Question

Describe the criteria of considering a half reaction to be a reference electrode in electrochemical measurement in a 3-electrode configuration.

Answer 1

Reference electrode allows you to measure the potential of the working electrode with out passing current through it while counter (auxiliary) electrode allows you to pass current. If oxidation occurs at the working electrode, reduction using the same magnitude of current is sustained at the counter electrode and hence there is no current flow between working and reference electrode(high input impedance) enabling us to follow changes in working electrode potential accurately. This is not possible in a two electrode system although we can get crude values by using certain types of electrodes which can act simultaneously as working and reference electrode.

If you tell what type of reference electrode you use, then it is very easy to explain l the electrochemical reaction occurring at the reference although this may be difficult for the case of quasi-reference electrodes. For example, if you are using a silver/silver chloride or saturated calomel electrode(SCE) this is very easy.

The counter electrode (also known as auxiliary electrode), is an electrode which is used to close the current circuit in the electrochemical cell. It is usually made of an inert material (e.g. Pt, Au, graphite, glassy carbon) and usually it does not participate in the electrochemical reaction. Because the current is flowing between the WE and the CE, the total surface area of the CE (source/sink of electrons) must be higher than the area of the WE so that it will not be a limiting factor in the kinetics of the electrochemical process under investigation.