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 reference electrode 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.

What is a reference electrode?

The definition that an expert electrochemist would give for a reference electrode is the following:

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.

Why do we need a reference electrode?

.Figure 1. Ag/AgCl reference electrode.

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?

Voltammetry without reference electrode

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.

Figure 2. Example of redox measurement in an electrochemical cell without reference electrode.

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.

The irreproducible potential solution: the reference electrode

Figure 3. Pourbaix diagram for a silver electrode in aqueous solution. A stable potential for the redox pair Ag/Ag+ can be observed up to pH 12.

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 thir 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 reference electrode is either Ag/Ag+ or Ag/AgCl, since it has a stable redox potential over a wide pH range.

Reference electrodes for aqueous environments

There are several reference electrodes 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 reference electrodes, the potential axis needs to be mathematically converted.

Standard Hydrogen Electrode (SHE)

This reference electrode is based on the reduction of protons to hydrogen gas as follows:

2 H+ (aq.) + 2 e- —> H2 (g)

In this reference electrode, platinum is used due to its ability to catalyze proton reduction.

The platinum electrode is immersed in a solution of 1 M H+ as hydrogen gas is injected in the solution, which makes the solution to bubble.

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

Saturated Calomel Electrode (SCE)

Calomel, or mercurous chloride, reference electrodes are legacy reference electrodes that are not used as often these days. Since they use a mercury based electrode, the toxic concerns have shifted the attention from SCE to other reference electrodes using less toxic materials such as Ag/AgCl.

SCE is based on the reaction between elemental mercury and mercurous chloride as follows:

Hg2Cl2 (s) + 2 e- –> 2Hg+ (aq.) + 2 Cl- (aq.)

Thanks to this reaction, the SCE provides a stable potential of 0.241 V vs SHE.

However, it must be noted that, since the reaction requires chlorine ions, the obtained potential will depend on the concentration of these species in solution. Therefore, this electrode is enclosed in a saturated solution of potassium chloride.

Copper Sulphate Electrode (CSE)

This reference electrode is not very common in electrochemistry labs, but it is used in the field to meadure corrosion in metalic structures.

This electrode consists in a copper electrode immersed in a saturated solution of copper sulphate.

The CSE reference electrode works vie the following reaction:

Cu2+ (aq) + 2 e- –> Cu (s)

Thanks to this reaction,the CSE provides a stable reference potential of 0.314 V vs SHE.

Silver/Silver Chloride Electrode (Ag/AgCl)

The Ag/AgCl reference electrode is the most common reference electrode used today for aqueous measurements. This is due to its lower toxicity compared to the SCE and the lower complexity when compared to the SHE.

This reference electrode is based on the following reaction between silver and chloride ions:

AgCl (s) + e- –> Ag+ (aq) + Cl- (aq)

Thanks to this reaction, the Ag/AgCl reference electrode provides a stable potential of 0.197 V vs SHE.

It must be noted though that, since the reaction is dependent on chlorine ions, the concentration of chlorine ions affects the potential provided by the Ag/AgCl reference electrode. For this reason, if possible, the Ag/AgCl reference electrode is immersed in a saturated KCl solution or a solution where the Cl- concentration is kept constant.

Reference electrode potential table

Reference ElectrodePotential vs SHE, V
SHE0
SCE (Saturated KCl)0.241
SCE (3.5 M KCl)0.250
SCE (1 M KCl)0.280
SCE (0.1 M KCl)0.334
CSE (Saturated CuSO4)0.314
Ag/AgCl (Saturated KCl)0.197
Ag/AgCl (3.5 M KCl)0.205
Ag/AgCl (0.1 M KCl)0.288
Table 1. Reference Electrode Potentials vs SHE.

Non Aqueous Reference Electrodes

So far, the reference electrodes we discussed were designed to work in water based electrolytes. However, these electrodes are not suitable for organic based electrolytes like the ones used in batteries.

Working with organic based electrolytes requires either a quasi-reference electrode or a metallic reference electrode.

Quasi-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.

Metallic electrodes

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.

These electrodes work on the following reaction:

M (s) + X e- –> M X+ (lq)

Like quasi-reference electrodes, the potentials provided by these electrodes depend on the organic solvent used for the electrolyte. Thus, the results can’t be compared between different solvents.

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