Ohmic drop, electronic compensation

Now it is possible to assemble microelectrodes with extremely short response times. Nevertheless, an additional problem for the reduction of the ohmic drop is that for short times high currents arise from the large concentration surface gradients. This leads to the use of on-line and real-time electronic compensation of the cell resistance combined with the use of microelectrodes [53]. 

The ohmic potential drop should not be a source of major error in this type of measurement, since it is a constant. In principle, it is possible to perform the experiment without any iR compensation, measure this correction term independently, and apply an appropriate correction to the result. Better sensitivity and accuracy can be achieved, however, if iR is measured first and its value subtracted from the measured potential electronically, particularly when it is large compared to the measured activation overpotential. The reason for this should be apparent from a comparison of the curves obtained with and without electronic compensation, as seen in Fig. 16K. 

The majority of commercially available potentiostats have a facility for electronically compensating for the ohmic drop due to the solution resistance between the Luggin capillary and the electrode. The Luggin probe is placed far enough away into the solution to prevent shielding of the electrode, and part of the output signal from the current follower is fed back into the potentiostat to compensate for the resistance between the Luggin tip and the electrode. A typical circuit is shown in Fig. 11.12. 

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.

These equations are important for localized corrosion and the ohmic drop within small corrosion pits, which will be discussed in detail in Sec. 1.6.2.2. Due to the concave geometry, the ohmic drop within pits and in the vicinity of pits is enlarged by a factor of three (Fig. l-9b). This value has been estimated and later confirmed by computer simulation (Vetter and Strehblow, 1970a Newman et al., 1974). Due to the intense local dissolution in the individual pits, the ohmic drop may not be compensated electronically for the whole electrode. 

At small values of voltage sweep rate, typically below 1 mV/s, the capacity effects are small and in most cases can be ignored. At greater values of sweep rate, a correction needs to be applied to interpretations of ip, as described by Nicholson and Shain. With regard to the correction for ohmic drop in solution, typically this can he handled adequately by careful cell design and positive feedback compensation circuitry in the electronic instrumentation. 

The related ohmic drop AU = iR may lead to an appreciable difference between the actual potential and that chosen for the experiment. It can be minimized by an appropriate position of the Haber-Luggin (HL) capillary of the RE, close to the surface of the working electrode (WE). However, it should not be too close in order to avoid the partial blocking of the metal surface and the formation of crevices. As a compromise, the distance should be about three times the diameter of the capillary. The ohmic drop may be also compensated electronically to about 90% of its value by an appropriate circuit built in the potentiostat as shown later in the block diagram of Figure 1.26a. The ohmic drop may be minimized by a small size of the electrode. 

The recorded current is caused not only by the heterogeneous electron transfer to the substrate (the Faradaic current ), but also by the current used to charge the electrical double layer, which acts as a capacitor. The measured potentials include the potential drop caused by the ohmic resistance in the solution, the iR drop. Both the charging current ic and the iR drop grows with the sweep rate it is always desirable to compensate for ic and iR drop, but it becomes imperative at higher sweep rates. There exist different ways to compensate electrically for these phenomena, and this makes it possible to operate up to about 103 V sec-1. It is assumed below that the data are obtained with proper compensation. 

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