Liquid membranes, selective electrodes

Abstract The problem of validation criteria for developing ion-selective membrane electrodes for the analysis of pharmaceuticals arises from the connection between the reliability of ion-selective membrane electrodes construction and the reliability of the analytical information. Liquid membrane selective electrodes are more suitable for validation than the solid variety. The influence of the stability of... 

Table 5.10 summarizes the presently available electrodes categorized as glass, ion-exchange membrane, crystal membrane, and liquid membrane. These electrodes can be used either for direct potentiometric measurements of ionic activity after calibration of the Nemst expression for the particular electrode or to monitor a potentiometric titration when a selected reaction that involves the monitored ion is available. Table 5.10 also indicates the common interfering ions. Several instrument companies are endeavoring to develop potentiometric-membrane electrodes to monitor directly ions in body fluids. 

Well-established potentiometric devices used in bioanalysis are based on macroscopic electrodes that use gas-permeable polymer, solid or liquid membranes selective to CO2, NH3, pH, or various alkali, alkaline earth, or halide ions [7]. The membranes separate the external analyte solution from the inner reference solution, and these electrodes are used in conjunction with an integral or external reference electrode. The selectivities of these membranes can be excellent, for example, the main interferent in the fluoride ISE that uses a solid Lap3/Eup2 membrane is hydroxide, with a Kfjoh value of 0.1 thus, over the pH range 0-8.5, the electrode responds in a Nernstian fashion to fluoride concentrations above 10 M [9]. Solid Ag2S membranes doped with AgX, where X is Cl , Br , I , CN , or SCN , are used for the determination of these anions. 

Nonactin was suspended in Nujol/2-octanol and valinomycin was suspended in diphenylether. These solutions were respectively incorporated into liquid membrane potentiometric electrodes. The usual commercial glass electrodes exhibit a K to Na selectivity on the order of 30 1. Simon s valinomycin electrode displayed a 3800 1 selectivity for K to Na", and an 18,000 1 selectivity for K with respect to H. Thus the electrode should be useful in strongly acid media in which glass electrodes cannot be used, or are ineffective. 

One example of a liquid-based ion-selective electrode is that for Ca +, which uses a porous plastic membrane saturated with di-(n-decyl) phosphate (Figure 11.13). As shown in Figure 11.14, the membrane is placed at the end of a nonconducting cylindrical tube and is in contact with two reservoirs. The outer reservoir contains di-(n-decyl) phosphate in di- -octylphenylphosphonate, which soaks into the porous membrane. The inner reservoir contains a standard aqueous solution of Ca + and a Ag/AgCl reference electrode. Calcium ion-selective electrodes are also available in which the di-(n-decyl) phosphate is immobilized in a polyvinyl chloride... 

The properties of several representative liquid-based ion-selective electrodes are presented in Table 11.3. An electrode using a liquid reservoir can be stored in a dilute solution of analyte and needs no additional conditioning before use. The lifetime of an electrode with a PVC membrane, however, is proportional to its exposure to aqueous solutions. For this reason these electrodes are best stored by covering the membrane with a cap containing a small amount of wetted gauze to... 

Faraday s law (p. 496) galvanostat (p. 464) glass electrode (p. 477) hanging mercury drop electrode (p. 509) hydrodynamic voltammetry (p. 513) indicator electrode (p. 462) ionophore (p. 482) ion-selective electrode (p. 475) liquid-based ion-selective electrode (p. 482) liquid junction potential (p. 470) mass transport (p. 511) mediator (p. 500) membrane potential (p. 475) migration (p. 512) nonfaradaic current (p. 512)... 

This experiment describes the preparation and evaluation of two liquid-membrane Na+ ion-selective electrodes, using either the sodium salt of monensin or a hemisodium ionophore as ion exchangers incorporated into a PVG matrix. Electrodes prepared using monensin performed poorly, but those prepared using hemisodium showed a linear response over a range of 0.1 M to 3 X 10 M Na+ with slopes close to the theoretical value. 

STUDIES ON A Pb -SELECTIVE ELECTRODE WITH MACROCYCLIC LIQUID MEMBRANE. POTENTIOMETRIC DETERMINATION OF Pb + IONS... 

Despite the fact that a great lot of ion-selective electrodes (ISEs) with liquid and film polymeric membranes for the determination of physiologically active amines (PhAA) has been described, the factors responsible for their selectivity have not yet been studied sufficiently. In this work, the influence of plasticizer and ion-exchanger nature on the selectivity of ISEs reversible to PhAA cations of various stmctures has been discussed. 

The discussion of Section 5-1 clearly illustrates that the most important response characteristic of an ISE is selectivity. Depending on the nature of the membrane material used to impart the desired selectivity, ISEs can be divided into three groups glass, liquid, or sohd electrodes. More than two dozen ISEs are commercially available and are widely used (although many more have been reported in the literature). Such electrodes are produced by firms such as Orion Research, Radiometer, Coming Glass, Beckman, Hitachi, or Sensorex. 

Many other cyclic and noncyclic organic carriers with remarkable ion selectivities have been used successfiilly as active hosts of various liquid membrane electrodes. These include the 14-crown-4-ether for lithium (30) 16-crown-5 derivatives for sodium bis-benzo-18-crown-6 ether for cesium the ionophore ETH 1001 [(R,R)-AA -bisd l-ethoxycarbonyl)undecyl-A,yVl-4,5-tctramcthyl-3,6-dioxaoctancdiamide] for calcium the natural macrocyclics nonactin and monensin for ammonia and sodium (31), respectively the ionophore ETH 1117 for magnesium calixarene derivatives for sodium (32) and macrocyclic thioethers for mercury and silver (33). 

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). 

One barrel-tip contains the organic membrane phase and an internal reference electrode the other constitutes a second reference electrode. A four-barrel configuration with a 1-pm tip in which three barrels are liquid membrane electrodes for Na, Ca and and the fourth is a reference electrode has been reported Some representative applications of ion-selective electrodes for intracellular measurements are shown in Table 3. 

Ion-selective membranes derive their permselective properties from either ion exchange, solubility or complexation phenomena. Current ion-selective electrodes contain membranes which consist of glass, solid or liquid phases. 

Liquid Membrane Ion-Selective Electrodes Response Mechanisms Studied by Optical Second Harmonic Generation and Photoswitchable lonophores as a Molecular Probe... 

The purpose of this chapter is to describe these experimental approaches for understanding the molecular mechanism of the membrane potentials for ionophore-incorpo-rated liquid membrane ion-selective electrodes. 

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. 

Curve 1 in Fig. 5 gives an example of the oscillation of membrane current observed with the liquid membrane system as in Eq. (3) by applying a constant AFwi-w2 of —0.48 V and measuring the time course of the current through the LM, /wi-w2- The cell used was the same as that used for the measurement of the potential oscillation, except a tetraphenyl-arsonium ion selective electrode [26,27], TPhAsE, was employed as a reference electrode in LM of NB ... 

I.3.2.2. Liquid membrane electrodes. The members of this class can be divided into direct or indirect working. In terms of ion selectivity, the direct working group are completely comparable to the solid-state sensors, so that we... 

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. ... 

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