What is a Reference Electrode?

In electrochemistry it is very common to work with so-called 3 electrode cells. These cells have:

• A working electrode (WE)
• A counter electrode (CE)
• A reference electrode (RE)

The RE is perhaps the one that is most difficult to understand for those starting in electrochemical research.

In this post we explain in detail what is a reference electrode, what are the different types, as well as some considerations to take into account when performing electrochemical measurements.

A reference electrode is an electrode used to measure, in a reliable and reproducible way, the potential experienced by the working electrode.

This definition, while precise, does not explain why a reference electrode is needed. Therefore, we explain below why we need one and what would happen if we don’t have one in our electrochemical cell.

In electrochemistry, we study reactions that occurr at certain electrical potentials.

The most common of these reactions is the oxidation and reduction of compounds.

To measure these reactions, we need to apply a potential difference betweeen 2 electrodes.

However, if we just use 2 electrodes, the results tend to be irreproducible.

Even during the same experiment. But… why?

When we work with 2 electrodes, one of them works as the working electrode while the other works as both counter and reference electrode. Therefore, the “0 V” that we measure depend on the potential difference between the two.

If we start measuring with cyclic voltammetry a reversible redox reaction like ferri/ferrocyanide, we will see that each voltammogram shows this reaction at a different potential.

This potential change that we see is due to our reference. In short, the “0 V” shift as time goes by. This occurs for 2 reasons:

1. When an electrode experiences a current, we are modifying its state. Therefore, its resting potential changes.
2. During the measurements, the redox potential of the electrolyte also changes as time goes by.

The combination of these 2 effects makes it very challenging to obtain reproducible measurements in 2 electrode electrochemical cells.

To obtain reproducible potential measurements we need to add a third electrode. The reference electrode.

This electrode, unlike the working or counter electrodes, will not experience any current. It will just serve as a “0 V” reference. This way, we can control the applied potential to the working electrode.

Thanks to this configuration, we can eliminate the first effect discussed earlier. As a result, the reliability and reproducibility is greatly improved.

Just having a third electrode improves results. But to get the best results, this third electrode must also counter-act the second effect: the fluctuations of the electrolyte’s redox state during the measurement.

To achieve this, reference electrodes are made using special materials. These materials must have 2 important characteristics:

• They have their own redox potential
• Their redox potential is stable to common electrolyte changes such as pH.

For applications in water-based environments, the most common RE is either Ag/Ag+ or Ag/AgCl, since it has a stable redox potential over a wide pH range.

There are several REs for aqueous environments. However, each one has its redox potential in a different place. Thus, the “0 V” recorded will be different accross them.

Therefore, it is important to bear in mind that to compare results acquired with different REs, the potential axis needs to be mathematically converted. If you would like to know more about reference electrode conversion, we have a dedicated page that includes an online reference electrode converter.

The potential given by the SHE is the one normally used as a “0 V” reference to compare the different reference electrodes.

This type of reference electrodes are based on a redox molecule, normally ferrocene or cobaltocene, rather than a metal like we have seen so far.

Unlike traditional reference electrodes, quasi-reference electrodes provide a different reference potential depending on the electrolyte used. For this reason, this reference electrode is composed of 2 parts:

• An aqueous reference elecctrode
• A redox molecule as an internal standard

With this configuration, the redox molecule signal is used to correct the deviations experimented by the aqueous reference electrode.

However, there are 2 issues with using quasi-reference electrodes.

First, if the reaction that we want to study occurs in the same range as the internal redox standatd, it will interfere with the study.

And second, since the potential provided by the redox internal standard depends on the organic solvent used, the acquired results can’t be compared with result obtained with electroytes using different solvents.

Another solution typically used in organic based electrolytes is using pure metallic electrodes.

The most common of these metals is lithium, since organic based electrolytes are used in electrochemical research involving energy storage devices. However, it is also common to see other metals commonly used in electrochemical energy storage systems such as sodium or potassium.