Potentiometric sensors half-cell

ISSs, as with any potentiometric sensor, can be treated as a galvanic half-cell and represented schematically ... 

The potential profiles in this PEVD system are illustrated in Figure 17. Although there is no driving force due to a difference in the chemical potential of sodium in the current PEVD system, the applied dc potential provides the thermodynamic driving force for the overall cell reaction (62). Consequently, electrical energy is transferred in this particular PEVD system to move Na COj from the anode to the cathode of the solid electrochemical cell by two half-cell electrochemical reactions. In short, this PEVD process can be used to deposit Na CO at the working electrode of a potentiometric CO sensor. 

Most sensors are potentiometric and are based on concentration cells. A typical arangement is shown schematically in Figure 17. The solid electrolyte, b, is sandwiched between two electrodes, c. One is exposed to the test gas and the other either to a known concentration of gas or some reference half cell. The potential... 

In Type III, there is an auxiliary phase attached on the surface of the solid electrolyte so as to be sensitive to the gas, and it is produced by a compound that contains the same ionic species as derived from the gas. The auxiliary phase can act as a sort of poor ion-conducting solid electrolyte, which forms a half cell of Type I or II as shown in Figure 1.1 [1]. Type III sensors can be divided into three subgroups depending on the types of the half cells combined [6]. Since a NASICON solid electrolyte potentiometric gas sensor using alkali metal carbonate as an auxiliary phase solid electrolyte is known to be sensitive to CO2, Type III sensors have been of immense importance as sensors for oxygenic gases such as CO2, NOx, and SOx... 

Potentiometric sensors can be classified based on whether the electrode is inert or active. An inert electrode does not participate in the half-cell reaction and merely provides the surface for the electron transfer or provides a catalytic surface for the reaction. However, an active electrode is either an ion donor or acceptor in the reaction. In general, there are three types of active electrodes the metal/metal ion, the metal/insoluble salt or oxide, and metal/metal chelate electrodes. 

Noble metals such as platinum and gold, graphite, and glassy carbon are commonly used as inert electrodes on which the half-cell reaction of interest takes place. To complete the circuitry for the potentiometric sensor, the other electrode is usually a reference electrode on which a noninterference half-cell reaction occurs. Silver/silver chloride and calomel electrodes are the most commonly used reference electrodes. Calomel consists of Hg/HgClj and is less desirable for biomedical systems in terms of the toxicity of mercury. 

The essential component of a potentiometric measurement is an indicator electrode, the potential of which is a function of the activity of the target analyte. Many types of electrodes exist (see Table 9.1), but those based on membranes are by far the most useful analytical devices. The broader field of potentiometry has been reviewed recently (1). The potential of the indicator electrode cannot be determined in isolation, and another electrode (a reference electrode) is required to complete the electrochemical cell. Undoubtedly the best known of the potentiometric indicator electrodes is the glass pH electrode, the operation and use of which has been adequately discussed (2). Ion-selective electrodes (ISEs) are also commonplace, and have been the subject of several books (3-5) there is even a review journal for ISEs (6). Unfortunately, the simplicity of fabrication and use of ISEs has given rise to the idea that ISEs are chemical sensors. At the best this is a half-truth certainly, they can behave like chemical sensors under well-controlled laboratory conditions, but in the real world their performance leaves much to be desired. Moreover, from a manufacturing point of view important features of a sensor are that it can be fabricated in relatively large numbers, and that each device is identical to all the others. Although some ISEs can be mass-produced , many cannot, and even those that do lend themselves to this form of production invariably require calibration before use. Nonetheless, in spite of the limitations of ISEs, transducers based on potentiometric membrane electrodes have much to contribute to the field of chemical sensing. 

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