Recent trends in the design of potentiostats capable of correcting for ohmic losses occurring in electrochemical cells are reviewed. The various effects of ohmic drop occurrence and their influence on diverse electroanalJ.tica1 techniques are considered, as well as potentiostat configurations and stabilig., with emphasis in the three-electrode cell configuration. Electronic correction methods are discussed according to the following classifications: ( 1) positive feedback, ( 2 ) negative impedance, (3) current pulse, ( 4 ) analogic subtraction. and (5) current interruption.
KEY WORDS: Potentiostat, Ohmic drop, Instrumentation
SCOPE OF THE PROBLEMIn electrochemistry, controlled-potential experimem are performed b! . using a potentiostat. This instrument makes possible the automatic maintenance of the potential of the working electrode (WE) at a programmed value relative to the reference electrode (RE), irrespective of changes in the current passing through the electrolytic cell and/or in the solution resistance. In a three-electrode cell. such operation is continuously achieved by comparing the cell and reference signals, changing the potential of the counter electrode (CE) as necessary to compensate for any difference arising. That is, the cell current is increased or decreased to maintain the equaliK between rhe cell potential and the reference signal.Yet, the electrical characteristics of the electrochemical system have not been considered. From an electronic point of view, a three-electrode electrochemical cell can be regarded [ 11 as the network of impedances shown in Figure 1, where Z, and Zll. represent the interfacial impedances at CE and WE and the solution resistance is split into two portions, Rn and R,,, depending on the position of the reference electrode's probe in the current path. Ideally, the potentiostat should control the interfacial potential only, without interference of ohmic losses. However, the potentiostat is capable of compensating the ohmic drop in the solution except for its fraction occurring between RE and WE. That is, the controlled voltage contains a portion 1. R, of the total voltage drop in the solution. The presence of this uncompensated resistance loss keeps the potentiostat from giving an accurate control of the desired potential. Thus, a change of potential between these electrodes corresponds to a change of potential across the double layer of UC'E only if RE is not polarized and the 1 . R,, drop between them is negligible. But the potential-measuring probe in the electrol).te cannot measure at a point directly on the solution side of the double layer, thus causing a voltage drop that adds to the measured potential [l-31. The accurate control of potential is most important in cyclic voltammetn. The existence of the uncompensated resistance causes the potential sweep to be nonlinear: the distortion may be large enough so as to invalidate subsequent analysis in terms of a preconceived model ( e g , the expected dependence of current on the square root of scan rate for ...
