StandardWasserstoffElektrode

The StandardWasserstoffelektrode, commonly referred to as the standard hydrogen electrode (SHE), is a reference electrode used in electrochemistry. By convention its standard reduction potential is defined as zero volts under standard conditions, serving as the baseline for measuring standard potentials of other redox couples.

Construction and operational principle: The electrode consists of a platinum (Pt) electrode in contact with hydrogen gas at a partial pressure of 1 atmosphere, immersed in an aqueous solution in which the activity of the hydrogen ion is unit (1 M H+). The half-reaction at the interface is 2 H+ + 2 e- ⇌ H2(g). An equilibrium between the gaseous hydrogen, the Pt surface, and the dissolved ions sets the electrode potential.

Potential definition and relation: Under standard conditions (temperature near 298 K, activities a(H+) = 1, P(H2) = 1 atm), the standard potential E° of the SHE is defined as 0 V. The Nernst equation relates the electrode potential to the actual conditions: E = E° − (RT/2F) ln [P(H2) / a(H+)^2]. When P(H2) = 1 atm and a(H+) = 1, E = 0. Deviations in temperature, hydrogen pressure, or ion activity shift the observed potential away from zero.

Use and limitations: The SHE provides a universal reference for reporting standard reduction potentials (E°) of other couples. In practice, realizing and maintaining a true SHE requires careful control of hydrogen purity, pressure, temperature, and surface condition of the Pt electrode. Because of practical challenges, laboratories often use alternative reference electrodes (for example, Ag/AgCl or saturated calomel electrodes) for routine measurements, while SHE remains a fundamental concept in electrochemistry.