How to select the right reference electrode?

Introduction

In electrochemistry, a 3-electrode set-up is generally used and consists of a working electrode, a counter electrode and a reference electrode. By using a reference electrode, the half-cell properties can be studied, instead of the full-cell properties obtained using a 2-electrode set-up.

The role of the reference electrode is to provide a stable, known potential, so that any changes in the cell can be ascribed to the working electrode [1]. It is essential that the reference electrode potential is fixed, in order to ensure accurate and valid measurements and to be able to compare experimental data.

The potential of a reference electrode depends on its chemical redox pair(s). These need to be stable, with well-defined chemistries (known and calibrated potential), and to remain unchanged in contact with the system of interest (no potential drift, deterioration, or interference) [2].

As they are based on different chemistries, reference electrodes are not compatible with all experimental conditions. Guidelines to help you to select the reference electrode that matches your experimental system are provided below.

Are you working in aqueous or non-aqueous media?

• In aqueous media, silver electrodes (Ag/AgCl) and calomel electrodes (Hg-based) are generally used.
• In non-aqueous media, Ag/Ag⁺ electrodes are used with an internal solution containing Ag⁺ salt and supporting electrolyte added to the same non-aqueous solvent as the solution sample to be measured. Refer to RE-7N and RE-7SN internal solution preparation to have an example.
• Reversible Hydrogen Electrode (RHE) works in every electrolyte.

What is the pH range of your media? [3]

• For alkali media, Hg/HgO electrode is recommended.
• For acidic media, Hg/Hg sulfate electrode is recommended.
• For neutral media, Ag/AgCl and Hg/Hg₂Cl₂ electrodes are recommended.
• Reversible Hydrogen Electrode (RHE) is pH-independent on a wide pH range (-2 to 16)

Are you working with salted water?

• In salted water, Reversible Hydrogen Electrode (RHE) is recommended.
• The use of Hg/HgO, Ag/AgCl and Ag/Ag⁺ electrodes is acceptable.

Are you working with hydrofluoric acid?

• Reversible Hydrogen Electrode (RHE) is recommended.
• Note: HF is highly corrosive and presents a risk of damaged for all other types of electrodes.

Are you working at a temperature other than room temperature?

• Ag/AgCl and calomel electrodes are designed to be used at room temperature (around 25 °C). Ag/AgCl and calomel electrode potential is dependent on temperature, potential drift is especially noticeable at high temperature. Hg/Hg₂Cl₂ can’t be used above 50 °C. [4]
• Reversible Hydrogen Electrode (RHE) is not sensitive to temperature changes (-20 to 210 °C).

Is your system susceptible to contamination?

• Reversible Hydrogen Electrode (RHE) does not contaminate the system and is mercury free. RHE is recommended for biological or environmental study.
• Ag/AgCl electrode is mercury-free. Chloride ions can contaminate your system as small leakage of inner electrolyte through the frit enables electric contact of Ag/AgCl reference electrode with test solution.
• Calomel electrode contains mercury. Hg/Hg sulfate electrode can be used to avoid chloride ions pollution.

Are you performing Electrochemical Impedance Spectroscopy (EIS) measurements?

• Reversible Hydrogen Electrode (RHE) is recommended for EIS measurement. RHE resistance is about 1 Ω whereas it is between 1 kΩ and 10 kΩ for traditional electrodes. There is a trick to add a capacitor in parallel to minimize the effect of the reference at high frequencies.

Checking and validating reference electrodes (Electrochemistry basics series) – BioLogic Learning Center.

Tips/Troubleshooting

• Check for bubbles.
  For traditional reference electrode, air bubbles in the internal solution of the reference electrode can disturb measurements. Wrong or noisy potential can be obtained if large bubbles are present. Remove them by flipping or shaking the electrode.
  For RHE, bubbles of hydrogen of the electrode itself do not disturb measurement, unless it sticks to the bottom of the electrode. In this case, hang the hydrogen electrode transversely.
• Check metal pins to ensure good contact (no trace of corrosion).
  Note: Incorrect storage of traditional electrodes can lead to corrosion of metal pins. Storage solution must not be in contact with metal pins and parafilm must protect metal pins from crystal deposition.
• Check for traces of contamination or deposition within the frit of the reference electrodes to avoid measurement issues (potential shift, oscillation of potentiostat due to external noise…).

Note: When using Ag/AgCl electrode, if perchlorate ions are present in the test solution, NaCl as the inner electrolyte (instead of traditional KCl electrolyte) has to be used to avoid deposition of potassium perchlorate within the frit and deterioration of Ag/AgCl electrode.

• Artifacts in EIS at high frequency. Check your reference electrode, please refer to the article Checking and validating reference electrodes (Electrochemistry basics series) – BioLogic Learning Center.

Conclusion

Different types of reference electrodes are available (silver-based electrode, calomel electrode, reversible hydrogen electrode, etc,…). As they are based on different chemistries, reference electrodes are not compatible with all experimental conditions. A reference electrode has to be selected carefully to avoid measurement errors, contamination of the system of interest, or deterioration of the reference electrode.

References

[1] BARD, Allen J., FAULKNER, Larry R., et WHITE, Henry S. Electrochemical methods: fundamentals and applications. John Wiley & Sons, 2022.

[2] ALNOUSH, Wajdi, BLACK, Robert, et HIGGINS, Drew. Judicious selection, validation, and use of reference electrodes for in situ and operando electrocatalysis studies. Chem Catalysis, 2021, vol. 1, no 5, p. 997-1013.

[3] ANANTHARAJ, Sengeni, SAGAYARAJ, Prince JJ, YESUPATHAM, Manova Santhosh, et al. The reference electrode dilemma in energy conversion electrocatalysis:“right vs. okay vs. wrong”. Journal of Materials Chemistry A, 2023, vol. 11, no 33, p. 17699-17709.

[4] SPITZER, Petra, WUNDERLI, Samuel, MAKSYMIUK, Krzysztof, et al. Reference electrodes for aqueous solutions. In : Handbook of Reference Electrodes. Berlin, Heidelberg : Springer Berlin Heidelberg, 2013. p. 77-143.