained by reaction of the neutral carrier with water dissolved in the membrane and with potassium ions diffusing from the aqueous solution into the membrane. Later, impedance measurements [7] showed that the bulk resistance of the membrane became practically constant after a few minutes of soaking. This con- tradicted the above assumption, since if it were true the bulk resistance would decrease substantially while the above reac- tion proceeded. Since the reaction would probably take a day or more to complete, as could be inferred from our earlier work [8-10], this model is an unlikely explanation of the ori- gin of ion-exchanger sites. The impedance measurements gave more support to the assumption that low levels of ionic impurities in the membrane constituents could be responsi- ble for the ion-exchanger behaviour of the membranes. It appeared unlikely, however, that these could be low molecu- lar weight hydrophilic ions or else they would have been expected to be leached out from the membranes in normal use after a few days. In the present work radiotracer experiments are de- scribed which confirm in several ways the ion-exchange character of neutral carrier PVC ISEs and point to the possi- ble origins of ion-exchange capacity. The work is an exten- sion of our preliminary results published earlier [8-10]. A review of the subject within a considerably wider frame has been recently published [11]. Experimental Plasticized PVC membranes of 0.2-0.3 mm thickness were made as described by Craggs et al. [12]. They contained 33 weight% PVC (Corvic $704 from ICI), 66,5 % bis(2-ethylhe- xyl)sebacate (Fluka) plasticizer (abbreviated as DOS) and 0.5 % valinomycin (Fluka). All other chemicals were analyti- cal grade. Water was doubly distilled in fused silica appara- tus.