Membrane electrode systems, types

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-... 

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. 

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. 

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). 

These selection and evaluation criteria were applied systematically to four technological fields, three of which contribute to new energy-efficient solutions. Passive houses, for example, with their major components of insulation solutions, window systems, ventilation and control techniques are close to market diffusion within the next ten years. Fuel cells for mobile uses in vehicles, however, are still a long way from market introduction, for instance, because of unresolved problems regarding the deactivation of the membrane electrode assembly (MEA) and the need for cost reductions by about one order of magnitude. Other types of fuel cells for stationary uses may be closer to market introduction, owing to less severe technical bottlenecks and better economic competitiveness. 

In the previous papers(12,13), we reported on the vessel access type, i.e. tubular type, glucose sensor. It consisted of a glucose electrode system with a GOX enzyme immobilized Nylon membrane and a glucose semipermeable membrane, and a reference oxygen electrode system. The sensor could directly measure up to 700 mg/dl of BGL in an arterial blood stream when it was placed into an external A-V shunt. This sensor, however, has some problems such as thrombus during in vivo testing without heparin and clinical complexity associated with implanting the sensor in a blood stream. 

Bipolar electrolysis systems are characterized by the type of electrolyte. The proton exchange membrane (PEM) system, developed by the General Electric Compare (GE), uses as the electrolyte a thin membrane of sulfonated fiuorocaibon (Nation ) that conducts electricity when saturated with water. Electrodes are formed by depositing a thin platinum film on opposite sides of the merrtbrane to form a bipolar cell. An electrolyzer is made by stacking 50-200 cells in series, with srritably formed separators to direct the exhaust gases into charmels at the sides. Since the membrane is the electrolyte, only pine water needs to be supphed to the cell. When the cell oper-... 

The different procedures based on the measurement of electromotive force that may be applied in many varied ways in the equilibrium chemistry of aqueous solutions have a much more restricted use in non-aqueous solutions. It is difficult to construct measuring cells that have small and readily reproducible diffusion potentials or that are without a liquid junction. The glass electrode, perhaps the most extensively used type in aqueous solutions, does not function at all in some non-aqueous solutions, and with very low accuracy in others. Evaluation of the data obtained with its use is hampered by the limits of the acidity scales employed in such systems. The most promising type of ionselective membrane electrodes, the liquid ionic exchange membrane electrode, virtually cannot be employed in non-aqueous solutions. 

Therefore, the identification of the most suitable electrolyte media, where the passivation rates remain confined to a limited extent, is essential for the development of long-life LPBs. The answer may again be provided by replacing the simple PEO-LiX system with the composite membranes of the type of those previously described. By finely dispersing in the polymer structure ceramic additives which have high affinity for the electrolyte impurities, and in particular for water impurities, a network of trapping centres for the impurities themselves can be provided and thus ensure their removal from the interface with the final result of controlling the corrosion rate of the electrode. 

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