Membrane types, ion-selective electrodes

Liquid membrane type ion-selective electrodes (ISEs) provide one of the most versatile sensing methods because it is possible to customize the sensory elements to suit the structure of the analyte. A wealth of different synthetic and natural ionophores has been developed, in the past 30 years, for use in liquid membrane type ISEs for various inorganic and organic ions [1], In extensive studies [2-4], the response mechanism of these ISEs has been interpreted in terms of thermodynamics and kinetics. However, there have been few achievements in the characterization of the processes occurring at the surface of ISEs at molecular level. 

When ions dissolved in W are not hydrophilic enough, the partition of these ions into ELSB can participate in the distribution equilibrium and can shift from the value given by Eq. (4.2.1) in a way similar to the interference by foreign ions in ion-exchange-membrane-type ion-selective electrodes [30, 40]. To estimate the degree of interference, we need to know for a particular ionic liquid used for an ILSB the values of which are thermodynamically not accessible in principle and their... 

A 1 1 mixture of an IL and PVDF-HFP has been used for ILSB. There is no difference in performance of gelled and non-gelled ionic liquids for ILSB [23]. A gelled ILSB membrane can be mounted in a membrane holder, as the one used for liquid-membrane-type ion-selective electrodes [66]. 

In 1970 Thomas et al. described for the first time the use of plasticized PVC membranes in ion-selective electrodes [30] and within a few years this was followed by the construction of the first K" " selective MEMFET [14]. The working mechanism of the plasticized PVC-type ion-selective electrode has been studied extensively [31] and from this work some general features for the design of ion-selective membrane materials can be defined ... 

Ion-selective membranes attain their permselectivity from ion-exchange, dissolution, or complexation phenomena. Different types of membranes are available for the construction of ion-selective electrodes glass and other solid state rods (crystals), liquid or polymer ion ecchangers, or dissolved ionophores. Many electrodes are commercially available with selec-tivities for different ions, mainly H, alkali metal cations, heavy metal ions, and halides or pseudohalides. Also gas-sensing electrodes may be constructed from an ion-selective electrode and a gas-permeable membrane [182]. Ion selective electrodes and gas-selective electrodes... 

Ionophore-based solvent polymeric membranes are widely used as sensing membranes in ion-selective electrodes (ISEs) [24,25]. This type of potentiometric sensor has attracted great interest in the last decade due to the extraordinary improvement in the detection limit down to picomolar (10-12 M) levels [26,27], Furthermore, solid-contact ISEs have been developed by using various conducting polymers, including PEDOT, as the ion-to-electron transducer [28-31],... 

The capability of neutral-carrier-type ion selective electrodes is considered to be dependent on its functional characteristics as an ionophore of the neutral carrier itself and on the compatibility of the carrier into the PVC membrane containing membrane solvent and lipophilic salt. At first, effects of membrane solvents on the sensitivity and selectivity of the Na" " selective electrodes based on six of the calix[4]arene derivatives 1-6, were determined. The membrane solvents tested were NPOE and FPNPE as the phenyl ether type solvents and DOP as the diester type one. Selectivity coefficients for Na" " ion with respect to other alkali and alkaline-earth metal ions, NH4 and H on the electrodes based on derivative 2 are summarized in Table I. 

When first developed, potentiometry was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. In 1906, Cremer discovered that a potential difference exists between the two sides of a thin glass membrane when opposite sides of the membrane are in contact with solutions containing different concentrations of H3O+. This discovery led to the development of the glass pH electrode in 1909. Other types of membranes also yield useful potentials. Kolthoff and Sanders, for example, showed in 1937 that pellets made from AgCl could be used to determine the concentration of Ag+. Electrodes based on membrane potentials are called ion-selective electrodes, and their continued development has extended potentiometry to a diverse array of analytes. 

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

Thus, the behavior of the selective membrane p N is completely equivalent to that of an electrode of metal N. Hence, membranes of this type are called ion-selective electrodes, and in the particular case discussed, the membrane is called an -selective electrode. Sometimes the term is extended to the entire left half of cell (23.5), which in addition to the membrane contains the standard solution and the reference electrode. 

Glass electrodes are used for the analysis of hydrogen ions various other types of ion-selective electrodes are used for the other ions. Electrodes with ion-selective solvent membranes have become very popular. These electrodes are made in the form of thin glass capillaries (about 1 rm in diameter), which in the lower part contain a small volume of a liquid that is immiscible with water the remainder of the capillary is filled with electrolyte solution (e.g., 3M KCl). 

