Electrode auxiliary/counter

Figure 6.17. Schematic diagram of apparatus for galvanostatic measurements P, constant current power supply e, test electrode e2, reference electrode counter (auxiliary)-electrode V, potential-time recording instrument.

The working electrode is kept at zero potential by connecting it to the virtual ground of the electronics. A counter electrode CE (auxiliary electrode ae) is used to regulate the potential difference between the reference electrode and the working electrode, necessary for electrolysis of the analyte. 

Define working electrode, reference electrode, counter electrode, and auxiliary electrode. 

The way in which the three main processes (electrode reaction, doublelayer charging, and conduction) at one electrochemical interface concomitantly influence the relation between current and voltage is illustrated in Fig. 1. In the experiments, the total electrochemical cell contains two such interfaces. For kinetic studies, however, this complication is usually eliminated by making the surface area of the electrode of interest (the working electrode or indicator electrode ) much smaller than that of the second electrode (the auxiliary electrode or counter electrode ). 

Electrochemical detectors are constructed with three electrodes. The electrolysis of interest takes place at the working (marked W in Fig. 4.5) electrode at a potential measured by the reference electrode. The auxiliary (or counter) electrode potential is controlled to maintain the reference potential. The reference electrode is usually saturated calomel or silver/silver chloride. Platinum or glassy carbon is generally used for the auxiliary electrode [67, 53]. 

In experiments where iR may be high (e.g., in large-scale electrolytic or galvanic cells or in experiments involving nonaqueous solutions with low conductivities), a three-electrode cell (Figure 1.3.10) is preferable. In this arrangement, the current is passed between the working electrode and a counter (or auxiliary) electrode. The auxiliary electrode can be any convenient one, because its electrochemical properties do not... 

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. 

The relationship between electrode potential and current is determined by the electrochemical reaction taking place at the working electrode. Measurements are done usually in a three-electrode arrangement with a reference electrode to control the potential of the working electrode (typically no current is in practice allowed to flow through the reference electrode and its potential is constant) and a counter (auxiliary) electrode where a counterbalancing but not rate-deterrnmirig electrode process takes place. In cyclic voltammetry for a reversible electrode reaction, the cathodic, pc and anodic, pa peak potentials depend on the formal potential, E ... 

The counter (auxiliary) electrode is used in the three-electrode system only. In this system, the current flows between the working and the counter electrode. Either a piece of platinum foil or a platinum or titanium wire is usually employed as the counter electrode. Carbon rods are also used. It is recommended that the area of the counter electrode is substantially larger than that of the working electrode. If this condition is met, the counter electrode should not affect the current measurement due to, e.g. passivation, deactivation and blocking of the surface. 

The section of Fig. 24 to the left is marked Polari2ation Control and is involved in electrical control of the cell potential or current. Note that the instrument can operate as either a potentiostat or galvanostat and the potential or current applied to the cell is programmed by summing the internal sources, marked DC Ref and Sweep, with an external polarity input. The box marked Feedback/Bandwidth control represents actual control circuitry similar to that in Figs. 8 and 11. Relays and electronic switches, controlled by the internal microprocessor, allow switching between potentiostatic or galvanostatic mode. Tbe microprocessor also sets the control loop bandwidth, which allows the experimenter to trade bandwidth for increased control loop stability. Other inputs to the feedback control circuitry are the RE potential and IR compensation (if necessary). A power amplifier is inserted at the counter (auxiliary) electrode connection. This allows currents of up to 2 A to be applied to the cell. 

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

Two-electrode and three-electrode electrochemical cells used for the electrochemical measurements are shown in Figures 8(a) and (b), respectively. In both cell configurations, lithium foil is selected as the counter (auxiliary)... 

Auxiliary electrode. The auxiliary electrode is a conductor that completes the cell circuit. The auxiliary (counter) electrode in lab cells is generally an inert conductor like platinum or graphite. In field probes it is generally another piece of the working electrode material. The current that flows into the solution via the working electrode leaves the solution via the auxiliary electrode. 

Fig. 6.4 Combination of an RF heating unit (framed arrangement, right) with a common potentiostat (/ ) to form a hot-wire electrochemical device. The triggering facility is necessary only for pulse heating. Potentiostat inputs REF reference electrode, WORK working electrode, AUX auxiliary (counter) electrode...

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