Neutral exchange carriers

In contrast to ISEs with neutral ion carriers in the membrane, not even qualitative rules have been formulated for the solvent effect on the behaviour of ISEs with ion-exchanger ions in a liquid membrane. A basic condition for the ion-exchanger ions is that they be strongly hydrophobic. It must hold for the standard Gibbs energy of transfer of the ion-exchanger ion X and the deter-minand Y that... 

Although one might think of neutral anion carriers for use in membrane electrodes, I cannot offer good suggestions. It is possible, however, to use charged ligands (classical ion exchangers) as membrane components. This makes accessible the measurement of, for example, Cl- and HCO in blood serum, without serious mutual interference. 

A membrane can be either a liquid or a solid. Its electrical properties arise when it allows transport of an ion of one charge but not that of another. Membranes are usually sufficiently thick that one can distinguish an inside region and two outer boundary regions which are in contact with electrolyte solutions. Two types of membranes are considered here (1) membranes of solid and glassy materials (2) liquid membranes with dissolved ion-exchanging ions or neutral ion carriers (ionophores). In fact all of these membranes are involved in ion exchange. It is important to understand how this process affects the potentials which develop in the system at both sides of the membrane. 

The versatility of ISEs was enhanced considerably by the introduction of membranes containing neutral ion carriers (ionophores). The first ISE of this type, with a membrane containing valinomycin and selective for potassium ions, was described by Stefanac and Simon in 1966. There are many liquid chemical systems that interact highly selectively with ions through, e.g., ion exchange, ion association, or solvent extraction. Practically useful ISEs based on these systems and on neutral ionophores have been obtained due to the gradual perfection of the technology of plasticized poly(vinyl chloride) (PVC) matrix membranes. 

S-2.2.2 Neutral Carrier Electrodes hi addition to charged liquid ion exchangers, liquid-membrane electrodes often rely on the use of complex-forming neutral carriers. Much effort has been devoted to the isolation or synthesis of compounds containing cavities of molecular dimensions. Such use of chemical recognition principles has made an enormous impact upon widespread acceptance of ISEs. The resulting neutral carriers can be natural macrocyclic molecules or synthetic crown... 

Japanese lacquer, have also been employed. The partitioning of ions into this phase is selectively aided by either ion exchangers or neutral carriers. 

FIGURE 4.6 Schematic view of the equilibria between sample, ion-selective membrane, and inner filling solution for three important classes of solvent polymeric ion-selective membranes. Top electrically neutral carrier (L) and lipophilic cation exchanger (R ) center charged carrier (L-) and anion exchanger (R+) and bottom cation exchanger (R-). 

Solvent polymeric membranes conventionally consist of ionophore, ion exchanger, plasticizer, and polymer. The majority of modem polymeric ISEs are based on neutral carriers, making the ionophore the most important membrane component. Substantial research efforts have focused on the development of highly selective ionophores for a variety of analytes [3], Some of the most successful ionophores relevant to biomedical applications are depicted in Fig. 4.1. 

P.C. Meier, W.E. Morf, M. Laubli, and W. Simon, Evaluation of the optimum composition of neutral-carrier membrane electrodes with incorporated cation-exchanger sites. Anal. Chim. Acta 156,1—8 (1984). 

Inspection of Eq. (13.6) shows that the selectivity behavior of a liquid membrane is specified completely by the membrane selectivity constant, Ky, which in turn is dependent on the equilibrium constant of Eq. (13.5) and on the mobility of ions i and j within the membrane. For the case in which the membrane consists of a neutral carrier [129], the exchange reaction can be presented as ... 

They are classified by membrane material into glass membrane electrodes, crystalline (or solid-state) membrane electrodes, and liquid membrane electrodes. Liquid membrane electrodes are further classified into liquid ion-exchange membrane electrodes and neutral carrier-based liquid membrane electrodes. Some examples are shown in Fig. 5.36 and Table 5.3. If the membrane is sensitive to ion i of charge Z and the activities of i in the sample and internal solutions are equal to (i) and a2(i), respectively, the membrane potential, m, which is developed across the membrane, is... 

There are two kinds of ionophores charged ones, which are called liquid ion exchangers, and neutral carriers. Because they are mobile in both the free and in the complexed form, mobilities of all species are again part of the selectivity coefficient together with the ion-exchange equilibrium constant. The best-known neutral ionophore is valinomycin (Fig. 6.15a) which shows a 1,000 1 selectivity for K+ in preference to Na+ and no pH dependence. In its uncomplexed form, it is electrically neutral. A better-known representative is di(n-octyl phenyl) phosphonate (Fig. 6.15c), which shows good selectivity for calcium ion and is relatively pH insensitive. 

The most widely used sensor for chloride ions in clinical analyzers is based on an ion-exchanger, a quaternary alkylammonium chloride, dispersed in a plastic membrane. It is not an ideal sensor due to the interference of lipophilic anions (e.g., salicylates, bromides) and lip-ophylic cations (e.g., bacteriostatic agents, anesthetics) and a relatively poor selectivity towards hydrogen carbonates (bicarbonates). However, compared to charged anion- and neutral carrier-based membranes that have been tested, it is still the best-suited for automated analyzers. 

The PVC membrane compositions consist of a neutral carrier and ion exchange ionophores as indicated in Table I. The K membrane includes valinomycin as the ionophore and is plasticized by 2-ethyl-hexyladipate (Fluka). The Na+ membrane contains methyl monensin and tris(2-ethylhexyl)phthalate. Tri-n-dodeclyamine is the ionophore for pH and tridodecylmethyammonium chloride is the ion exchanger for Cl. HC03 utilizes 4-butyltrifluorophenone for C03 detection as well as trioctylammonium chloride ion exchanger. Both anion membranes are plasticized with dioctyladipate. 

---