Understanding the Differences Between Standard Hydrogen Electrode (SHE) and Normal Hydrogen Electrode (NHE)

The terms Standard Hydrogen Electrode (SHE) and Normal Hydrogen Electrode (NHE) are commonly encountered in electrochemical studies. Although these terms are often used interchangeably, there are subtle but important differences that researchers need to be aware of. This article will delve into the definitions, characteristics, and practical applications of both SHE and NHE.

Standard Hydrogen Electrode (SHE)

The Standard Hydrogen Electrode (SHE) is a widely used reference electrode in electrochemical studies. It is based on a half-cell reaction involving hydrogen ions (H ) and hydrogen gas (H2). The conditions under which the SHE operates are precisely defined, making it an ideal reference for standardization.

Definition: The SHE is a reference electrode that operates under standard conditions, typically at a temperature of 25 °C (298 K) and a pressure of 1 atm for hydrogen gas, with a concentration of 1 M for hydrogen ions (H ). Reference Potential: The SHE is assigned a potential of 0.00 V. This makes it a standard reference for measuring the electrode potentials of other half-cells. Characteristics: The SHE is considered ideal because the hydrogen ions do not interact with other ions in the solution, and the hydrogen gas is pure. Note: The SHE does not actually exist in practice. Instead, it serves as a theoretical reference point. A Reversible Hydrogen Electrode (RHE) is often used, which does not require the proton concentration to be exactly one and whose potential is pH-dependent.

Normal Hydrogen Electrode (NHE)

The Normal Hydrogen Electrode (NHE) can be considered conceptually similar to the SHE and often refers to the same setup. However, it is sometimes used to describe the electrode under slightly different conditions. In some contexts, the NHE may imply a more practical application in laboratory settings.

Definition: The NHE, like the SHE, involves a platinum (Pt) electrode immersed in a solution of 1N (1 mol/L) hydrochloric acid (HCl) and bubbling hydrogen gas (H2) at 1 atm through the solution. However, the NHE is not strictly reversible and may change over time. Differences from SHE: The primary difference between the NHE and the SHE is the electrolyte and the conditions under which they operate. Electrolyte Solution: In the NHE, the concentration of the electrolyte is unity (1 M), whereas in the SHE, the activity of the electrolyte is maintained at unity. Practical Application: The NHE is more practical for use in laboratory settings due to its ease of construction and lower cost.

Practical Implications

In practice, both the SHE and NHE refer to the same fundamental concept of a hydrogen electrode used as a reference, with the primary distinction lying in the electrolyte solution. While the SHE is defined under ideal conditions, the NHE is often a more practical and commonly used electrode in real-world research and experimentation.

Understanding the subtle differences between these two electrodes is crucial for accurate and reliable electrochemical measurements. Researchers must be aware of the conditions under which they are using the electrode to ensure precise and effective results.

Conclusion

The distinction between the Standard Hydrogen Electrode (SHE) and the Normal Hydrogen Electrode (NHE) lies primarily in the conditions under which they operate and the accuracy of the reference potential they provide. Both electrodes serve as valuable tools in electrochemical studies, but their differing characteristics must be considered to achieve accurate and meaningful results.