Three-electrode circuit, ohmic drop

A potentiostatic, three-electrode circuit allows the separation of both functions physically for the reference potential, a non-polarisable electrode is used (a calomel or AglAgCl reference electrode), while the electrical-current conducting electrode is an inert metal electrode. With electrochemical, direct-current methods, the effect of this modification is limited to a reduction of the so-called IR-drop (or ohmic-drop), which is caused by... 

Ohmic drop in three-electrode circuits. In modem coulometry and voltammetry the use of a potentiostat and a three-electrode configuration is the routine practice. The three electrodes are usually called the working, reference, and counter (or auxiliary) electrodes (see Figure 5.2). The cell current passes between the working electrode immersed in the test solution and the counter electrode, which may be in the test solution but is usually isolated from it by a single- or double-junction glass frit. 

The extent to which a three-electrode circuit can compensate for ohmic drop... 

For measurements involving current flow, three-electrode cells (Fig. ll.lb) are more common they contain both an AE and a RE. No current flows in the circuit of the reference electrode, which therefore is not polarized. However, the OCV value that is measured includes the ohmic potential drop in the electrolyte section between the working and reference electrode. To reduce this undesired contribution from ohmic... 

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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