Liquid membrane-type electrodes

Figure 2. Experimental system for the measurement of membrane potential by a PVC matrix liquid-membrane-type electrode (reprinted with permission from Yakugaku Zasshi 1995, 115, 432. Copyright 1995 The Pharmaceutical Society of Japan).

In these titrations,a cationic (Q ) or anionic (X ) species is titrated with an oppositely charged titrant (X or Q ), respectivelly. If the substance determined or the titrant (or both) has adequate lipophilic character, the poorly soluble ionpair precipitates, + X = QX. Further, the same ion pairs can be used as active substances for liquid membrane-type electrodes because of their good extractability into... 

Liquid membrane type ion-seleetive 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. 

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 electrodes of the liquid-membrane type can be used for the determination of the ions such as Cl , C104, N03, Cu, Pb " and BF4. ... 

There is considerable interest in the design of biosensors for drugs of abuse (cocaine, heroin, morphine, etc.), as well as for prescription pharmaceutical compounds (see 267,325, and 326 for reviews). Typically these sensors are of the liquid membrane type, containing an organic compound reactive to the drug. A cocaine sensor, for example, consists of a PVC membrane plasticized with dibutyl phtalate, and containing tetraphenylborate as the active substance. A strychnine sensor uses a liquid membrane electrode based on ion pairs of strychnine with picrolonate or tetrakis (3-methylpheny)borate. 

The size requirements for ISEs utilized for intracellular measurements varies greatly depending on the size of the cells under investigation. Historically, the term micro-ISE has been used to describe electrodes with tip diameters ranging from 200 p.m down to 0.1 p.m. Fabrieation of such devices, particularly those <10 pm in tip diameter is a real art, and highly specialized equipment is required. Over the years, micro-ISEs of the glass, solid-state, and liquid membrane classes have been proposed, although the liquid membrane type are the ones most extensively employed at this time. 

At present, the most widely used type of micro-ISEs are the liquid membrane type. As has been the case with the previously mentioned clinical chemistry systems and catheter designs, the need for many of the glass membrane electrode systems has been obviated thanks to the increasing number of highly selective neutral carrier molecules that have become avail-... 

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

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... 

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]. 

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. ... 

Heterogeneous liquid membrane electrodes. This type, which has become of considerable practical importance, consists of a liquid ion-exchange layer or a complex-forming layer within a hydrophobic porous membrane of plastic (PTFE, PVC, etc.), sintered glass or filtering textile (glass-fibre, etc.). The construction of such an electrode is depicted in Fig. 2.12. 

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. 

Calculations for the liquid-membrane electrode are performed in exactly the same manner as those used for the pH or fluoride electrode, i.e. with an equation of the following type ... 

Table 6. Selectivities for different types of liquid membrane electrodes and restrictions used...

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