Standard Hydrogen ElectrodeEdit
The Standard Hydrogen Electrode (SHE) is a foundational reference point in electrochemistry. It provides a universal baseline for comparing the thermodynamic potentials of redox couples, allowing scientists to tabulate standard electrode potentials (E°) in a consistent framework. The electrode is defined by the half-reaction 2H+ + 2e− ⇌ H2, carried out under carefully controlled conditions: hydrogen gas at a pressure of 1 atmosphere above the solution, hydrogen ions with unit activity (pH 0, conventionally 1 M H+ in the standard state), and a temperature of 25°C (298 K). Under these standard conditions, the electrode potential is set to 0 volts, and the potentials of all other half-reactions are measured relative to this anchor. This reference framework underpins a great deal of practical work in batteries, corrosion science, electroplating, and sensor development, where precise control of redox thermodynamics matters.
In practice, realizing a perfect SHE in the laboratory is challenging. The definition assumes a stable H2 atmosphere, a clean and inert electrode surface (typically a platinum electrode), rigorous temperature control, and a solution where the activity of H+ is precisely unity. Real-world implementations often approximate the SHE or adopt closely related references, and researchers frequently calibrate other electrodes against a known standard. The thermodynamic potential of the H+/H2 couple is then used to derive standard potentials for a wide range of redox couples through the Nernst equation. The SHE thus functions as the theoretical bedrock of the standard electrochemical potential scale, even as laboratories employ alternative references for routine work. Key related concepts include the Nernst equation, the general framework of electrochemistry, and the role of the standard electrode potential in predicting reaction spontaneousness.
Principle
The core idea behind the Standard Hydrogen Electrode is the equilibrium between dissolved hydrogen, protons, and electrons at a defined interface. The half-reaction 2H+ + 2e− ⇌ H2 describes how electrons are exchanged with hydrogen ions and molecular hydrogen at the electrode surface. The electrode potential is governed by the Nernst equation, which for this couple can be written in terms of the activities (or effective concentrations) of H+ and H2. With the hydrogen gas activity set to 1 (1 atm H2) and the hydrogen ion activity set to 1 (pH 0) at 298 K, the expression yields E = E°, where E° is defined as zero volts by convention. If either the hydrogen pressure, temperature, or proton activity deviates from the standard values, the observed potential shifts according to the Nernst relationship.
The electrode material is typically platinum, chosen for its inertness and catalytic ability to facilitate rapid exchange of electrons with hydrogen gas at the surface. The exact surface state and cleanliness of the Pt electrode influence practical measurements, but under standard conditions the intrinsic thermodynamics of the H+/H2 couple determine the reference potential. In the broader context, the SHE defines the zero-point for standard reduction potentials, serving as the reference against which the potentials of other redox couples are tabulated. For background on how these potentials relate to reaction feasibility, see Standard electrode potential and Redox potential.
Construction and use
A conventional SHE apparatus consists of a Pt electrode intersecting a closed cell, with hydrogen gas supplied at 1 atm to equilibrate at the gas–liquid interface. The electrolyte contains hydrogen ions at unit activity (pH 0), typically achieved with a strong acid solution at the standard concentration. The Pt electrode is connected to a measurement circuit, paired with a counter electrode and, in many setups, another reference electrode for operational convenience. The design ensures that the interfacial reaction 2H+ + 2e− ⇌ H2 is the sole determinant of the electrode potential under the defined conditions.
In practice, two practical considerations often arise. First, maintaining a stable, pure hydrogen atmosphere and a contamination-free Pt surface is essential to prevent drift in the measured potential. Second, laboratory measurements seldom realize the ideal SHE exactly; instead, researchers use calibrated reference electrodes that approximate the SHE under defined conditions and then account for deviations due to temperature, pressure, or solution composition. As a result, many experiments report potentials relative to a nearby reference electrode (for example, a Saturated calomel electrode or an Ag/AgCl electrode) and then convert to the standard hydrogen scale when comparing with published E° data. The standard hydrogen reference remains the conceptual anchor for the thermodynamics of redox processes and the basis for the tabulated standard potentials found in reference books and databases. See also the discussions around the Nernst equation for how these values shift with nonstandard conditions.
Applications of the SHE span chemistry, materials science, and related fields. It is used to calibrate other reference electrodes, determine the thermodynamic favorability of redox transformations, and underpin the construction of electrochemical cells for energy storage, corrosion studies, and sensor technologies. In nonaqueous or specialized media, researchers may employ SHE-like references or alternative standards more suitable to the solvent or system, but the SHE retains its central role in defining the standard potential scale and guiding interpretation of electrochemical data.