Types of membrane electrodes

In coulometry these exchange membranes are often used to prevent the electrolyte around the counter electrode from entering the titration compartment (see coulometry, Section 3.5). However, with membrane electrodes the ion-exchange activity is confined to the membrane surfaces in direct contact with the solutions on both sides, whilst the internal region must remain impermeable to the solution and its ions, which excludes a diffusion potential nevertheless, the material must facilitate some ionic charge transport internally in order to permit measurement of the total potential across the membrane. The specific way in which all these requirements are fulfilled in practice depends on the type of membrane electrode under consideration. 

Table 2.3 shows the various types of membrane electrodes, a few of which are shown schematically in Fig. 2.10. 

At present other types of membrane electrodes are not yet of real importance with regard to non-aqueous media however, with developments in the field of crowns ethers (see p. 70), extensively reviewed by Kolthoflf127, this may change in the near future. 

Ion-selective electrodes (ISE) are membrane-based devices with internal filling solution and internal reference electrode or with internal solid contact. Ion-selective electrodes may be classified according to the nature of the ion-selective membrane or the shape and size of the electrode arrangement. The main types of membrane electrodes are ... 

Various types of membrane electrodes have been developed in which the membrane potential is selective toward a given ion or ions, just as the potential of the glass membrane of a conventional glass electrode is selective toward hydrogen ions. These electrodes are important in the measurement of ions, especially in small concentrations. Generally, they are not poisoned by the presence of proteins, as some other electrodes are, and so they are ideally suited to measurements in biological media. This is especidly true for the glass membrane ion-selective electrodes. 

Membrane electrodes are a class of electrodes that respond selectively to ions by the development of a potential difference (a type of junction potential) across a membrane that separates the analyte solution from a reference solution. The potential difference is related to the concentration difference in the specific ion measured on either side of the membrane. These electrodes do not involve a redox reaction at the surface of the electrode as do metallic electrodes. Because these electrodes respond to ions, they are often referred to as ion selective electrodes (ISEs). The ideal membrane allows the transport of only one kind of ion across it that is, it would be specific for the measurement of one ionic species only. As of this writing, there are no specific ISEs, but there are some highly selective ones. Each electrode is more or less selective for one ion therefore, a separate electrode is needed for each species to be measured. In recent years, many different types of membrane electrodes have been developed for a wide variety of measurements. 

Since the sample solutions always contain similar concentrations of an analyte ion and its counter ions, the primary requirement for Nemstian responses is selective membrane permeability for the analyte against the aqneous counter ions. The selectivity can be obtained simply by doping a membrane with ionic sites that have a charge opposite to that of the analyte. This type of membrane electrode based on ionic sites is called ion-exchanger-based ISEs or more specifically, ionophore-free ion-exchanger-based ISEs because any liquid membrane ISE has an ion-exchange capability for Nemstian responses. Table 7.1... 

Two types of membrane electrode systems have been developed that act selectively toward certain types of molecules. One of these is used for the determination of dissolved gases, such as carbon dioxide and ammonia. The other, which is based on biocatalytic membranes, permits the determination of a variety of organic compounds. such as glucose and urea. 

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. 

Another important point in coulometric titration is the necessity in many instances for internal generation to keep the electrolyte around the auxiliary electrode separated from the analyte solution around the working electrode by means of a diaphragm or a special type of membrane we describe below two important examples of this situation. 

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 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. 

The catalytic effect of the type of mixed electrodes occiu s also in a ntunber of bio-redox reactions on bio-membranes and also on enzymes as shown in Fig. 11-4 ... 

Very little work (relative to research of electrode materials and electrolytes) is directed toward characterizing and developing new separators. Similarly, not much attention has been given to separators in publications reviewing batteries.A number of reviews on the on cell fabrication, their performance, and application in real life have appeared in recent years, but none have discussed separators in detail. Recently a few reviews have been published in both English and Japanese which discuss different types of separators for various batteries. A detailed review of lead-acid and lithium-ion (li-ion) battery separators was published by Boehnstedt and Spot-nitz, respectively, in the Handbook of Battery Materials. Earlier Kinoshita et al. had done a survey of different types of membranes/separators used in different electrochemical systems, including batteries."... 

Bacterial electrodes [11, 31, 33, 46, 48, 49, 60] In this type of electrode, a suspension of suitable bacteria is placed between the sensor proper and a dialysis membrane that prevents passage of high-molecular substances (see fig. 8.3). The sensor is usually a gas probe. In the simple types of bacterial electrode, the determinand is converted by a suitable strain of bacteria into a product sensed by the gas probe. Thus it is possible to determine arginine [46], glutamine [48],/.-aspartic acid [31],/.-histidine [60] and nitrate [33]. Hybrid bacterial - enzyme electrodes contain both a bacterial strain and a suitable enzyme. For example, an extract from ivingas Neurospora chossa can be used as a source of NAD nucleosidase and an Escherichia coli culture as a source of nicotinamide deaminase, so that the electrode responds to NAD [49] as a result of the series of reactions... 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

There are two main classes of membrane equilibria [14] general equilibria and specific equilibria. The general equilibria are ion-exchange equilibria, Donnan equilibria and extraction-re-extraction equilibria. The specific equilibria are, for example, redox equilibria, complexation-de-complexation equilibria, etc. These specific equilibria determine the type of the electrode. For EPMEs, the main equilibria that take place are extraction-re-extraction and complexation-de-complexation. 

There are three basic types of selective electrode those based on glass membranes, on inorganic salt solid membranes, and on ion exchange. Other more complex electrodes are sensitive to dissolved gases and enzymes. These various types are now described. 

In this type of selective electrode, the membrane is an ionic solid which must have a small solubility product in order to avoid dissolution of the membrane and to ensure a response that is stable with time. Conduction through the membrane is principally ionic and is due to point defects in the crystal lattice, relying on the fact that no crystal is perfect. 

Conventional reference electrodes consist of a solid reversible electrode and an aqueous electrolyte solution. To measure the individual contributions from the anode and the cathode of a PEM fuel cell, the electrolyte solution of the reference electrode must either be in direct contact with one side of the solid proton exchange membrane or be located in a separate compartment with electrical contact between the reference electrode and the solid membrane by means of a salt bridge [66], As a result, two different types of reference electrode configurations are employed for the study of fuel cells internal and external. 

The external type of reference electrode is connected to the membrane via a liquid electrolyte bridge, such as a sulphuric acid solution, as shown in Figure 5.45. Compared with the internal reference electrode configuration, the external type is easier to use in a normal PEM fuel cell set-up because it needs minimal modifications. However, attention must also be paid to ensure that the liquid electrolyte has good contact with the membrane and does not flow into the cell. Furthermore, the use of a liquid electrolyte in an acid bridge can induce non-uniform hydration and a proton concentration gradient in the membrane, therefore interfering with the fuel cell electrodes. 

As can be seen in these figures, the low-frequency arc, which is dependent on electrode potential, is the well-known kinetic arc. But close examination of the high-frequency region (1-20 kHz) in the AC spectra reveals a certain distortion of the kinetic arc from a perfect semicircle, which is the result of superimposition of a small potential-independent impedance branch on the kinetic arc. This behaviour was observed for all types of membranes, experimental conditions, and electrodes employed in Paganin et al. s [4] work. 

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