What Is a Standard Hydrogen Electrode?

The standard hydrogen electrode is the standard measurement of electrode potential for the thermodynamic scale of redox potentials. The standard hydrogen electrode is often abbreviated as SHE or may be known as a normal hydrogen electrode (NHE). Technically, a SHE and NHE are different. The NHE measures the potential of a platinum electrode in a 1 N acid solution, while the SHE measures the potential of a platinum electrode in an ideal solution (current standard of zero potential at all temperatures).

The standard is determined by the potential of a platinum electrode in the redox half-reaction
2 H⁺(aq) + 2 e^- → H₂(g) at 25 °C.

Construction

A standard hydrogen electrode has five components:

1. Platinized platinum electrode
2. Acid solution that has a hydrogen ion (H⁺) activity of 1 mol/dm³
3. Hydrogen gas bubbles
4. Hydroseal to prevent interference from oxygen
5. Reservoir to attach the second half-element of the galvanic cell. Either a salt bridge or a narrow tube to prevent mixing may be used.

The redox reaction takes place at the platinized platinum electrode. When the electrode is dipped into the acidic solution, hydrogen gas bubbles through it. The concentration of the reduced and oxidized form is maintained, so the pressure of hydrogen gas is 1 bar or 100 kPa. The hydrogen ion activity equals the formal concentration multiplied by the activity coefficient.

Why Use Platinum?

Platinum is used for the SHE because it is corrosion-resistant, catalyzes the proton reduction reaction, has a high intrinsic exchange current density, and yields reproducible results. The platinum electrode is platinized or coated with platinum black because this increases the electrode surface area and increases reaction kinetics because it adsorbs hydrogen well.

Sources

• Ives, D. J. G.; Janz, G. J. (1961). Reference Electrodes: Theory and Practice. Academic Press.
• Ramette, R. W. (October 1987). "Outmoded terminology: The normal hydrogen electrode". Journal of Chemical Education. 64 (10): 885.
• Sawyer, D. T.; Sobkowiak, A.; Roberts, J. L., Jr. (1995). Electrochemistry for Chemists (2nd ed.). John Wiley and Sons.