What is the role of the standard hydrogen electrode in defining standard reduction potentials?

• A. It serves as a fixed potential for all redox couples.
• B. It measures the total cell potential of a galvanic cell.
• C. It serves as the reference half-cell with H+/H2 at unit activity.
• D. It is optional and not used in practice.

Answer: (C)

The key idea here is that the standard hydrogen electrode sets the zero point for the reduction-potential scale. Under standard conditions—hydrogen ions at unit activity (1 M), hydrogen gas at 1 bar, and a defined temperature—the H+/H2 couple is assigned an E° of zero volts. This makes the electrode a stable, reproducible reference half-cell.

When you pair any other redox couple with the standard hydrogen electrode under those same standard conditions, the measured cell potential equals the standard reduction potential of that other couple. In other words, all standard reduction potentials are defined relative to the SHE’s zero point. The role isn’t to fix a universal potential for every couple, but to provide a consistent reference against which every E° is measured. In practice, other reference electrodes are used in experiments, but the potentials are always defined with respect to the SHE.