Sensing electrode auxiliary

The proposed operating mechanism of a CO2 sensor based on soUd Sc2(W04>3 and YSZ together with an auxiliary Li2C03-sensing electrode is as follows. (Auxiliary sensing electrode)... 

Since Sca(W04)3 and LiaCOs are the base electrolyte and auxiliary sensing electrode and Lig(W04)3 is formed between the LiaCOs and the Sc " ion-conducting solid electrolyte, the activities (a) of these soUds should be unity. In addition, as the entire sensor cell is exposed to the same atmosphere (as shown in Fig. 17.1), O2 is exactly equal to Oa . Based on this, the Nemst equation (17.6) is simplified as follows ... 

Sensors may also use multiple electrolytes (referredto as electrolyte chains), and two examples of these are shown in Figure 13.Id and e. In both examples, a Na2SO4 auxiliary electrode is used with a sodium ion conductor so that the sensing electrode reaction is given by Equation (13.5). The difference between the two cases is that, in Figure 13.Id, the sodium ion conductor is used with another cation (strontium) conductor, whereas in Figure 13.le the sodium ion conductor is used with an anion (oxygen) conductor. In both cases, an equilibrium reaction is required to relate the... 

Electrochemical analytical techniques are some of the oldest in chemistry and can be divided into potentiometry, voltammetry and conductimetry. They are most important as detectors after chromatographic separations and as chemical and biological sensors. They generally involve the use of electrodes that are housed in electrochemical cells. All electrochemical cells contain two electrodes but some have three. The first electrode is the actual working electrode (also called a sensing or indicator electrode) and the second is a combined reference electrode and auxiliary (counter) electrode. If there are three electrodes, the reference and counter electrodes are separate. 

There are two generic types of device used for earth leakage detection those that are voltage-operated and those that are current-operated. The voltage-operated devices are no longer used but, for completeness, they consisted of a coil connected in series in the earthing conductor or between the metalwork of the installation and an auxiliary earth electrode. The device sensed a voltage rise in the metalwork with respect to earth and, when this occurred, tripped the circuit breaker. 

In electrochemical kinetics, the concept of the electrode potential is employed in a more general sense, and designates the electrical potential difference between two identical metal leads, the first of which is connected to the electrode under study (test, working or indicator electrode) and the second to the reference electrode which is in a currentless state. Electric current flows, of course, between the test electrode and the third, auxiliary, electrode. The electric potential difference between these two electrodes includes the ohmic potential difference as discussed in Section 5.5.2. 

This sensing model also implies that the geometric properties, instead of solely the physical properties of the auxiliary phase at the working electrode, are critical to kinetic performance. Accordingly, the four most important geometric criteria for an auxiliary phase are as follows ... 

Maruyama et al.[57] of Tokyo Institute of Technology, Japan, reported COj sensing characteristics of galvanic cells composed of Na" conductor like NASICON or P-Al Oj, and NajCOg as an auxiliary electrode. The cell composition is expressed as follows. 

The pxjtential measured between WE and RE may be substantially influenced by em IR drop, in particular if high current densities are applied or an electrolyte of low conductivity is used. This is due to the fact that the reference electrode is connected with a point in the solution some distance away from the electrochemical double layer. Thus an ohmic resistance - the so-called uncompensated resistance (R j) - is included in the solution between the tip of the reference electrode and the surface of the working electrode (Fig. 6.1). As a result an error will be introduced in the measurement of the potential difference in such a way that the potential difference between the working and reference electrodes is not as large in an absolute sense as indicated by the potentiostat or an auxiliary voltmeter. 

The area of steel sensed by the electrodes is obviously far smaller than the whole rebar network, however we cannot assume that the area measured is that directly below the electrode as the current fans out as shown in Figure 4.15. Assuming a 1 1 relationship between probe size and area of steel polarized can lead to errors of up to lOOX in our measurement (Fliz et al., 1992). However, this is mainly at low corrosion rates. One device uses a large electrode with a conductive foam pad as the auxiliary electrode to minimize wetting of the concrete and ensure that area of measurement is directly below the electrode as shown in Figure 4.16. 

Gas sensors and biosensors are obtained by fixing an auxiliary chemical or biochemical system over the ISE membrane. The analyte reacts with the auxiliary system with production or consumption of the ion that is sensed by the ISE. Two basic types of gas sensor, the Severinghaus electrode and the air-gap electrode, are described elsewhere in this encyclopedia. 

Fig. 3.3 Active microfluidic transport system with integrated components for optical sensing based on ECL a completed chip b planar layout of the entire system c magnified view of the mixing area showing electrodes for the transport, mixing, and generation of the ECL. The dimensions of the chip are 14 x 20 mm. W.E., working electrode R.E., reference electrode A.E., auxiliary electrode (Reprinted with permission from Ref. [17]. Copyright 2006 Elsevier B.V)...

It is worth recalling that amperometric measurements were first employed to follow redox titrations [1] in which (1) either the reactant or the relevant reaction product or (2) either the titrant or the relevant reaction product are electroactive. Although the titrant volume in amperometric titration measurements is an example of independent variable, it is obvious that time or potential of the working electrode constitute the most useful and frequently encountered variables in amperometric sensing. The i vs. t or i vs. E plots, where i is the current flowing in the working plus auxiliary electrode circuit, constitute the signal to analyse in chronoamperometric and voltammetric measurements, respectively. 

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