Selectivity coefficients values for K - and Na -ISFETs with the optimized ion-sen-sing membranes encapsulating valinomycin and bis(12-crown-4) are summarized in Fig. 9. The selectivity coefficient for with respect to Na in the K -ISFET is 2 x 10 " and that for Na with respect to in the Na -ISFET is 3 x 10. The selectivity coefficient values are similar to those for the ISFETs and ion-selective electrodes with the previous membrane materials containing the same neutral carriers. The high sensitivity and selectivity for the neutral-carrier-type ISFETs based on sol-gel-derived membranes can last for at least 3 weeks. 

Two aqueous phases separated by a liquid membrane, EM, of nitrobenzene, NB, were layered in a glass tube, which was equipped with Pt counterelectrodes in W1 and W2 and reference electrodes in three phases as in Eq. (1). Reference electrodes set in W1 and W2 were Ag/AgCl electrodes, SSE, and those in LM were two tetraphenylborate ion selective electrodes [26,27], TPhBE, of liquid membrane type. The membrane current, /wi-w2 was applied using two Pt electrodes. The membrane potential, AFwi-wi recorded as the potential of SSE in W2 vs. that in W1. When a constant current of 25 /aA cm was applied from W1 to W2 in the cell given as Eq. (1), the oscillation of membrane potential was observed as shown in curve 1 of Fig. 1. The oscillation of AFwi-wi continued for 40 to 60 min, and finally settled at ca. —0.40 V. 

Various types of research are carried out on ITIESs nowadays. These studies are modeled on electrochemical techniques, theories, and systems. Studies of ion transfer across ITIESs are especially interesting and important because these are the only studies on ITIESs. Many complex ion transfers assisted by some chemical reactions have been studied, to say nothing of single ion transfers. In the world of nature, many types of ion transfer play important roles such as selective ion transfer through biological membranes. Therefore, there are quite a few studies that get ideas from those systems, while many interests from analytical applications motivate those too. Since the ion transfer at an ITIES is closely related with the fields of solvent extraction and ion-selective electrodes, these studies mainly deal with facilitated ion transfer by various kinds of ionophores. Since crown ethers as ionophores show interesting selectivity, a lot of derivatives are synthesized and their selectivities are evaluated in solvent extraction, ion-selective systems, etc. Of course electrochemical studies on ITIESs are also suitable for the systems of ion transfer facilitated by crown ethers and have thrown new light on the mechanisms of selectivity exhibited by crown ethers. 

Together with active metal electrodes, the membrane electrodes represent the best known ion-selective electrodes (ISEs) however, the membrane type has the advantages of insensitivity to redox agents and surface poisons. As the... 

Liquid membranes in this type of ion-selective electrodes are usually heterogeneous systems consisting of a plastic film (polyvinyl chloride, silicon rubber, etc.), whose matrix contains an ion-exchanger solution as a plasticizer (see Fig. 6.5). 

Among cations, potassium, acetylcholine, some cationic surfactants (where the ion-exchanger ion is the / -chlorotetraphenylborate or tetra-phenylborate), calcium (long-chain alkyl esters of phosphoric acid as ion-exchanger ions), among anions, nitrate, perchlorate and tetrafluoro-borate (long-chain tetraalkylammonium cations in the membrane), etc., are determined with this type of ion-selective electrodes. 

This type of membrane consists of a water-insoluble solid or glassy electrolyte. One ionic sort in this electrolyte is bound in the membrane structure, while the other, usually but not always the determinand ion, is mobile in the membrane (see Section 2.6). The theory of these ion-selective electrodes will be explained using the glass electrode as an example this is the oldest and best known sensor in the whole field of ion-selective electrodes. 

The history of ion-selective electrodes (ISEs) [1] starts from the discovery of the pH response of thin film glass membranes by Cremer in 1906, thus making ISEs the oldest class of chemical sensors. They still are superior over other sensor types in a variety... 

The liquid-membrane electrode is another important type of ion-selective electrode. The internal filling solution contains a source of the ion under investigation, i.e. one for which the ion exchanger is specific, while also containing a halide ion to allow the reference electrode to function. The physico-chemical behaviour of the ISE is very similar to that of the fluoride electrode, except that ise and the selectivity are dictated by the porosity of a membrane rather than by movement through a solid-state crystal. 

At the turn of the century, considerable attempts were being made to find suitable membrane models. These models fall into two groups compact, usually liquid ( oil ) and soUd membranes [10, 33, 62, 75] and porous membranes [9]. At the very beginning of the study of compact membranes, the glass electrode was discovered [ 18, 34], whose membrane represented the first observation of marked selectivity for a particular type of ion, here the hydrogen ion. It is interesting that this first ion-selective electrode remains the best and most widely used of all such electrodes. 

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