Cell and Circuit Switching
Last Updated: 5/20/19 by Tim Paschkewitz
Most electrochemical instruments are designed to perform a wide range of electrochemical techniques. Different techniques make use of different portions of the circuitry within the instrument and this means that circuit switching is likely to occur when configuring the instrument for a particular method.
The feedback loop circuitry for a galvanostatic method, for example, actively monitors the working electrode current, but for a potentiostatic method, the feedback circuitry must monitor the working electrode potential. Different feedback paths are switched in and out of the circuit depending upon whether the working electrode is operating in potentiostatic or galvanostatic mode. Still, other methods require passive measurement of the open circuit potential, meaning that the active feedback loop must be switched out of the circuit entirely. Circuit switching also occurs when changing the current measurement sensitivity (i.e., the current range) or the amount of stability filtering in the feedback loop (i.e., the response time).
The term cell switching refers to making (or breaking) the connection between the instrument and the external electrochemical cell. In some cases, this switching occurs manually (as you physically connect/disconnect cell cables), but in many cases this switching occurs under software control (via internal instrument relays).
Each time a cell or circuit switching action occurs, the instrument is likely to lose control of the external electrochemical cell for a brief period (typically on the order of a few milliseconds). As the instrument regains control of cell, various current transients may occur at the working electrode. These transients are often harmless and decay away rather rapidly. If your electrochemical system is particularly sensitive, however, these transients may interfere significantly with your experiments.
For systems where such transients are harmless, a one to three second long induction period
at the beginning of an experiment assures that the random transients will not affect the data recorded during an experiment. In other cases, special steps (see below) must be taken to prevent such transients.
2Minimizing Switching and Transients
The best way to minimize unwanted and potentially harmful current transients is to carefully make sure that the signal applied to the working electrode at the beginning and end of the experiment is exactly the same as the signal applied to the working electrode during the idle periods in between experiments.
When no experiment is being performed, the instrument is said to be in its idle mode. Although the instrument is idle, it can still maintain a steady potential (or current) at the working electrode. The precise signal levels can be adjusted from the Instrument Status window. In general, the idle signal level should be set at a potential (or current) at which your electrochemical system is stable.
When specifying a new experiment to be performed, carefully adjust the initial conditions and the induction period
conditions so that they exactly match the idle conditions which prevail before the experiment begins. This means that the initial signal level should match the idle signal level. On those instruments which permit adjustment of the idle current range and idle filtering settings, you should further strive to match the idle filter and range settings so that they match those which will be used during the experiment.
Finally, each electrochemical technique allows you to specify the idle conditions that will prevail after the experiment is completed.
These post-experiment idle conditions should also be selected so that they exactly match the final conditions which prevail during the relaxation period
at the end of the experiment.