NHe

Normal Hydrogen Electrode (NHE) is a reference electrode used in electrochemistry to measure electrode potentials. It is based on the hydrogen redox couple H+ + e− ⇌ 1/2 H2. In a typical NHE setup, hydrogen gas is bubbled at 1 atmosphere into contact with an aqueous solution in which the activity of hydrogen ions is unity (approximately 1 M H+ under standard conditions), with a platinum electrode providing the interface to the solution. The cell is usually gas-tight to prevent hydrogen loss.

Under standard conditions at 25°C, with H2 at 1 atm and H+ activity equal to 1, the potential of this hydrogen couple defines the zero of the potential scale, so the NHE is often treated as the reference with E = 0 V. In practice, the measured potential of the H+/H2 couple varies with temperature and the pH of the solution. The hydrogen reference is thus sensitive to experimental conditions, and comparisons across different pH values are commonly made using the reversible hydrogen electrode (RHE), which relates to the NHE by E_RHE ≈ E_NHE + 0.0591 pH at 25°C.

Applications and limitations: NHE is widely used as a reference in studies of hydrogen evolution and oxidation reactions, electrochemical catalysis, and other redox measurements. However, it requires a hydrogen gas supply and a gas-tight setup, and its potential can drift with temperature, gas purity, and electrolyte composition. For routine or portable measurements, other references such as Ag/AgCl or SCE are often preferred.

See also: standard hydrogen electrode, reversible hydrogen electrode, pH, electrochemistry.