Electrochemical sensors electrolytes

FICs are useful as electrochemical sensors, electrolytes and electrodes in batteries and in solid state displays (Farrington Briant, 1979 Ingram Vincent, 1984). If a FIC material containing mobile M ions separates two compositions with different activities of M, a potential is set up across the FIC that can be related to the difference in the chemical activities of M. By fixing the activity on one side, the unknown activity on the other can be determined. This principle forms the basis of a number of ion-selective electrodes LaFj doped with 5% SrF2 is used for monitoring fluoride ion concentration in drinking water. Similarly, calcia-stabilized-zirconia is used in cells of the type... 

Electrochemical Microsensors. The most successful chemical microsensor in use as of the mid-1990s is the oxygen sensor found in the exhaust system of almost all modem automobiles (see Exhaust control, automotive). It is an electrochemical sensor that uses a soHd electrolyte, often doped Zr02, as an oxygen ion conductor. The sensor exemplifies many of the properties considered desirable for all chemical microsensors. It works in a process-control situation and has very fast (- 100 ms) response time for feedback control. It is relatively inexpensive because it is designed specifically for one task and is mass-produced. It is relatively immune to other chemical species found in exhaust that could act as interferants. It performs in a very hostile environment and is reHable over a long period of time (36). 

Electrochemical sensors with a liquid electrolyte are widely used for the detection of corrosive or toxic gases in the workplace. Portable monitors are used in short time measurements of exhaust gases as well. These sensors work amperometri-cally - an external voltage supply is connected with the electrode on both sides of the measuring cell. 

In the early part of this century, many types of solid electrolyte had already been reported. High conductivity was found in a number of metal halides. One of the first applications of solid electrolytes was to measure the thermodynamic properties of solid compounds at high temperatures. Katayama (1908) and Kiukkola and Wagner (1957) made extensive measurements of free enthalpy changes of chemical reactions at higher temperatures. Similar potentiometric measurements of solid electrolyte cells are still made in the context of electrochemical sensors which are one of the most important technical applications for solid electrolytes. 

Generally, in solid electrolytes, ionic conductivity is predominant (( = 1) only over a limited chemical potential. The electrolytic conductivity domain is an important factor limiting the application of solid electrolytes in electrochemical sensors. 

One area where the relationship between the structure of the polymer matrix and the physical processes of the thin layer has been studied in detail is that of electrodes modified with polymer films. The polymer materials investigated in these studies include both conducting and redox polymers. Such investigations have been driven by the many potential applications for these materials. Conducting polymers have been applied in sensors, electrolytic capacitors, batteries, magnetic storage devices, electrostatic loudspeakers and artificial muscles. On the other hand, the development of electrodes coated with redox polymers have been used extensively to develop electrochemical sensors and biosensors. In this discussion,... 

However, the interplay between electrolyte and polymer layer needs to be considered when optimizing the performances of polymer-modified electrodes. Structural factors will influence the interfacial ion transport and this will have a direct effect on the mechanism and location within the polymer layer of the mediation process. The following discussion will show that the nature of the mediation process can be changed dramatically by changing the electrolyte, from a situation where an electrochemical sensor with good sensitivity is obtained, to a situation where the sensitivity obtained is not much better than that observed for the bare electrode. 

Figure 6.5. Cross-sectional view of an electrochemical sensor.Key 1. Membrane 2. Thin-film electrolyte 3. Working electrode 4. Counter electrode 5. Electrolyte. (Courtesy of Enmet Corporation, Ann Arbor, Ml.)...

Electrolytes are used in electrochemistry to ensure the current passage in -> electrochemical cells. In many cases the electrolyte itself is -> electroactive, e.g., in copper refining, the copper(II) sulfate solution provides the ionic conductivity and the copper(II) ions are reduced at the - cathode simultaneous to a copper dissolution at the - anode. In other cases of -> electrosynthesis or - electroanalysis, or in case of - sensors, electrolytes have to be added or interfaces between the electrodes, as, e.g., in case of the -> Lambda probe, a high-temperature solid electrolyte. 

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. 

The performance of each type of electrochemical sensor described above depends on the selection of appropriate materials, for both the electrolyte and electrode. The most appropriate materials used depend not only on the sensor type, but also on the species being detected. 

Cation-conducting electrolytes (see Chapter 7) are also used in electrochemical sensors, one of the most widely used being 3 alumina [55, 56]. The 3 alumina structure consists of relatively densely packed spinel blocks that are separated by less densely packed planes through which ionic conduction occurs. The most common example is sodium 3 alumina, which is a Na+ ion conductor, although the sodium can be exchanged with other ions to create electrolytes that conduct other cations [57], as well as other species, such as O [58] (see also Chapter 8). 

Zosel, J. and Guth, U. (2004) Electrochemical solid electrolyte gas sensors-hydrocarbon and NOx analysis in exhaust gases. Ionics, 105 (5-6), 366—77. 

FIGURE 4.16 Instrumentation scheme for impedance measurements 1 generator, 2, 4, and 6 amplifiers 3 attenuator 5 filter 7 zirconia oxygen sensor 8 osdlloscope C capacitance and Z electrochemical sensor impedance. (From Zhuiykov, S., In-situ diagnostics of solid electrolyte sensors measuring oxygen activity in melts by developed impedance method, Meas. Sci. Technol. 17 (2006) 1570-1578. With permission.)... 

Thus, in Ca() Zr() 901 9 (consistent with the ionic charges Ca2+, Zr4+ and O2-) the ratio of anions to cations is less than the value 2 1 required for the normal Zr02 lattice, so that oxygen vacancies are present. Doped Zr02 is used as a solid electrolyte in electrochemical sensors and in fuel cells. One important application is in sensors that measure the 02 concentration of exhaust gases from... 

The manufacture of electrochemical sensors has evolved considerably and their production has diversified. The three electrodes of the sensor shovm in the example of Figure 20.7 result from a manufacturing technique characteristic to microelectronic circuits. Completed by a membrane and an electrolyte, this sensor is used to measure chloride dissolved in water. Based upon the oxidation of the CIO (hypochloric acid) ion its detection limit is 1 ppb. 

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