Ohmic drop method

Experimental arrangement [1, 5, 6, 13,14, 15, 17, 19, 20] In contrast to electrochemical methods utilizing metal electrodes, the ohmic drop of the potential... 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

This AE can be measured directly since there is no ohmic drop at t > tx. The coulostatic impulse method is, therefore, particularly suitable for kinetic studies in low-conducting media. For further details, the reader is referred to the extensive reviews of van Leeuwen [39]. 

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... 

One can see from Eq. (24) that at L 3> 1, m0 D/a (as for a microdisk electrode alone), but at L -C 1, m0 D/d, which is indicative of the TLC type behavior. By decreasing d, the mass-transport rate can be increased sufficiently for quantitative characterization of fast ET kinetics, while preserving the advantages of steady-state methods, that is, the absence of problems associated with ohmic drop, adsorption, and charging current. 

There are two advantages of the coulostatic method in the study of kinetics of electrode reactions. First, the ohmic drop is not of importance, therefore the measurements can be carried out in highly resistive media. Second, since Ic = IF, Q does not interfere in the measurement. By the help of this technique jo values up to about 0.1 A cm-2 and - standard rate constants up to 0.4cms 1 can be determined. A detailed discussion of coulostatic techniques can be found in Ref. [vi]. 

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. 

It is evident from the foregoing considerations that there exists an influence of the ohmic drop, which will be dealt with further on, on the determination of the electrochemical parameters and the correct application of the methods of numerical analysis. Moreover, experience has shown that the success of numerical analysis depends also on the way the contribution of the ohmic drop to electrode overvoltage is reduced. In this respect, it may be mentioned, for example, that in the case of iron and carbon steels serious difficulties are met with the anedysis of polarization curves performed in uninhibited HCl solutions at temperatures above 65 °C [40] because the corrosion current density assumes very high values. 

Finally, the presence of a contribution of the ohmic drop to electrode overvoltage becomes more important when one tries to evaluate 1 using the three different versions of the three-point method [43, 44, 45]. In its simplest form, the theory of this method. 

A numerical study of the influence of the ohmic drop on the evaluation of electrochemical quantities has been conducted, for example, over the AE interval [-20, 20] mV by means of the IRCOM program, which makes use of a polynomial of the sixth degree, considering some experimental polarization curves and taking the values of the electrochemical parameters obtained by the NOLI method. The examples examined have shown that the representation of experimental data by a polynomial of the sixth degree is very good and that the evaluation of the correct order of magnitude of the corrosion current density, in the presence of an ohmic contribution to the electrode potential, requires that the actual values of the Tafel slopes be known. 

The necessity of determining the value of R, with a good degree of accuracy becomes more pressing when computerized systems are employed or when experimental data must be processed in real time and with no operator, using non-linear numerical methods, because in this case the reduction in the ohmic drop contribution to the electrode potential might influence the convergence of the numerical methods very unfavourably. 

Laviron55 has recently noted that linear potential sweep or cyclic voltammetry does not appear to be the best method to determine the diffusion coefficient D of species migrating through a layer of finite thickness since measurements are based on the shape of the curves, which in turn depend on the rate of electron exchange with the electrode and on the uncompensated ohmic drop in the film. It has been established that chronopotentiometric transition times or current-time curves obtained when the potential is stepped well beyond the reduction or oxidation potential are not influenced by these factors.55 An expression for the chronopotentiometric transition has been derived for thin layer cells.66 Laviron55 has shown that for a space distributed redox electrode of thickness L, the transition time (r) is given implicitly by an expression of the form... 

Although ohmic drop cannot be eliminated completely it can be minimized, while the remaining effect can be taken into account, either by active methods, e.g. by some means of instrumental compensation or passively, by calculation and subsequent correction. 

In practice a combination of these methods is often used i.e., a good cell design to minimise the ohmic drop, instrumental compensation of the greater part of the remaining error and, finally, removal of the last part by evaluation, calculation and correction of the experimental data. 

A number of active methods are available in which the compensation of the ohmic drop is necessarily incorporated in the control system of the potentiostat. 

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 method is based on a setup which has been succesfully used to eliminate ohmic potential drop at gas-evolving electrodes at high current densities. With modem equipment for elimination of ohmic drop as previously described it is possible to make rapid corrections in solutions with conductivities below 1 JlS cm ... 

This heading covers all methods in which the ohmic drop is measured or calculated, followed by mathematical correction afterwards when the experimental data have been collected [15,16,25, 33]. 

The measurement of the ohmic drop can be perfonned by a number of methods, the most well-known being ... 

When performing polarization measurements an error due to the ohmic drop over the uncompensated resistance will be included in the potential between the working and the reference electrode. The significance of this error is decided by the ratio between the value of the uncompensated resistance and the polarization resistance of the system. The uncompensated resistance can be minimized by careful design of the cell and the positioning of the electrodes. Several methods of instrumental compensation of the ohmic drop are available, of which the interrupt methods are the most versatile. Such methods are applied during the polarization measurements. 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

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. 

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