Resistance, electronic compensation

For low HF concentrations in the order of 0.1%, the behavior of the interface is not oscillation, but rather resonant if the potential is set to a fixed value and time is allowed for stabilization, a steady-state constant current is finally reached. Addition of a series resistor in the order of 1 kD crrf2 leads to sustained potentiostatic oscillations [Ch5], For higher HF concentrations of about 2-5% aqueous HF, the system is self-oscillating, if the series resistivity of the electrolyte itself is not electronically compensated. For even higher concentrations the periodicity is lost and... 

There are electronic compensation circuits available to reduce this error, and if the current density is low enough (or the solution highly conducting), it may be negligible. The high resistance of nonaqueous solutions could provide a difficulty (which, however, is not present in the decay transient approach). 

There are four general responses to the problem of solution resistance. First, if only qualitative information is sought in the experiment, a certain amount of iR error can be tolerated, perhaps 100 mV. Second, electronic compensation of solution resistance can be applied, and this is often quite successful and will allow accurate data to be obtained even with macroelectrodes. Nevertheless, problems of potentiostat stability and signal distortion must be addressed. 

Examples of experimental voltammograms are shown in Figs 6.22-6.24. These are all from our current research on the oxidation of substituted pyrroles [39,42]. Numerous other examples may be found in the literature [1, 2]. The voltammograms shown are recorded with a potentiostat without electronic compensation of the solution resistance in order to illustrate better some of the problems caused by the solution resistance. The peak potentials below are given with a precision of 10 mV, which is typical for values reported in the literature. These values originate from the data files and cannot be determined with this precision directly from the figures reproduced below. 

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 next step was the electronically compensated pump. All pumps speed the motor as resistance increases to maintain a constant solvent slow. These pumps also add a major plunger speed-up during refill and repressurization. With this modification, a pump with a single pump head and a pulse dampener could give 90% of the performance of a two-headed pump for 50% of the cost. An overall dramatic price reduction for the dual-pump HPLC system resulted. 

Any additional resistance in the working electrode itself, for example due to the formation of resistive films, will be included in the uncompensated resistance and can only be reduced by electronic compensation. 

Schone and Wiesbeck [684] proposed using two working electrodes (disk-ring) with electronic compensation of the solution resistance and frequency analyzer without a potentiostat. The potentiostat was used only for slow dc polarization of the working electrodes. 

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. 

Five-membered heterocycles with two heteroatoms have the jr-electron deficiency of Y-type heteroatoms compensated by the jr-electron excessive character of the X-type atoms therefore, this category includes some of the most stable heterocycles. For example, NMR spectral data and chemical behavior (e.g., resistance to oxidation by potassium permanganate) suggest that pyrazole and imidazole have delocaliza-... 

It should be emphasized that this design of the three-electrode cell gives good results in the majority of cases. However, as mentioned, in fast electrochemical techniques in non-aqueous solvents, iRnc can assume values which compromise the accurate control of the potential of the working electrode and hence the achievement of reliable electrochemical data. In such cases one must employ electronic circuits which compensate for the resistance of the solution. 

The fact that either the peak-to-peak separation, AEp, somewhat departs from the value of 59 mV or the current function ipJvl/2 is not rigorously constant seems to contrast with the diagnostic criteria (illustrated in Chapter 2, Section 1.1.1) for an electrochemically reversible one-electron process. This can be largely attributed to the non-compensated resistance given by the dichloromethane solution, which is a low conducting solvent. 

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