Ultramicroelectrodes ohmic drop

C. Amatore, C. Lefrou, and F. Pfltiger, On-Line Compensation of Ohmic Drop in Submicrosecond Time Resolved Cyclic Voltammetry at Ultramicroelectrodes, J. Electroanal. Chem. 270 43-59 (1989). 

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

The peak-potential difference A p depends mainly on the kinetic parameter i/t, as illustrated in Table 2. By measurement of A p as a function of v for a given system, k° can be estimated. However, great care should be exerted to ensure that uncompensated resistance does not contribute to the value of A p, since this would hamper the procedure. Clearly, the use of ultramicroelectrodes can be recommended for this kind of measurements, as the ohmic drop is much smaller here compared to microelectrodes of normal size. This is particularly true when high sweep rates are required for determining large values of k° (see Section 2.4)... 

However, it regains interest with the development of ultramicroelectrodes. Indeed, because of their micrometer (or smaller) sizes, the currents are of the order of nanoamperes. Thus even with R of several kilo-ohms, the ohmic drop term is totally negligible. 

This discussion and presentation of ohmic drop problems should lead to the conclusion that, with maybe an exception for ultramicroelectrodes, the resistivity of electrochemical solutions must be decreased as much as possible. 

Finally, it is worth mentioning that LaCroix et al [28] reported in 1989 ultrafast electrical response data on a timescale of 100 ps for pANI film (ca. 0.2 pm thickness) in 2 m sulphuric acid using an ultramicroelectrode (ca. 0.2 mm area) with appropriate correction for uncompensated ohmic drop. However, the optical response data are reported in arbitrary units and thus they are difficult to evaluate with a view to practical uses. In general, it would seem that 100 ps are not a viable reference timescale for electrochromic response, at least for conventional electrochromic devices. 

The ultramicroelectrodes allow the investigation in highly resistive media due to extensive decrease of the ohmic drop. However, the decrease of the supporting electrolyte concentration results in an increase of the migration contribution to the mass transport in the region adjacent to the electrode. 

From what precedes, it is understood that whenever the goal is to obtain general kinetic informations, in relevance to "homogeneous" chemistry, it is highly desirable that such informations could be derived under conditions that reproduce as closely as possible those encountered for the "homogeneous" system of interest. Unfortunately, up to the introduction of ultramicroelectrodes, this seemed to have to remain a wish. However, the considerable decrease of ohmic drop at these electrodes has offered some experimental reality to this wish. Indeed, over the past decade, several groups have established experimentally as well as theoretically, that at these electrodes, meaningful electrochemical data could be obtained under conditions that were almost unthinkable before the past decade. For example, and within the context of the above discussion, one may recall that electrochemistry without added electrolyte or in solvents with low dielectric constants such as arenes or hexane is no more a fantasy. 

Finally, to conclude this introduction, and to avoid any possible confusion in the terminology, we wish to define briefly what is an ultramicroelectrode (at least in our sense ). When their interfacial properties are to be considered identical with those of any other electrode of a larger dimension, ultramicroelectrodes must remain much larger than the double layer thickness. This sets a lower dimension of a few tens of A for ultramicroelectrodes.On the other hand, if diffusional steady state voltammetry has to be observed without significant interference of convection, they must be smaller than convective layers, which sets an upper limit of a few tens of /im. Between these limits, all ultramicroelectrodes possess identical intrinsic physico-chemical properties. However, their behavior (viz. ohmic drop, steady state or transient currents, etc) obviously depends on the medium and the time-scale considered. ... 

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. ...

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).

The considerable decrease of ohmic drop at ultramicroelectrodes has allowed significant electrochemical data to be obtained from voltammetry in highly resistive media. Thus voltammetry can be performed in usual electrochemical solvents, but without purposefully added supporting electrolyte." Also, steady state voltanunograms can be obtained in solvents with very low constants,such as alcanes or arenes, if a small concentration of an inert electrolyte is added. In the following we want to present two examples of application of voltammetry under these conditions, to the unravelling of organometallic reactivity. 

Taking advantage of the reduc ohmic drop at ultramicroelectrodes, the oxidative addition of a series of / ara-substituted iodobenzenes to Pd (PPh3)4 could then be monitored in toluene, (e = 2.4) by following the decay of the zerov ent oxidation wave (compare Figure 8, previous page), and the results compared to those reported previously for THF (e = 7.6). The reaction was thus shown to have almost identical activation parameters (e.g., AH = 18 1 kcal.moW, AS = 1.7+1 cal.moH.K- in toluene) and lead to identical Hammett correlations (e.g., p = 2.3 0.2, in toluene) in both solvents, which supports a non-ionic mechanism. 

Amatore C, Lefrou C, Pfliiger F (1989) On-line compensation of ohmic drop in submicrosecond time resolved voltammetry at ultramicroelectrodes. J Electroanal Chem 270 43-59. doi 10.1016/0022-0728(89)85027-2... 

Garreau, D. Hapiot, P. Saveant, J.M. (1990). Fast cyclic voltammetry at ultramicroelectrodes Current measurement and ohmic drop positive positive feedback compensation by means of current feedback operational amplifiers. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 281,73-83. 

In this connection let us recall that the recent availability of ultramicroelectrodes has opened new frontiers to electrochemistry, besides the obvious direct access to subnanosecond time scales. Indeed owing to the considerable decrease in ohmic drop, a large variety of media... 

The behavior of array electrodes depends on the ratio of the diameter of the individual electrode to the spacing between electrode features. High diffusion current densities conditioned by radial flows and low values of ohmic potential drop are most prominent in the case of ultramicroelectrodes [13, 14]. By replacing a single macroelectrode by an array of ultramicroelectrodes, the current density can be increased by orders of magnitude as well as the ratio of faradaic to capacitive currents. 

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