IR drop effect

Owing to the small current measured, Ohmic drop (also called IR-drop) effects in solution are much less pronounced. This includes that ultra-micro electrodes can be used in highly resistive media (including non-aqueous solution) and/or without the use of supporting electrolyte. 

The reference electrode is sometimes located at a position remote from the pipeline, recommended because currents do not penetrate remote areas, and hence, IR drop effects are avoided. Actually, the potential measured at a remote position is a compromise potential at some value between that of the polarized structure and the polarized auxiliary or sacrificial anode. These potentials differ by the IR drop through the soil and through coatings. The potential measured at a remote location, therefore, tends to be more active than the true potential of the structure, resulting in a structure that may be underprotected. 

As explained in Sects. 3.4.2.1 and 3.4.2.2, application of the external reflectance technique requires the use of a thin layer of solution between the working electrode and the IR window. Because of this experimental approach, there has been some reticence to use the IR method under conditions of current flow. Criticism was centered on IR-drop effects and reactant-depletion problems in the solution thin layer. However, although IR drop and reactant depletion indeed occur, these problems can be experimentally minimized and should not be overestimated. On one side, usually employed solutions... 

CVFIT was utilized, which theoretically corrects for iR drop effects on the CV waveshape. 

This crosstalk is a result of the uncompensated IR drop effect [18] that has a more complex nature in two working electrode systems compared to a standard three-electrode cell. 

From an electrochemical viewpoint, stable pit growtli is maintained as long as tire local environment witliin tire pit keeps tire pit under active conditions. Thus, tire effective potential at tire pit base must be less anodic tlian tire passivation potential (U ) of tire metal in tire pit electrolyte. This may require tire presence of voltage-drop (IR-drop) elements. In tliis respect the most important factor appears to be tire fonnation of a salt film at tire pit base. (The salt film fonns because tire solubility limit of e.g. FeCl2 is exceeded in tire vicinity of tire dissolving surface in tlie highly Cl -concentrated electrolyte.)... 

Another parameter essential for quantitative applications of micropipettes is the internal ohmic resistance, R. It is largely determined by the solution resistance inside the narrow shaft of the pipette, and can be minimized by producing short (patch-type) pipettes. The micropipette resistance has been evaluated from AC impedance measurements. Beattie et al. measured the resistance of micropipettes filled with aqueous KCl solutions (0.01, 0.1, and 1 M) [18b]. The value obtained for a 3.5/am-radius pipette was within the range from 10 to 10 As expected, the tip resistance was inversely proportional to the concentration of KCl in the filling solution. In ref. 18b, the effect of pipette radius on the tip resistance was evaluated using a constant concentration of KCl. The pipette resistance varied inversely with the tip radius. The iR drop was found to be 4.5-8 mV for the pipette radii of 0.6 to 19/rm when 10 mM KCl was used. 

The cyclic voltammograms of these systems display quasi-reversible behavior, with AEv/v being increased because of slow electrochemical kinetics. Standard electrochemical rate constants, ( s,h)obs> were obtained from the cyclic voltammograms by matching them with digital simulations. This approach enabled the effects of IR drop (the spatial dependence of potential due to current flow through a resistive solution) to be included in the digital simulation by use of measured solution resistances. These experiments were performed with a non-isothermal cell, in which the reference electrode is maintained at a constant temperature... 

UMEs decrease the effects of non-Earadaic currents and of the iR drop. At usual timescales, diffusional transport becomes stationary after short settling times, and the enhanced mass transport leads to a decrease of reaction effects. On the other hand, in voltammetry very high scan rates (i up to 10 Vs ) become accessible, which is important for the study of very fast chemical steps. For organic reactions, minimization of the iR drop is of practical value and highly nonpolar solvents (e.g. benzene or hexane [8]) have been used with low or vanishing concentrations of supporting electrolyte. In scanning electrochemical microscopy (SECM [70]), the small size of UMEs is exploited to locahze electrode processes in the gm scale. 

Immediate benefits of switching to the larger-volume cell (Fig. 25) include the formation of larger-area deposits, 2-3 cm, versus the 0.1-0.2 cm deposits obtained using the thin layer flow cell. The thicker layer of solution eliminated problems with IR drops, bubbles, and edge effects except at the top of the deposit. Some nonuniformity at the top of the deposit resulted from small variations in the levels of different solutions pumped into the cell. Rinse solution levels, for instance, were always raised to a slightly higher level than the reactant solutions in order to make sure that the reactants were completely removed after each deposition. 

