The Measurement of Potential in Electrochemical Reactions

We have already referred above to the measurement of current density in electrochemical reactions. This is a very simple measurement so long as one does not want to do it at very low times, e.g., in the microsecond range, but under the normal conditions of ordinary steady state measurements where the time may be seconds or longer, a simple 

The measurement of potential, and correspondingly overpotential, is, however, by no means simple, and in fact it involves quite special reasoning, and even conventions, which would herewith be presented. 

First of all, it must be understood that one of the weaknesses of the subject of electrochemistry is that there is no experimental method available (or even possible) by which the absolute solution electrode p.d. can be measured. This may seem surprising, considering that electrochemists talk about potential as the center of their arguments, but it is true and not controversial, and the reasoning and details concerning the why and the wherefore can be read in a text such as that of Bockris and Reddy. 

FIGURE 7. The three-electrode system required to measure electrode overpotential, i.e., A l — A(l g. The potential between the working electrode and the reference electrode when both A l and correspond to the same reaction is equal to the overpotential rj. The tube joining reference electrode and working electrode is called a Luggin capillary. It helps diminish the inclusion of the illicit IR drop in the measurement. 

The essence of this reference electrode is that it presents a well-known and stable potential, in comparison with which the potential of the working electrode can be measured. Thus, suppose one has a reference electrode which is some arbitrary standard set up so that the conditions which lead to its reproduction become easy. In practice, two reference electrodes are chosen. The first and more fundamental is the hydrogen electrode, which is the electrode produced by bubbling hydrogen gas at one atmosphere over a highly catalytically active surface (which is in the solution of proton, of activity unity) for the reaction of hydrogen to equilibrate protons in solution and electrons (platinized platinum is often used) with a hydrogen gas. The potential of this electrode (it could be used as a reference) is arbitrarily taken to be zero volt. 

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