IR compensation

The electrode potential was controlled with an EG G Princeton Applied Research (PAR) model 173 potentiostat/galvanostat and is referenced to a saturated calomel electrode (SCE). A PAR model 276 current-to-voltage converter allowed monitoring of current during the ORC and SERS experiments and it also provided for positive feedback iR compensation for accurate potential control. 

Fig. 5.3 Effect of Ru content on Tafel plot of fresh Ru/Ti oxide electrodes in 5M NaCI (pH of approximately 3.5) at room temperature (with IR compensation).

Fig. 5.4 Time dependence of HE profiles during chlorine evolution at a 40 at.% Ru electrode in 5 M NaCI + 0.1 M HCI at room temperature (without IR compensation), subjected to square-wave potential cycling (from 1.35 to -0.32 V versus SCE at 60s cycle-1). The numbers in the figure refer to the time of electrolysis in hours. Each Tafel plot is shifted to the right by 20 mV to avoid overlapping. 

Any deviation from the above criteria is indicative of kinetic complications and should be treated individually. However, one case is worthy of note. In non-aqueous solutions, it is commonly observed that AEP, for example, has typical values between 70 and 100 mV owing to the so-called IR drop resulting from the uncompensated and relatively large solution resistance. While IR compensation techniques are available, they are not always reliable, and it is more convenient to compare the measured AEP with that of a known reversible reaction measured under similar conditions. 

Positive feedback iR compensation in three-electrode measurements As described in Section 5.3, the influence of iR-drop is serious in two-electrode polarography or voltammetry. The influence is eliminated considerably with three-electrode instruments, if the tip of the reference electrode is placed near the surface of the indicator electrode. However, there still remains some iR-drop, which occurs by the residual resistance at... 

E. R. Brown, H. L. Hung, T.G. McCord, D. E. Smith, and G. L. Booman, A Study of Operational Amplifier Potentiostats Employing Positive Feedback for iR Compensation II. Application to AC Polarography, Anal. Chem. 40 1411 (1968). 

In Figure 8 the positive slope is approximately ten times steeper than a normal irreversible system should exhibit. This excessive effect is probably caused by capacitance and resistance effects. Excessive capacitance currents (Figure 9) are indicated by linear variations of Ep vs v when IR compensation was not used and Ep vs vl/2 when IR compensation was used. 

Figure 9. Effect of capacitance current (a) DCD 13 B, IR compensation off (b) DCD 12, IR compensation used.

Figure 11 Transport- and IR-compensated 02 reduction currents as a function of potential recorded on nuclear fuel (U02) in 0.1 mol dm3 NaC104 (pH = 9.5). The electrode was cathodically reduced before the experiment. (1) Data recorded from the most negative to the most positive potential, showing the behavior on a reduced U02 surface. (2) Data recorded from the most positive to the most negative potential after corrosion in aerated solution, illustrating the behavior on an oxidized U02+x surface.

Fig. 7D Variation of current density with time during linear potential sn eep measurement. Quinhydrone (5 mM) in 1 mM sulfuric acid. V -75 mVis. (a) with dynamic iR compensation (b) without iR compensation. From Gileadi, Kirowa-Eisner and Penciner, "Interfacial Electrochemistry - An Experimental Approach", Addison Wesley, 1975, with permission.

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. 

Consider now the effect of uncompensated iR on the shape of the potentiostatic transients. This was shown in Fig. 6D. The point to remember is that although the potentiostat may put out an excellent step function - one with a rise time that is very short compared to the time of the transient measured - the actual potential applied to the interphase changes during the whole transient, as the current changes with time (cf. Section 10.2). This effect is not taken into account in the boundary conditions used to solve the diffusion equation, and the solution obtained is, therefore, not valid. The resulting error depends on the value of R, and it is very important to minimize this resis-tance, by proper cell design and by electronic iR compensation. 

Fig. 16K Galvanostatic transients, (a) without iR compensation (b) with 95% iR, compensation. Note the difference in the scale of potential.

Polarizability of ITIES has given us a thermodynamic degree of freedom for controlling the potential drop across the interface. The nonideal polarizability at an actual ITIES does not, however, necessarily imply ambiguity in controlling the potential drop across the interface. From an experimental point of view, the potential control in the presence of a residual current is primarily determined by the capability of the potentiostat, in particular its fast response and IR compensation. Appropriate design of a potentiostat [38-41] and of a measuring cell [42, 43] assures the precise control of the potential drops across the ITIES, even in the presence of appreciable flow of current across the interface. 

Figure 3. Cyclic and linear sweep voltammograms (IR compensated) of methanol...

Corrected with an IR compensation insnument (vs. Ag/AgCl) t>Trace cNot detected. 

Although the potentiostat has an adjustable IR compensation circuit, this was not used. When it was used, the scan rate varied. Compensation did not seem to have much effect. Some researchers have developed their own IR compensation circuit for the high resistance solution ( ), some added a supporting electrolyte (12 11) And some simply ignored it (H). Since the behavior of electrolytes under supercritical conditions is not well known, no supporting electrolyte was added to eliminate the IR drop in this experiment. Furthermore, the IR circuit was not used for this study either, since the desired electrochemical data, such as exchange current density, open circuit potential, and transfer coefficients, can be obtained from the polarization curves without IR compensation. 

All measurements will be carried out under an inert atmosphere at 25 °C. Be sure to purge your solution with your inert gas for 5 min prior to scanning. Polish your Pt working electrode, and perform an IR compensation prior to each run. 

The anionic Ni(III) carborane, (Ni[C2B,H,(CH3)jJ2] , can be reversibly reduced at — 0.9 V in CHjCN to the dianion and cyclic voltammetry data at a hanging-Hg drop are given in Table 2. To minimize resistance losses, a luggin probe is used for the reference electrode and positive feedback iR compensation is employed. At each scan rate, the measured value of AEp is used to obtain >j/ from Table 1. Then, using a value of a calculated from the scan rate and Eq. (1), a value of kj/D is obtained at each V. The average of values at different scan rates is calculated D must be measured independently (from, e.g., the polarographic I value) and in the present example is 6.4 X 10 cm s . This yields ... 

Potential Error Correction (iR Compensation), Technical Note 101, EG G Princeton Applied Research, Princeton, NJ, 1986... 

It should be realized that overcompensation does not always result in "instability" and so great care is needed to avoid it. Positive feedback in IR-compensated potentiostats has been discuss in detail by Britz [15] and by Me Kubre and MacDonald [13]. 

---