Why Isn't the Standard Hydrogen Electrode Used as a Reference Electrode?

In the field of electrochemistry and analytical chemistry, the need for accurate and reliable reference electrodes is paramount. One of the most common types of reference electrodes is the standard hydrogen electrode (SHE). Despite its widespread use in academic and research settings, it is not commonly used as a primary reference electrode in practice due to certain limitations. This article will delve into the reasons why, explore alternative reference electrodes, and discuss the practical considerations involved.

Introduction to the Standard Hydrogen Electrode (SHE)

The standard hydrogen electrode is a primary reference electrode used in electrochemistry to measure the reduction potential of other species. It is defined as the electrode with a reduction potential of 0.000 V at 25°C under standard conditions (1 atm of H2, 1 M H solution). The idea of using a hydrogen electrode as a reference electrode is straightforward; the hydrogen evolution reaction is known, and the reduction potential is exactly 0.0 V, providing a standard to compare other redox couples.

Challenges of Using a Hydrogen Electrode as a Reference

While theoretically elegant, the practical application of a hydrogen electrode is fraught with challenges.

Bulkiness and Cost

The hydrogen electrode requires a cylinder of hydrogen gas at 25°C to operate accurately. This means that a hydrogen electrode is bulky, making it less convenient to use in many experimental setups. Additionally, handling and storing hydrogen gas can be hazardous, which adds an extra layer of safety concerns to the experiment. The cost of procuring and maintaining a hydrogen gas supply can also be significant, making it economically prohibitive for many laboratories.

Temperature Dependence

The Nernst equation indicates that the reduction potential of a hydrogen electrode varies with temperature. At 25°C, the standard reduction potential of the hydrogen electrode is 0.000 V. However, this value deviates at different temperatures, which can introduce cumbersome corrections in experimental setups where precise control over temperature is not feasible.

Maintenance and Durability

The hydrogen electrode requires regular calibration and maintenance to ensure its accuracy. Moreover, the electrode's functionality can degrade over time due to the effects of water impurities, adsorption of impurities, and changes in the polarization behavior of the hydrogen evolution reaction. This necessitates frequent checks and adjustments, adding to the experimental workload.

Alternative Reference Electrodes

Given the limitations of the hydrogen electrode, there are several alternative reference electrodes that are widely used in practice. Among these, the silver/silver chloride (Ag/AgCl) and copper/copper sulphate (Cu/CuSO4) electrodes are the most popular choices.

Silver/Silver Chloride (Ag/AgCl) Electrode

The Ag/AgCl electrode is a versatile and stable reference electrode. It operates on the principle of the chloride ion exchange reaction, with a reduction potential defined under standard conditions as -0.241 V. This electrode is less prone to temperature variations and is more resistant to contamination compared to the hydrogen electrode.

Copper/Copper Sulphate (Cu/CuSO4) Electrode

The Cu/CuSO4 electrode is another popular alternative. It operates on the copper(II) ion reduction reaction, with a reduction potential of 0.340 V under standard conditions. Like the Ag/AgCl electrode, the Cu/CuSO4 electrode is also less sensitive to temperature changes and provides a linear calibration across a wide range of concentrations.

Comparison and Practical Considerations

The choice between the hydrogen electrode and these alternatives depends largely on the specific requirements of the experimental setup. The Ag/AgCl and Cu/CuSO4 electrodes offer several advantages over the hydrogen electrode, including reduced bulk, ease of maintenance, and better stability. However, it is important to note that the reduction potential of these electrodes is not exactly defined as zero, which can introduce small variations in the experimental data.

Conclusion

In summary, while the standard hydrogen electrode has a theoretical advantage in providing a precise reference point, its practical application is limited by its bulkiness, cost, and temperature dependence. Alternative reference electrodes such as the Ag/AgCl and Cu/CuSO4 provide a more convenient and reliable solution for many electrochemical analyses. Understanding these limitations and weighing the pros and cons of each option can help researchers and lab technicians choose the most suitable reference electrode for their specific needs.

Keywords

Standard hydrogen electrode, reference electrode, hydrogen electrode, silver/silver chloride electrode, copper/copper sulphate electrode