Electrode: auxiliary working

FIGURE 3-23 Schematic of a carbon-fiber amperometric detector for capillary electrophoresis A, fused silica capillary B, eluent drop C, stainless steel plate RE, reference electrode WE, working electrode, AE, auxiliary electrode. (Reproduced with permission from reference 58.)... 

Electrochemical measurements usually concern not a galvanic cell as a whole but one of the electrodes, the working electrode (WE). However, a complete cell including at least one other electrode is needed to measure the WE potential or allow current to flow. In the simplest case a two-electrode cell (Eig.l2.1a) is used for electrochemical studies. The second electrode is used either as the reference electrode (RE) or as an auxiliary electrode (AE) to allow current to flow. In some cases these two functions can be combined for example, when the surface area of the auxiliary electrode is much larger than that of the working electrode so that the current densities at the AE are low, it is essentially not polarized and thus can be used as RE. 

To overcome these difficulties one must use a three-electrode cell, which is shown schematically in Figure 8. Here, a third electrode, auxiliary electrode (AE) is inserted together with the working and the reference electrodes. 

Voltammetry is the second most utilized technique for electronic tongue devices (see Fig. 2.6). It is a d)mamic electroanalytical method, that is, a current flow passes through the measurement cell (z 0). Voltammetry consists of the measurement of current at a controlled potential constant or, more frequently, varying. In the classic three-electrode cell configuration, the current flows between two electrodes, called working and counter (or auxiliary) respectively, while the potential is controlled between the working and a third electrode, the reference (Kissinger and Heineman, 1996). 

Gaseous Catalyst working Solid electrolyte Counter-electrode Auxiliary gas... 

Figure 9.1 Equivalent circuit of an electrochemical cell. A, Auxiliary electrode R, reference electrode W, working electrode Rc, compensated resistance R , uncompensated resistance Rr, reference electrode impedance Zf, faradaic impedance Cdl, doublelayer capacitance.

This would be accomplished by immersing the chemically modified electrode, a reference electrode, and an auxiliary electrode into an appropriate electrolyte solution (e.g., 0.1 M NaC104 in acetonitrile). The potential difference between the modified electrode (the working electrode) and the reference would then be adjusted to a value appropriate to drive this reaction, using a commercially available potentiostat, and the resulting anodic current would be measured. 

Rule 1. The first rule is the requirement of the closed electrical circuit. This means that at least two electrodes must be present in the electrochemical cell. From a purely electrical point of view, it means that we have a sensor electrode (the working electrode) and a signal return electrode (often called the auxiliary electrode). This requirement does not necessarily mean that a DC electrical current will flow in a closed circuit. Obviously, if we consider an ideal capacitor C in series with a resistor R (Appendix C), a DC voltage will appear across the capacitor, but only as a transient DC current will not flow through it. On the other hand, if an AC voltage is applied to the cell, a continuous displacement charging current will flow. 

Prepare a solution of 0.01 M aniline+O.lM HC1. Assemble a three-electrode cell by using the polished GC disk electrode as working electrode, a bare GC rod as auxiliary electrode and a Ag/AgCl/3M KC1 as reference electrode. Deaerate the cell solution (0.01 M aniline+0.1M HC1) by nitrogen (or argon) bubbling for ca. 

The three electrodes the working microelectrode, the saturated calomel reference electrode and the platinum auxiliary electrode are immersed in a 5 mmol L-1 deaerated ferricyanide solution. 

In the meantime, set the chronoamperometric unit with the Ag/ AgCl as reference electrode, the platinum as auxiliary electrode and the modified GEC electrode as working electrode. Set the potential fixed at — 0.100 V. 

Figure 1. Thin-layer electrochemical detector ( (A) auxiliary electrode (R) reference electrode (W) working electrode)...

In cyclic voltammetry, both the oxidation and reduction of the metal complex (called the analyte from now on) will take place in one electrochemical cell. This cell houses the analyte solution as well as three electrodes, the working electrode, the auxiliary electrode and the reference electrode. Electron transfer to and from the metal complex takes place at the working electrode surface (Fig. A.2.2) and does so in response to an applied potential, /iapp, at the electrode surface. During the experiment, current develops at the surface as a result of the movement of analyte to and from the electrode as the system strives to maintain the appropriate concentration ratio (0, through electron transfer, as specified by the Nemst equation. 

Figure 2-9 Typical EIS device. Auxiliary, working, and reference electrodes are shown in gold, bine, and green, respectively. In a solntion (gray transparent cube), a redox compound snch as Fe(CN)g will complete the circnit between the auxiliary and working electrodes, bnt upon application of a protein that wonld bind to a SM immobilized on the working electrode, the access of the redox solnte may be severely limited, resnlting in impedance changes. (See insert for color representation of figure.)...

Fig. 2. Pictures of transducer body construction ( A, B ) GECE-M preparation ( C, D ) system of three electrodes, from left to right, auxiliary, working and reference immersed into electrochemical cell (E) electrochemical analyzer Autolab PGSTAT 20 connected to a personal computer, at which DPV electrochemical detection of AuNPs was carried out (F).

B, central platinum auxiliary electrode C, Luggin capillary for reference electrode D, working electrode lead F, quartz tube. [Reprinted with permission from R. D. Allendoerfer, G. A. Martinchek, and S. Bruckenstein, Anal. Chem., 47, 890 (1975). Copyright 1975, American Chemical Society.]... 

The auxiliary electrode is used to complete the electrochemical circuit allowing current to flow between the working and auxiliary electrodes so that E is more accurately measured between the working and reference electrodes. Auxiliary electrodes have small surface areas, like a wire, for analysis methods and large surface areas for BE methods. While the redox reaction of the analyte takes... 

Detector cells can be made which contain only two electrodes, a working electrode and a reference electrode. A preselected potential equal to or greater than the half wave potential of interest is applied constantly across the electrodes. However, two electrode systems give a non-linear response as the voltage drops across the eluant as the current flow changes. Thus electrochemical detectors typically employ a three electrode cell. The additional electrode known as the auxiliary or counter-electrode, serves to carry any current generated in the flow cell thus enabling the reference electrode to ensure a fixed potential despite the decrease in the internal resistance of the detector cell. 

Denomination of electrodes h Working electrode (WE) Counter-electrode (or auxiliary electrode, CE) Reference electrode (Ref)... 

FIGURE 4,3.16. Block diagram of the circuit for cyclic voltammetry. A Auxiliary electrode W Working electrode R Reference electrode. 

Reference electrode Auxiliary electrode Working electrode Outlet... 

Instruments suitable for voltammetry and am-perometry consist of three basic components a wave-form generator, some form of potential control, and an electrochemical cell. Modem electro-analytical systems employ a three-electrode arrangement for the electrochemical cell. A device called a poleiuiosiat is used to maintain a programmed or fixed potential difference between the two current-carrying electrodes (the working electrode and the auxiliary electrode) relative to a third electrode (reference electrode), the function of which is to provide a fixed potential reference in the cell [64], [65]. 

Electrochemical detector. The electrochemical detector responds to substances that are either oxidizable or reducible, and the electrical output results from an electron flow caused by the chemical reaction that takes place at the surface of the electrodes. The detector normally has three electrodes— the working electrode (where the oxidation or reduction takes place), the auxiliary electrode and the reference electrode (which compensates for any change in the electrical conductivity of the mobile phase). 

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