Membrane indicator electrodes

Activity Versus Concentration In describing metallic and membrane indicator electrodes, the Nernst equation relates the measured cell potential to the concentration of analyte. In writing the Nernst equation, we often ignore an important detail—the... 

The most reliable method is probably the potentiometric titration procedure first reported by Dilley [101]. This procedure has the added advantage of avoiding the use of trichloromethane. The procedure for the manufacture of the membrane-indicating electrode has been modified and a simplified description is given below. Commercial variants are also becoming available. 

Combes and Tremillon [154] studied the oxoacidic properties of tungsten(VI) oxide and the solubility of calcium tungstate in a molten equimolar KCl-NaCl mixture at 1000 K. A potentiometric cell with the membrane indicator electrode Ni,NiOlYSZ was used for the detection of the equilibrium oxide ion concentration. Investigation of the equilibria taking place in CaW04 solutions in KCl-NaCl allowed them to determine the solubility of CaO in the said melt at 1000 K as 0.084 mol%. The solubility of Scheelite (CaW04) was determined to be 10-3 5 mol kg-1, and the equilibrium constant of reaction (1.2.38) was estimated as 1010. 

A General Principles 659 23B Reference Electrodes 660 23C Metallic Indicator Electrodes 662 23D Membrane Indicator Electrodes 664 23E Ion-Selective Field-Effect Transistors 675 23F Molecular-Selective Electrode Systems 677 23G Instruments for Measuring Cell Potentials 684 23H Direct Potentiometric Measurements 686 231 Potentiometric Titrations 691 Questions and Problems 692... 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

Glass membrane pH electrodes are often available in a combination form that includes both the indicator and the reference electrode. The use of a single electrode greatly simplifies the measurement of pH. An example of a typical combination electrode is shown in Figure 11.12. 

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)... 

To measure the e.m.f. the electrode system must be connected to a potentiometer or to an electronic voltmeter if the indicator electrode is a membrane electrode (e.g. a glass electrode), then a simple potentiometer is unsuitable and either a pH meter or a selective-ion meter must be employed the meter readings may give directly the varying pH (or pM) values as titration proceeds, or the meter may be used in the millivoltmeter mode, so that e.m.f. values are recorded. Used as a millivoltmeter, such meters can be used with almost any electrode assembly to record the results of many different types of potentiometric titrations, and in many cases the instruments have provision for connection to a recorder so that a continuous record of the titration results can be obtained, i.e. a titration curve is produced. 

Neither the usual membrane ISEs nor the gas-sensing electrodes, in which their internal indicator electrode functions as a zero-current potentiometric half-cell, are under consideration here. 

The cell consists of an indicator and a reference electrode, the latter usually being the calomel or silver-silver chloride type. The potential of the indicator electrode is related to the activities of one or more of the components of the solution and it therefore determines the overall cell potential. Ideally, its response to changes of activity should be rapid, reversible and governed by the Nernst equation. There are two types of indicator electrode which possess the desired characteristics - metallic and membrane. 

Fig. 18a.l. Schematic diagram of a potentiometric cell with an ion-selective electrode (ISE) as the indicator electrode. EM is the electrical potential of the sensing membrane and IFS the internal filling solution. 

Two main groups of indicator electrodes are considered here. In one case, metal indicator electrodes that exhibit a potential difference as a consequence of a redox process occurring at the metal surface are examined. Later, ISEs that can respond to ionic species based on the principles of ion extraction across an active sensing membrane will be studied in detail. 

Epoxy-based membrane of 2-[(4-chloro-phenylimino)-methyl]-phenol reveals a far Nemstian slope of 43 mV per decade for Pb+2 over a wide concentration range CIO 6 to 10 1 mol dm-3). The response time of the electrode is quite low (< 10 sec) and could be used for a period of 2 months with a good reproducibility. The proposed electrode reveals very high selectivity for Pb(II) in the presence of transition metal ions such as Cu2+, Ni2+, Cr and Cd2+at concentrations l.()xl() 3 M and 1.0><10 4 M. Effect of internal solution concentration was also studied. The proposed sensor can be used in the pH range of 2.50 - 9.0. It was used as an indicator electrode in the potentiometric titration of Pb+2 ion against EDTA. 

