Electrodes Electrolyte conductivity sensor

Most of the analytes (pC02, Na , K, Ca , and pH) are determined by potentiometric measurements using membrane-based ion-selective electrode technology. The hematocrit is measured by electrolytic conductivity detection, and p02 is determined with a Clark voltammetric sensor (see Section 23B-4). Other results are calculated from these data. 

An electrochemical sensor is generally an electrochemical cell containing two electrodes, an anode and a cathode, and an electrolyte. Electrochemical sensors in general are classified, based on the mode of its operation, and they are conductivity sensors potentiometric sensors, and voltammetric sensors. Amperometric sensors can be considered as a special type of voltammetric sensors. The fundamentals of these sensors operational principles are described exceptionally well in several excellent electro-analytical books. In this entry, only the essential features are included. 

Solid-state reference electrodes for potentiometric sensors are currently under research. The main problem to be faced in developing this type of electrode lies in connecting the ionic conducting (usually aqueous) solution with an electronic conductor. Since the reference electrode has to maintain a defined potential, the electrochemical reaction with components of the electrolyte has to be avoided. Oxides, mixed oxides, and polyoxometalate salts of transition elements can be proposed for preparing solid-state reference electrodes. Tested compounds include tungsten and molybdenum oxides (Guth et al., 2009). 

One of the advantages of mixed potential sensors is that it is possible for both electrodes to be exposed to the same gas. The elimination of a need to separate the two electrodes simplifies the sensor design, which in turn reduces fabrication costs. Although this simpler planar design is often used, the electrodes are sometimes separated to provide a more stable reference potential. As with equilibrium potentiometric sensors, the minimum operating temperature is often limited by electrolyte conductivity. However, the maximum operation temperatures for nonequilibrium sensors are typically lower than those of equilibrium sensors, because the electrode reactions tend towards equilibrium as the temperature increases. This operating temperature window depends on the electrode materials, as will be discussed later in the chapter. 

Lithium carbonate can be used more directly as the auxiliary electrode with a lithium ion conductor, since the in situ formation of another carbonate phase is not required. Lithium ion conductors used with Li2CO3 include LISICON [163] and other Li3PO4-based electrolytes [164—173]. As with sodium ion conductors, Li2CO3-containing carbonates are used with lithium ion conductors [174, 175], The outputs of some examples of these lithium ion-conducting electrolyte-based sensors are shown in Figure 13.11 [163, 172-174]. 

Consequently, the proposed model allows the necessary information regarding the electrolyte-metal electrode interface and about the character of the electronic conductivity in solid electrolytes to be obtained. To an extent, this is additionally reflected by the broad range of theoretical studies currently published in the scientific media and is inconsistent with some of the research outcomes relative to both physical chemistry of phenomena on the electrolyte-electrode interfaces and their structures. Partially, this is due to relative simplifications of the models, which do not take into account multidimensional effects, convective transport within interfaces, and thermal diffusion owing to the temperature gradients. An opportunity may exist in the further development of a number of the specific mathematical and numerical models of solid electrolyte gas sensors matched to their specific applications however, this must be balanced with the resistance of sensor manufacturers to carry out numerous numbers of tests for verification and validation of these models in addition to the technological improvements. 

Electrochemical, such as ion-selective electrodes (ISE), ion-selective field affects transistors (ISEET), solid electrolyte gas sensors, semiconductor-based gas sensors, and conducting polymer sensors. Most electrochemical sensors are based on potentiometry, voltammetry, or amperometry although coulometry and conductimetry have also been utilized. 

The electrolyte conductance measurement technique, in principle, is relatively straightforward. However, the conductivity measurement of an electrolyte is often complicated by the polarization of the electrodes at the operating potential. Faradaic or charge transfer processes occur at the electrode surface, complicating the conductance measurement of the system. Thus, if possible, the conductivity electrochemical sensor should operate at a potential where no faradaic processes occur. Also, another important consideration is the formation of the double layer adjacent to each electrode surface when a potential is imposed on the electrochemical sensor. The effect of the double layer complicates the interpretation of... 

The most successful SECSs are those which use zirconia-based electrolytes to measure oxygen concentrations. The three most common applications of these electrolytes are to measure oxygen concentrations of steel melts and in combustion gas environments and to control the air-fuel ratio in automobile engines. In the latter two applications, there is increasing interest in lowering the sensor temperature below 600°C, the current minimum temperature of operation because of low ionic conductivity and slow charge-transfer reactions at electrode-electrolyte interfaces. 

Santhosh P, Manesh KM, Gopalan A, Lee K-P (2007) Novel amperometric carbon monoxide sensor based on multi-wedl carbon nanotubes grafted with polydiphenylamine-fabrication and performance. Sens Actuators B 125 92-99 Shai K, Wagner J (1982) Enhanced ionic conduction in dispersed solid electrolyte systems (DSES) and/or multiphase systems Agl-Al Oj, Agl-SiO, Agl-Ely ash, and Agl-AgBr. J Sohd State Chem 42 107-119 Shimizu Y, Yamashita N (2000) Solid electrolyte CO sensor using NASICON and perovskite-type oxide electrode. Sens Actuators B 64 102-106... 

Measurement Principles. The equipment used for measuring conductance usually consists of a conductivity sensor, a measuring attachment, and a temperature-compensation unit. Conductance values are obtained by establishing either the current flow between a set of electrodes subjected to a constant AC voltage, or the current induced in the secondary coil of a transformer connected to the primary coil by the electrolyte of interest. Capacitive coupled cells have been developed as well, but because of their expensive construction they are no longer of interest (98)-[101]. 

Parameter Severinghaus electrode Conductivity sensor Solid electrolyte sensor NDIR sensor... 

As can be seen in Fig. 15.1a, the first gas-diffusion electrodes were designed based on back-side metallized porous membranes and the presence of liquid electrolyte. However, there is other approach to diffusion electrode design (Knake et al. 2005). It was found that the liquid electrolyte in a GDE could be replaced by a solid, ionically conductive polymer. Thus, whole electrode-electrolyte assemblies consisting of porous electrodes and ion-exchange polymers were manufactured. This arrangement is depicted in Fig. 15.1b. When using SPE-membrane-based sensors, the electrodes face the sample gas and, therefore, essentially no diffusion barrier is present. Furthermore, they have usually been used in systems with a forced gas flow. 

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