Potentiostat, circuit diagram

The determination and evaluation of potentiodynamic curves can only be used as a preliminary assessment of corrosion behavior. The protection current requirement and the limiting value for the potential control can only be determined from so-called chronopotentiostatic experiments as in DIN 50918. in systems that react with spontaneous activation after the protection current is switched off or there is a change in the operating conditions, quick-acting protection current devices must be used. Figure 8-6 shows the circuit diagram for such a potentiostat. 

Fig. n.1.6 (a) Cell for cyclic voltammetric experiments with three-electrode configuration, (b) Schematic drawing of the simplified circuit diagram for a three-electrode potentiostat system... 

There are essentially two different coulometric processes, namely potentio-static and galvanostatic coulometry. The former functions with constant, controlled electrode potential, whereas the galvanostatic method - also called coulometric titration - functions with constant current strength and uncontrolled potential. Fig. 13 shows the basic circuit diagram for potentiostatic coulometry. 

Although it is not necessary to understand electronics in order to use an EC detector, it is useful to have some idea of the circuitry involved, in the event that a detector is to be modified for a specific application. Simple circuit diagrams for potentiostats have been reported. They have two elements, viz. potential control and current monitoring. A circuit for a self-metering detector built from available integrated circuits has also been described. Circuits for dual and pulsed detectors will obviously be more complex. Since many modem EC analysers are computer controlled, it is no doubt possible to do the same for HPLC-ED detectors, but such detectors do not seem to be commercially available. 

Files containing circuit diagrams, part numbers, etc., for the potentiostat and fast amplifier are accessed using URLs http //www.inst.bnl.gov/cgi-bin/view io. pl table=ionumbers id=45 and http //www.inst.bnl.gov/cgi-bin/view io.pl table=ionumbers id=553 respectively. Click on the numeral in table entry Archive CD and follow instructions to download the desired files. 

Potentiostatic circuits make use of operational amplifiers (OA) to control voltage and to measure current. Operational amplifiers have a number of inputs, but in circuit diagrams the power supply connections are understood and only three connections are considered (Figure 2-28). Based upon their action, these connections are the inverting input (—), the noninverting input (-h), and the output. In normal use, OAs are used in a feedback mode that is, feedback loop is introduced fi-om the output or from some other source to the inverting input. The action of the OA in electrical currents can be understood largely on the basis of a few characteristics. 

Figure 1-11. Block diagram of a potentiostatic circuit serving an electrochemical cell.

The overall design of the USB-based portable cycUc voltammetric analyzer is depicted in Figure 5.42. A miniaturized three-electrode system is connected to the built-in ADC of the PIC18F4550 microcontroUer-based data acquisition unit containing a home-made potentiostat. The overall electronic circuit diagram of the portable USB-based electrochemical analyzer is depicted in Figure 5.43. 

Figure 1. Schematic diagram illustrating Langmuir-Blodgett transfer under potentiostatic conditions. A gold-coated glass slide is acting as a working electrode in a three-electrode potentiostatic circuit. Below, an inset shows the pattern of the vapor deposited gold film. The central rectangular area (A = 0.20 cm ) is coated with an LB monolayer as the substrate is withdrawn from the subphase. The two lines mark the initial and the final position of the water meniscus in the LB experiments.

Flgure 7. Schematic diagram of the cell and current distribution with one (iK = 1a = 0) two (Ik 0 and iA = 0), or three (Ik 0 and 0) electrical circuits. A, ammeter P, potentiostat V, power supply a, porous working electrode b, auxiliary counterelectrode c, porous insulator d, fritted glass separator Er, reference electrode, electrode flow circuit, i ed = k v io = 1a + iv (Reprinted from Ref. 67 by permission of Chapman and Hall.)... 

Figure 1. A schematic diagram (a) and a partial equivalent circuit (b) are given for the LAPS. The components 4, Ci, Cdf Re, Vref and Vchem, respectively represent the applied bias potential, the insulator and depletion layer capacitances, the electrolyte resistance, the potential across the reference electrode, and a chemically sensitive surface potential. Ip represents the photogeneration of hole-electron pairs, and I the measured alternating photocurrent. Solution potential is maintained by a potentiostat using a Pt controlling electrode (CTL) and Ag/AgCl reference electrode (REF). The potential is defined as the potential from the output of the reference electrode to ground.

Figure 3.13. Diagram of the electrical circuit for a controUed-potential electrochemistry-ES emitter. Floated potentiostat common is that of the ES high-voltage supply. W, R, and A represent the working, reference, and auxiUary electrodes in the electrochemical ceU, respectively, /w, Aux, fe, ext are the currents in the working electrode, auxiUary electrode, ES spray current, and upstream external current loop, respectively. In standby mode the electrodes remain connected to the ES high voltage, but the worki ng electrode potential is not controlled with the potentiostat. (Adapted with permission from Ref. 89. Copyright 2005, American Chemical Society.)...

The related ohmic drop AU = iR may lead to an appreciable difference between the actual potential and that chosen for the experiment. It can be minimized by an appropriate position of the Haber-Luggin (HL) capillary of the RE, close to the surface of the working electrode (WE). However, it should not be too close in order to avoid the partial blocking of the metal surface and the formation of crevices. As a compromise, the distance should be about three times the diameter of the capillary. The ohmic drop may be also compensated electronically to about 90% of its value by an appropriate circuit built in the potentiostat as shown later in the block diagram of Figure 1.26a. The ohmic drop may be minimized by a small size of the electrode. 

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