Tissue electrodes [2, 3, 4, 5, 45,57], In these biosensors, a thin layer of tissue is attached to the internal sensor. The enzymic reactions taking place in the tissue liberate products sensed by the internal sensor. In the glutamine electrode [5, 45], a thick layer (about 0.05 mm) of porcine liver is used and in the adenosine-5 -monophosphate electrode [4], a layer of rabbit muscle tissue. In both cases, the ammonia gas probe is the indicator electrode. Various types of enzyme, bacterial and tissue electrodes were compared [2]. In an adenosine electrode a mixture of cells obtained from the outer (mucosal) side of a mouse small intestine was used [3j. The stability of all these electrodes increases in the presence of sodium azide in the solution that prevents bacterial decomposition of the tissue. In an electrode specific for the antidiuretic hormone [57], toad bladder is placed over the membrane of a sodium-sensitive glass electrode. In the presence of the antidiuretic hormone, sodium ions are transported through the bladder and the sodium electrode response depends on the hormone concentration. 

Figure 4.17 — (A) Exploded view of a tubular flow-cell integrated microconduit system. I Ag/AgCl inner reference electrode M sensitive membrane S internal reference solution. (B) Detail of the integrated microconduit shown within the dotted lines in C. (C) Integrated-microconduit FI manifold for potentiometric measurements C carrier stream R reference electrode solution P pump V injection valve I indicator electrode R reference electrode I pulse inhibitor G ground W waste. (Reproduced from [140] with permission of Pergamon Press).

Shen et al. have reported studies on procaine-selective electrodes embodying PVC membranes [57]. Various ion-pair complexes (procaine derivatives with tetraphenylborate, dipicrylamine, tetraiodomecurate, and reineckate) were incorporated into platinized PVC membranes, and with dinonyl phthalate as the solvent mediator, formed procaine selective electrodes. The efficiency and performance of these were compared, and it was found that procaine picrylamine and procaine tetraphenylborate were the best electroactive materials. The procaine picrylamine electrode exhibited a Nemstian response over the range 10 pM to 0.1 M, and was used as the indicator electrode for the potentiometric determination of procaine. The method recovery was found to be 99.8%, with a standard deviation of 0.9%. 

Selective ion electrodes (SIE). Selective ion electrodes are essentially variants of the well-known pH meter. They are membrane indicator types of electrodes in which a potential is developed across a membrane in the presence of the ion the size of the potential is related to the concentration and hence can be used to quantitatively detect and measure the species. However, instead of a glass membrane, as in the pH meter, the membranes consist of organics that are immersible in water. For example, anion-sensitive electrodes use a solution of an anion exchange resin in an organic solvent the liquid can be held in the form of a gel, for example, in polyvinyl chloride. The ion reacts with the organic membrane, setting up an equilibrium between the free ion in solution and the ion bound to the membrane, generating a potential difference, which is measured. 

By running a potentiometric precipitation titration, we can determine both the compositions of the precipitate and its solubility product. Various cation- and anion-selective electrodes as well as metal (or metal amalgam) electrodes work as indicator electrodes. For example, Coetzee and Martin [23] determined the solubility products of metal fluorides in AN, using a fluoride ion-selective LaF3 single-crystal membrane electrode. Nakamura et al. [2] also determined the solubility product of sodium fluoride in AN and PC, using a fluoride ion-sensitive polymer membrane electrode, which was prepared by chemically bonding the phthalocyanin cobalt complex to polyacrylamide (PAA). The polymer membrane electrode was durable and responded in Nernstian ways to F and CN in solvents like AN and PC. 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

Presently, about 20 ion selective electrodes (ISE) are commonly used and classified in several groups depending on the membrane they use. These electrodes are used in direct ionometry or as indicator electrodes for many measurements involving titrimetry and complexometry with automatic titrators. 

We will study two broad classes of indicator electrodes. Metal electrodes described in this section develop an electric potential in response to a redox reaction at the metal surface. Ion-selective electrodes, described later, are not based on redox processes. Instead, selective binding of one type of ion to a membrane generates an electric potential. 

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