We are told as undergraduates that when our electrodes are small, our electrolyte volumes are relatively large and the counter electrode is far way enough from the working electrode, then we can ignore what is happening on the counter electrode, and treat the working electrode as if it is isolated. These assumptions may not be the case in the PECC-2 where the working electrode is often relatively large, approximately 2 cm2, whilst the electrolyte volume is relativly small at 7.3 cm3. In the PECC-2 cell we therefore have a volume to area ratio of 4:1. If we contrast this with a more traditional disk electrode, then we may have a 0.2 cm2 electrode in a 10 cm3 volume and so we have a volume to electrode area of 50:1. Whether the volume to area ratio is an issue in the PECC-2 cell depends the average or approximate current you pass during your experiment and the time over which you do your experiment.
In the adjacent figure we have shown where the product from the anode and the cathode are diffusing between the electrode and are becoming available for reaction on the adjacent electrode; the result is a feedback loop which in constant potential experiment would be seen as the current on the working electrode increasing for no apparent reason. The question is though apparent how real is this phenomena in the PECC-2 cell?
Whether a scientist has the possibility of an electrochemical feeback loop in their experiment can be judged using Faraday's Law of Electrolysis, this Law says that the amount of chemical change in an electrochemical cell brought about by an electrolysis is proportional to the charge passed, we have summarized the three useful equations below.
Equation 1) Q = mnF
Equation 2) Q = it
Equation 3) it = mnF
Q = Charge passed, m = moles, F = Faraday's constant, i = current, t = time.
In simple terms Faraday's Law of Electrolysis says that if you pass a large current for a a long time you will produce a significant amount of chemical change in an electrolysis cell, whereas if you pass a small current for a short period of time you will produce an insignificant chemical change in the electrolysis cell.
When we combine Faraday's Law of Electrolysis with the PECC-2 Cell we can estimate the average current and the time the current is passed and calculate the amount of chemical change in the electrolysis cell. If the chemical change is significant relative to the reagents/chemicals in the PECC-2 Cell then one can have the possibility of a electrochemical feedback loop in the PECC-2 Cell, whereas if the amount of calculated chemical change in the PECC-2 Cell is relatively small then one can rule out the possibility of an electrochemical feedback loop. We have simplified this calculation by providing a calculator on this page. This calculator allows you to estimate the amount of chemical change produced within the PECC-2 Cell and judge whether you have an electrochemical feedback loop.
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