Ohmic drop effect

The ohmic drop effect we are discussing deals only with the Ru portion ofthe cell resistance (Figure 1.5c). Indeed, the action of the potentiostat makes the working electrode potential independent not only of the possible shift of the counter electrode potential as the current varies, but also independent of the ohmic drop in the Rc portion of the cell resistance. In the case of cyclic voltammetry, the equation above becomes... 

There are two ways of handling the ohmic drop effect. One consists of equipping the instrument with a positive feedback loop that subtracts from E a tension, Rei, proportional to the current, thus eliminating, at least partially, the effect of the ohmic drop.14 One may even get the impression that total compensation, or even more, overcompensation, could be achieved. In fact, before total compensation is reached, oscillations appear as a result of the bandpass limitations of the operation amplifiers. The entire instrument can indeed be represented by a self-inductance, La, that is a... 

It must be emphasized again that the mid-peak potential is equal to E° for a simple, reversible redox reaction when neither any experimental artifact nor kinetic effect (ohmic drop effect, capacitive current, adsorption side reactions, etc.) occurs, and macroscopic inlaid disc electrodes are used, that is, the thickness of the diffusion layer is much higher than that of the diameter of the electrode. 

What electrodes should be used to obtain a current density of 0.1 mA cm 2 if mass transfer and ohmic drop effects are neglected (Zinola)... 

A first parameter to be studied is the applied potential difference between anode and cathode. This potential is not necessarily equal to the actual potential difference between the electrodes because ohmic drop contributions decrease the tension applied between the electrodes. Examples are anode polarisation, tension failure, IR-drop or ohmic-drop effects of the electrolyte solution and the specific electrical resistance of the fibres and yarns. This means that relatively high potential differences should be applied (a few volts) in order to obtain an optimal potential difference over the anode and cathode. Figure 11.6 shows the evolution of the measured electrical current between anode and cathode as a function of time for several applied potential differences in three electrolyte solutions. It can be seen that for applied potential differences of less than 6V, an increase in the electrical current is detected for potentials great than 6-8 V, first an increase, followed by a decrease, is observed. The increase in current at low applied potentials (<6V) is caused by the electrodeposition of Ni(II) at the fibre surface, resulting in an increase of its conductive properties therefore more electrical current can pass the cable per time unit. After approximately 15 min, it reaches a constant value at that moment, the surface is fully covered (confirmed with X-ray photo/electron spectroscopy (XPS) analysis) with Ni. Further deposition continues but no longer affects the conductive properties of the deposited layer. 

The electrode size is another important variable to analyze since the use of microelectrodes is very relevant for experimental electrochemical studies enabling the reduction of capacitative and ohmic drop effects, as indicated in Sect. 2.7. Specifically, it is of great interest to check the behavior of the system when the size of the electrode is reduced. In Fig. 4.20, the influence of the electrode radius on the... 

Figure 40 Double loop EPR data for Type 304 stainless steel heated at 600°C for 100 h (solid line) and 1 h (dotted line). The extremely sharp active-passive transition at —0.5 V(SCE) is due to ohmic drop effects. Note the much larger i, for the sensitized (100 h) material. (Data courtesy of M. A. Gaudett, University of Virginia.)...

To conclude this introductory section concerning ohmic drop effects, let us consider the case of transient electrochemical techniques. In this case the currents are of the order of few milliamperes or less, and thus all these effects should cancel. Unfortunately, this is not the case because of the size of the working electrodes. Indeed, these are often of millimetric dimensions, which amounts to a considerable narrowing of the field lines near the tip of the working electrode. As a result the resistance increases, and most of... 

Such an expression justifies a posteriori the previous discussion of the ohmic drop effect in electrochemical cells (Sec. III.A.3). Indeed, for a given resistance of the solution in Eq. (138), the ohmic drop across the cell is expressed as in Eq. (139) ... 

Y. Bultel, L. Genies, O. Antoine, P. Ozil, and R. Durand, "Modeling Impedance Diagrams of Active Layers in Gas Diffusion Electrodes Diffusion, Ohmic Drop Effects and Multistep Reactions," Journal of Electroanalytical Chemistry, 527 (2002) 143-155. 

In order to obtain a correct value for /cor from the polarization resistance, it is necessary, on one hand, to measure under steady-state conditions (current stable with time) and, on the other hand, to avoid ohmic drop effects. Indeed, in presence of an ohmic drop between the reference and working electrodes, instead of r oi, one measures an apparent polarization resistance r -p given by ... 

Amatore C, Oleinick A, Svir 1 (2008) Theoretical analysis of microscopic ohmic drop effects on steady-state and transient voltammetry at the disk microelectrode a quasi-confotmal mapping modeling and simulation. Anal Chem 80 7947-7956... 

Crevice corrosion occurs in restricted transport zones where the access of reactants and the elimination of corrosion products can be very slow. The detrimental effect of a crevice is related to a very small volume of solution in contact with very large metal surfaces. The restricted transport path between crevice and bulk solution is responsible not only for low diffusion exchanges but also for the buildup of a potential difference between the free surfeces and the crevice that becomes more anodic (ohmic drop effect). 

Considerations Specific to the Potentiometric Mode 10.5.2.1 Ohmic Drop Effects... 

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