Ohmic drop distortion

Ohmic drop distortion — Distortion of an electrochemical response caused by uncompensated ohmic resistance (see - IRU (ohmicpotential) drop). 

Overall, microelectrodes offer three key advantages (1) high rate of steady-state mass transport, (2) decreased ohmic drop distortion and (3) decreased double layer charging... 

Solutionfftdrop - (ohmicpotential) drop, - IR drop compensation, and -> Ohmic drop distortion... 

Temptation has been strong, and not always resisted, to approach total compensation by damping the oscillations out with an appropriately placed capacitance so as to reach the ideal situation of total compensation with unconditional stability. In fact, the cure is worse than the disease. The additional response time accompanying introduction of the damping capacitance will indeed distort the Faradaic current in a more severe and undecipherable manner than does ohmic drop. 

FIGURE 1.11. Convolution of the cyclic voltammetric current with the function I j Jnt, characteristic of transient linear and semi-infinite diffusion. Application to the correction of ohmic drop, a —, Nernstian voltammogram distorted by ohmic drop , ideal Nernstian voltammogram. b Convoluted current vs. the applied potential, E. c Correction of the potential scale, d Logarithmic analysis. 

In voltammetric experiments, electroactive species in solution are transported to the surface of the electrodes where they undergo charge transfer processes. In the most simple of cases, electron-transfer processes behave reversibly, and diffusion in solution acts as a rate-determining step. However, in most cases, the voltammetric pattern becomes more complicated. The main reasons for causing deviations from reversible behavior include (i) a slow kinetics of interfacial electron transfer, (ii) the presence of parallel chemical reactions in the solution phase, (iii) and the occurrence of surface effects such as gas evolution and/or adsorption/desorption and/or formation/dissolution of solid deposits. Further, voltammetric curves can be distorted by uncompensated ohmic drops and capacitive effects in the cell [81-83]. 

A major advantage of microelectrodes is that electrochemical data is less distorted by ohmic drop than when recorded with electrodes of conventional size... 

Gas bubbles can also deactivate the electrode surface by forming a physical curtain [173,174], While bubble movement may enhance in situ mass transfer resulting in an increase of the current [175-178], the blocking effect of bubbles can distort measured parameters by introducing a sizable ohmic drop [179-183]. Bubble effects... 

Returning to the three-electrode setup, it could seem that no ohmic drop would affect the measurement of the potential difference between the working and reference electrodes, since there is practically no current flow between both electrodes. However, this is not totally true. The reference electrode is located at a given distance from the working electrode surface, and, as a result of this separation, the potential difference measured contains a part of the ohmic drop in the solution which is called residual ohmic drop, IRU (with I being the current and Ru the uncompensated resistance). For more details concerning the minimization of the ohmic distortion of the current-potential response, see Sects. 1.8 and 5.4. 

One of the main disadvantages of voltammetric techniques like CV is the distortion caused by the combination of the double-layer charging process with the ohmic drop, related to the uncompensated resistance of the solution, Ru (see Sect. 1.9). This distortion can be very significant for macroelectrodes. 

As can be seen in this figure, the combined effect of ohmic drop and double-layer capacitance is much more serious in the case of CV. The increase of the scan rate (and therefore of the current) causes a shift of the peak potentials which is 50 mV for the direct peak in the case of the CV with v = 100 V s 1 with respect to a situation with Ru = 0 (this shift can be erroneously attributed to a non-reversible character of the charge transfer process see Sect. 5.3.1). Under the same conditions the shift in the peak potential observed in SCV is 25 mV. Concerning the increase of the current observed, in the case of CV the peak current has a value 26 % higher than that in the absence of the charging current for v = 100 Vs 1, whereas in SCV this increase is 11 %. In view of these results, it is evident that these undesirable effects in the current are much less severe in the case of multipulse techniques, due to the discrete nature of the recorded current. The CV response can be greatly distorted by the charging and double-layer contributions (see the CV response for v = 500 V s-1) and their minimization is advisable where possible. 

In order to avoid the distortion caused by these two effects, the usual approach is to compensate the resistance Ru by a positive feedback loop (this is imperative in systems like plasticized membranes for which the uncompensated resistance can be of the order of megaohms [32-34]). Another possibility is to use microelectrodes, for which a decrease in the measured current is obtained which minimizes the ohmic drop and charging current distortion (see Sects. 2.7 and 5.4.1). 

As stated in Sect. 5.2.3.4, there is always a potential difference generated by the flow of faradaic current I through an electrochemical cell, which is related to the uncompensated resistance of the whole cell (Ru). This potential drop (equal to IRU) can greatly distort the voltammetric response. At microelectrodes, the ohmic drop of potential decreases strongly compared to macroelectrodes. The resistances for a disc or spherical microelectrode of radius rd or rs are given by (see Sect. 1.9 and references [43, 48-50]). 

A further example, which confirms the necessity of evaluating the resistivity of the medium very carefully, concerns the corrosion of rebars in reinforced concrete. In this caae the intensity of the current flowing between the anodic and cathodic zones of a macrocell depends on the resistivity of the concrete and the extent of the region involved. To determine the concrete resistivity various methods have been developed, which can be applied in the laboratory [14] as well as in the field [15]. It should be noted, however, that in the latter case most researchers have pursued the approach suggested by Wenner [16] for the evaluation of the resistivity of soils. The contribution of the ohmic drop to the electrode overvoltage cannot be neglected when the values of the corrosion rate of the rebars are appreciable, even if the current intensity is small within a given polarization potential interval, because under such conditions the interpretation of experimental results could be completely distorted. 

The importance of knowing the exact value of the ohmic drop or uncompensated resistance in an electrochemical system has been pointed out by many workers. In studies of the kinetics of electrode processes by potentiostatic techniques, the ohmic potential drop produces a distortion of the steady state polarization curve which, if uncorrected, will yield erroneous values of the characteristic parameters (Tafel slope, reaction orders) of the electrode reactions (Fig. 6.2). 

Thus far we have considered only the case of planar macroelectrodes. Although these are widely used for electrochemical experiments, they have some drawbacks mainly due to the distorting effects arising from their large capacitance and ohmic drop. In addition, mass transport in linear diffusion is quite inefficient such that in the case of fast homogeneous and heterogeneous reactions, the response is diffusion-limited and therefore it does not provide kinetic information. 

Fig. 5 Theoretical limitations on ultrafast cyclic voltammetry. The shaded area between the slanted lines represents the radius that a microdisc 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.

---