The most serious causes of error are (i) wave and peak distortion caused by excessively fast scan rates, which are themselves caused by diffusion being an inefficient mass transport mechanism, (ii) current maxima caused by convective effects as the mercury drop forms and then grows, and (iii) IR drop, i.e. the resistance of the solution being non-zero. Other causes of error can be minimized by careful experimental design. 

Lower Conductivity. The equivalent conductance of nonaqueous solutions a( infinite dilution is often comparable to that of aqueous systems, but it decreases with an increase in concentration more rapidly than the corresponding aqueous systems (the effect of the lower dielectric constant). Since the specific conductivity, K (that which determines the resistance between cathode and anode) is proportional to Ac, the equivalent conductance, the IR drop between the electrodes of a cell in which deposition from nonaqueous solutions is to lake place will be greater than that in aqueous solution (see Section 4.8.7). The electricity needed to deposit a given mass of metal is proportional to the total E between the electrodes, and this includes the IR between the electrodes, which is much greater in the nonaqueous than in the aqueous cases. Hence, nonaqueous deposition will be more costly in electricity (more kilowatt hours per unit of weight deposited) than a corresponding deposition in aqueous solution. The difference may be prohibitive. 

If the three-electrode instrument is equipped with an iR-drop compensator, most of the iT-drop caused by the solution resistance can be eliminated. However, in order to minimize the effect of the iT-drop, a Fuggin capillary can be attached to the reference electrode with its tip placed close to the indicator electrode. Moreover, for a solution of extremely high resistance, it is effective to use a quasi-reference electrode of a platinum wire (Fig. 8.1(a)) or a dual-reference electrode (Fig. 8.1(b)), instead of the conventional reference electrode [12]. 

To impose the diffusion-controlled conversion of O to R as described earlier, the potential E impressed across the electrode-solution interface must be a value such that the ratio Cr/Cq is large. Table 3.1 shows the potentials that must be applied to the electrode to achieve various ratios of C /Cq for the case in which Eq R = 0. For practical purposes, C /C = 1000 is equivalent to reducing the concentration of O to zero at the electrode surface. According to Table 3.1, an applied potential of -177 mV (vs. E° ) for n = 1 (or -88.5 mV for n = 2) will achieve this ratio. Similar arguments apply to the selection of the final potential. On the reverse step, a small C /Cq is desired to cause diffusion-controlled oxidation of R. Impressed potentials of +177 mV beyond the E° for n = 1 (and +88.5 mV for n = 2) correspond to Cr/Cq = 10"3. These calculations are valid only for reversible systems. Larger potential excursions from E° are necessary for irreversible systems. Also, the effects of iR drop in both the electrode and solution must be considered and compensated for as described in Chapter 6. 

Data at smaller scan rates and higher temperatures allowed determination of the rate constant for cleavage of nitrite from the radical anion of this and other v/c-dinitro compounds. In addition to the use of microelectrodes to reduce iR drop, another interesting feature of this study was the observation of the effect... 

Our last example does not involve the rate of a chemical reaction, but instead, the effect of temperature on diffusion rates [25]. One of the motivations for using microelectrodes as in the previous example is to allow fast experiments without appreciable iR drop. When used in the opposite extreme of very small scan rates, microdisk electrodes produce steady-state voltammograms that have the same sigmoidal shape as dc polarograms and RDE voltammograms (cf. Chap. 12). 

One of the first observations one is likely to make when carrying out low-temperature voltammetric measurements is that the effects of solution iR drop that may have been scarcely noticeable at room temperature are suddenly alarmingly pronounced. The iR drop, of course, is governed by the cell current and the effective resistance between the working electrode and the Luggin capillary of the reference electrode. For example, the solution resistance for an embedded circular disk electrode of radius r with a distant reference electrode is given by Equation 16.13, where p is the resistivity of the solution. 

Thus, as the resistivity increases, the iR drop can become excessive. This problem is somewhat ameliorated by the fact that diffusion coefficients also decrease with decreasing temperature so that mass-transport-controlled currents will be smaller. However, this effect is not large enough to offset significantly the deterioration in response due to increased resistance. 

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