Three-electrode devices

The apparatus for DC polarography usually consists of three parts, i.e. the circuit to control the potential of the indicator electrode (DME), the circuit to measure the electrolytic current, and the electrolytic cell. Classically two-electrode devices as in Fig. 5.8(a) were used, but now three-electrode devices as in Fig. 5.8(b) are predominant. In the latter, the electrolytic cell is equipped with three electrodes a DME, a reference electrode and a counter electrode (Fig. 5.9). The droptime of the modem DME is controlled mechanically, and is usually between 0.1... 

The electrical resistance of non-aqueous electrolytic solutions is often much higher than that of aqueous ones, and so polarographic and voltammetric measurements in non-aqueous solutions should be made with a three-electrode device. A computer-aided three-electrode instrument, equipped with a circuit for iR-drop compensa-... 

When three-electrode devices are used, reference electrodes similar to those in potentiometry (Section 6.1.2) are applicable, because no appreciable current flows through them. The reference electrodes used in non-aqueous solutions can be classified into two groups [1, 2, 5, 10]. Reference electrodes of the first group are prepared by using the solvent under study and those of the second group are... 

The three-dimensional quadupole field ion trap - or Paul trap is a three-electrode device [see Figure 4.5(b)]. Ions are injected into the device and collected in packets from an ESI or MALDI source. The ion trap analyzer is capable of MS, MS" (MS = MS-MS-MS) and high-resolution scans (R = 20,000). The ion packets enter through an entrance-end cap and are analyzed by scanning the RF amplitude of the ring electrode. The ions are resonated sequentially from low to high m/z and are ejected from the ion trap through the exit-end cap electrode to a detector. Unlike the triple quadrupole (QqQ) mass spectrometer discussed previously, the ion trap performs tandem mass spectrometry (MS-MS) scan modes in the same analyzer. 

A bilayer structure usually consists of two different films deposited on a substrate, one overlying the other. A typical system consists of a Pt substrate with an electro-deposited film of poly-[Ru(vbpy)3 ] on which a film of poly-[Os(bpy)2(vbpy)2" ] is elec-trodeposited (25). Another type of sandwich structure involves a pair of closely spaced electrodes such as in an electrode array (26), bridged by a polymer film. Alternatively, a different polymer can be deposited on each electrode of an array pair to form a bilayerlike arrangement having a junction where the films meet. Three-electrode devices of this type can produce a structure functionally equivalent to a field effect transistor (FET) (27). 

A second important use of op-amps and feedback circuits is to control the potential at a WE. The circuit that performs this task is called a potentiostat. Potentiostats and modern polarog-raphy equipment are called three-electrode devices because they attach to a WE, a reference electrode, and an auxiliary electrode, also called a CE (Eigure 15.14). The principle of operation is that the potential between the WE and the reference electrode is maintained by a feedback circuit. The applied voltage E ppi be a constant voltage or some signal generator voltage. 

Adaptation of usual electrochemical techniques (three electrode devices) to thin film polymer technology was identified as highly desirable in order to improve the understanding of polymer electrolyte cell behaviour. In addition, specific properties of polymer electrolytes, like low vapor pressure, should be exploited to adapt physical analytical techniques to observation of polymer electrlyte cell components. 

Figure 15. (a) SEM image of a three-terminal device with source, drain and gate electrodes. The inset shows in magnification the gap in... 

This method in some ways resembles the technique for ASV [321,322]. The analytical device is based on a three-electrode system (1) a glassy carbon electrode, which serves as a cathode (2) a saturated calomel electrode (SCE), which is the reference electrode and (3) a platinum counter-electrode during electrolysis. 

The ion trap is a device that utilizes ion path stability of ions for separating them by their m/z [53]. The quadrupole ion trap and the related quadrupole mass filter tvere invented by Paul and Steinwedel [57]. A quadrupole ion trap (QITor 3D-IT) mass spectrometer operates with a three-dimensional quadrupole field. The QIT is formed by three electrodes a ring electrode with a donut shape placed symmetrically between two end cap electrodes (Fig. 1.20). 

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). 

For their characterization, electrochromic compounds are initially tested at a single working electrode under potentiostatic control using a three-electrode arrangement. Traditional characterization techniques such as cyclic voltammetry, coulometry, chronoamperometry, all with in situ spectroscopic measurements, are applied to monitor important properties [27]. From these results, promising candidates are selected and then incorporated into the respective device. 

Fig. 5.8 Schematic diagram of polarographic (or voltammetric) circuits for two-electrode (a) and three-electrode (b) systems. WE(DME) indicator or working electrode (dropping mercury electrode in the case of polarography) RE reference electrode CE counter electrode DC voltage (V) DC voltage source Current (/) current measuring device.

To carry out amperometric or voltammetric experiments simultaneously at different electrodes in the same solution is not difficult. In principle, any number of working electrodes could be studied however, it is unlikely that more than two or three would ever be widely used in practice. The bulk of the solution can have only one controlled potential at a time (if there are significant iR drops, there will be severe control problems with multiple-electrode devices). It is necessary to use a single reference electrode to monitor the difference between this inner solution potential and the inner potential of W1 at the summing point of an operational amplifier current-to-voltage converter (this is the potential of the circuit common see OA-2 in Fig. 6.17). The potential difference between... 

Figure 6.18b depicts the classical three-electrode constant-current experiment. The counter electrode is now replaced by an auxiliary electrode and a reference electrode. The reference electrode enables one to monitor the working electrode potential as a function of time using a high-impedance (zero-cur-rent) measuring device. In many early experiments, carried out on a time scale... 

Figure 3. Schematic diagram of electrode geometry facilitating four-point voltage measuremenis. Three such devices are fabricated simultaneously on pre-cut glass microscope slides.

The ClinLab flow cell Sputnik [123] was originally developed for H PLC applications but is a separate device which in principle can be combined with every amperometric detector which is designed for three-electrode operation (Figure 4.74). Two PEEK capillaries and three electronic plugs are the only external interfaces. 

A 9] The thin-film three-electrode cell possesses a cell volume adjustable from 0.75 to 2.5 pi. The fluid dynamic design of the cell should prevent turbulence. The reference electrode, an Ag/AgCl cell element, can be filled manually with KC1. A transparent housing permits optical control of the liquid level. The operational pressure is limited to 5 bar. The supplier recommends this robust device for routine measurements when high selectivity is desired. 

A three-dimensional device was obtained by an hybrid assembly of comb-like multishank electrodes. The final structure resembled a bed of nails [69,70]. The array contains 256 electrodes with a lateral and horizontal spacing of 120 pm and 150 pm, respectively (Fig. 10). The assembly was sealed with glass and was implanted into the subarachnoidal space without any significant tissue damage. 

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