Ohmic drop compensation

Although, strictly speaking, total compensation cannot be achieved, partial compensation may well lead to a negligible residual ohmic drop, although the presence of damped oscillations does not yet prevent detection of the Faradaic current. Such a situation is typically reached for ARu = 417 in the system shown in Figure 1.8. This figure illustrates how the positive feedback ohmic drop compensation should be carried out in practice. The procedure may be summarized as follows ... 

C. Amatore and C. Lefrou, New Concept for a Potentiostat for On-Line Ohmic Drop Compensation in Cyclic Voltammetry Above 300 kV s 1, J. Electroanal. Chem. 324 33-58 (1992). 

Minimization of time scale measurements Ultrafast undistorted cyclic voltammetry may be performed at ultramicroelectrodes using an ultrafast potentiostat allowing on-line ohmic drop compensation. 

Ohmic drop compensation necessary at high scan rates... 

Kinetic data that have been obtained so far fall into three groups. The first comprises data measured without the proper ohmic drop compensation or subtraction and/or the ideal polarization of the liquid-liquid interface being considered. Thus, from kinetic measurements made by Gavach et al. [138] Buck and coworkers [121, 122], or Samec et al. [38], rather low values of the standard rate constant kl were... 

However, ohmic drop compensation by positive feed-back has some disadvantages it can only he used in the potentiostatic mode and it requires the use of a fixed current range. This is because, in order to reduce the effect of the presence of R, on the polarization potential of the working electrode, a voltage signal (E/) coming from the output of the... 

Figure 10. Schematic diagram of ohmic drop compensation by the positive feed-back technique in potentiostatic mode.

An analogue technique of ohmic drop compensation has been proposed by Gabrielli et al. [25], which avoids some stability problems related with positive feedback. 

Some of the more expensive commercially available potentiostats are, or can be, equipped vnth these forms of ohmic drop compensation technique. In principle, the method can be applied when the values of and the measuring resistance change during the measurements. 

The change of potential due to the ohmic drop is instantaneous since if / = 0, then IR = 0. (This is the basis of the ohmic drop compensation by interruption techniques.) It is obvious that from the E vs. t function C can be determined. This is the principle of the determination of pseudocapacitance (e.g., chemisorbed hydrogen on platinum) by the method of charging curves. In this case, after the adsorption of hydrogen, a not too high anodic current (j < jo) is applied and E is followed as a function of time. 

Figure 1. Range of ultramicroelectrodes radii (rQ in fim) to be used to obtain an undistorted voltanunogram at a given scan rate (v in V.s ) as adapted from ref. 16. (i) limit for least edge diffusion interference (5% error on peak current, from ref. 15). (ii) limit for least ohmic drop due to Faradaic current (10 mV). limit for least ohmic drop due to capacitive current (10 mV) or cell constant. For a 90% on-line ohmic drop compensation boundaries ii and iH are pushed upward to and uigQ%t respectively. All limits are established for a one electron transfer at 20 C, ba on errors given above and D = 10 cm. s p = 20fi.cm, C = 10 /iF.cm and C = 5 mM. Above limit (io-) coupling between diffusion layer and double layer is predicted to occur. The thick horizontal line represents the location of the voltammograms shown in Figure 2.

Figure 2. Cyclic voltammetry of anthracene, 10 mM, in acetonitrile, 0.6 M NEt4Bp4, at a gold disk (ro = 5 /im) ultramicroelectrode, with and without ohmic drop compensation, at different scan rates as indicated in kV.s on each set of curves. ...

The very exergonic proton transfers in eq.4 are extremely fast. However their rate constants could be determined experimentally if the standard oxidation potential of the arenes (eq.3) could be determined independently under the same experimental conditions (acetonitrile, 0.1 M ionic strength).2 26 jq achieve such measurements we resorted to fast scan cyclic voltammetry. Figure 3 illustrates the merit of on line ohmic drop compensation... 

Figure 4. On line ohmic drop compensated cyclic voltammetry of 2,5-di-(p-anisole)pyrylium perchlorate (3), 5 mM, in acetonitrile, 0.6 M NBU4BF4, at a 5 /xm radius gold disk ultramicroelectrode and a scan rate of 153 kV.s 20 C. (a) in the absence or (b) in the presence of 1. (c) Background subtracted voltammogram (b - a).

Lamy C, Herrmann CC (1975) A new method for ohmic-drop compensation in potentiostatic circuits stability and bandpass analysis, including the effect of faradaic impedance. J Electroanal Chem 59 113-135... 

Amatore C, Maisonhaute E, Simoimeau G (2000) Ohmic drop compensation in cyclic voltammetry at scan rates in the megavolt per second range access to nanometric diffusion layers via transient electrochemistry. J Electroanal Chem 486 141-155. doi 10.1016/80022-0728(00)00131-5... 

Wipf DO (1996) Ohmic drop compensation in voltammetry iterative correction of the applied potential. Anal Chem 68 1871-1876. doi 10.1021/ ac951209b... 

Figure 6.1.4.2 Theoretical limitations on ultrafast cyclic voltammetry. The shaded area between the slanted lines represents the radius that a microdisk must have if the ohmic drop is to be less than 15 mV and distortions due to nonplanar diffusion account for less than 10% of the peak current, (a) Without iR drop compensation by positive feedback, and (b) with 90 and 99% ohmic drop compensation. The dotted area in (a) and (b) represent the regions where transport within the double layer affects the voltammetric response. Limits are indicative and correspond approximately to a 5-mM anthracene solution in acetonitrile, 0.3 M tetrafluoroborate as supporting electrolyte. [Reproduced by permission of Marcel Dekker from C. Amatore, Electrochemistry at Microelectrodes, I. Rubenstein, Ed., 1995, Chapter 4, p. 198.]...

Fig. 3. Block scheme of the four-electrode potentiostat with a positive feedback for the ohmic drop compensation [30]...

Currently a four-electrode system with automatic ohmic drop compensation[5] has been used for accurate polarization measurements at ITIES. However, this rather complicated experimental set-up and the large area of the water/nitrobenzene interface (about 100 mm )[5] have not permitted the use of the fast pulse technique Therefore, we have developed a simpler three-electrode system with the hanging electrolyte drop electrode.[1]... 

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