Liquid membrane type ion-selective

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. 

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]. 

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... 

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. 

Other types of electrodes are listed in Table 8.9. The glass membrane is replaced by a synthetic single-crystal membrane (solid-state electrodes), by a matrix (e.g., inert silicone rubber) in which precipitated particles are imbedded (precipitate electrodes), or by a liquid ion-exchange layer (liquid-liquid membrane electrodes). The selectivity of these electrodes is determined by the composition of the membrane. All these electrodes show a response in their electrode potentials according to the Nemst equation. 

K13. Kudoh, M., Kataoka, M., and Kambara, T., Construction of liquid membrane type histamine ion-selective electrode. Bunseki Kagaku 28, 705-707 (1979). 

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... 

M. Kudoh, M. Kataoka, and T. Kambara, Construction of a liquid-membrane-type periodate ion-selective electrode and its application to the potentiometric titration of a-diols and a-aminoalcohols, Talanta, 27 (1980)495-498. 

To decipher this complexity, electrochemistry at the polarized liquid-liquid interface developed over the past two decades has been proven to be a powerful tool, as shown in elucidation of the mechanism of ion-pair extraction [1 ] and the response of ion-selective electrodes of liquid-membrane type to different types of ions [5 7]. Along this line, several attempts have been made to use polarized liquid liquid interfaces for studying two-phase Sn2 reactions [8-10], two-phase azo-coupling [11], and interfacial polymerizations [12]. Recently, kinetic aspects of complexation reactions in facilitated ion transfer with iono-phores and the rate of protonation of amines have been treated quantitatively [13-16]. Their theoretical framework, which was adapted from the theories of kinetic currents in polaro-graphy, can be directly applicable to analyze quantitatively the chemical reactions in the two-phase systems. In what follows is the introduction to recent advances in electrochemical studies of the chemical reactions at polarized liquid liquid interfaces, mainly focusing on... 

Semiconductor processing technologies have often been used to produce ISEs, particularly as field-effect transistors (FETs) with ion-selective layers like silicon oxide over the gate region. Such ion-selective FETs (ISFETs) are, in principle, solid ISEs, although sometimes the dielectric over the gate is covered with a second, liquid membrane-type layer to achieve different selectivities. 

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). 

Cells of the type in Scheme 10 represent the simplest case of an ion-selective liquid cell its EMF is often called a membrane, or monoionic, potential [3]. The first term is too narrow due to the fact that the membrane potential corresponds to the behavior of a number of cells, including those of Schemes 8 to 11, and to the cells with solid membranes and with Donnan equilibrium. 

We recently synthesized several reasonably surface-active crown-ether-based ionophores. This type of ionophore in fact gave Nernstian slopes for corresponding primary ions with its ionophore of one order or less concentrations than the lowest allowable concentrations for Nernstian slopes with conventional counterpart ionophores. Furthermore, the detection limit was relatively improved with increased offset potentials due to the efficient and increased primary ion uptake into the vicinity of the membrane interface by surfactant ionophores selectively located there. These results were again well explained by the derived model essentially based on the Gouy-Chapman theory. Just like other interfacial phenomena, the surface and bulk phase of the ionophore incorporated liquid membrane may naturally be speculated to be more or less different. The SHG results presented here is one of strong evidence indicating that this is in fact true rather than speculation. 

Homogeneous liquid membrane electrodes. This type, which is in limited use, is sometimes considered as a solid ion-exchange electrode as the electroactive species, e.g., calcium dioctylphosphate, after being dissolved in an ethanol-diethyl ether solution of collodion, is left to "dry and can function as an ion-selective pellet in an electrode tip. Orion37 use these electrodes with PVC-gelled membranes for Ca2+, K+, BF4 and N03. ... 

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). 

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. 

There are two general types of liquid-membrane ISEs, namely one which involves liquid-phase ion exchange, with the response being selective to the anion or cation under scrutiny (generally polyvalent ions), while the other type involves... 

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. 

The theoretical treatment of the selectivities of various types of liquid membranes is mainly due to the efforts of Eisenman and his school (18—25) and others (26,27). The membrane potential E is subdivided into two components (Eq. (4)), namely a boundary potential Eb, produced by the exchange equilibria at the phase boundaries between the membrane and the outside solutions, and a diffusion potential Ed, produced by the diffusion of ions in the membrane itself ... 

In spite of the simple theoretical treatment of the electrochemical alkali ion selectivity for liquid membranes incorporating electrically neutral ligands, nothing comparable has been achieved for negatively charged ligands. As shown in Table 6 for this type of membrane, two completely different expressions for the selectivity constant can be derived (27, 25, 26), depending on which assumptions are made ... 

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