Electrochemistry studies the relationship between electrical energy and chemical change. Measuring electrical potential requires a fixed reference point. The Standard Hydrogen Electrode (SHE) provides this necessary zero reference point for all electrochemical measurements. The SHE potential is defined as exactly 0.00 volts (V), a convention essential for creating a consistent, comparative scale for all other electrode potentials.
The Standardized Definition of Zero Volts
The specific value of 0.00 V for the SHE is not a measured quantity but a convention established by the International Union of Pure and Applied Chemistry (IUPAC). This convention creates a unified scale to measure the tendency of chemical species to gain or lose electrons. The “Standard” refers to the precise physical and chemical conditions under which this zero potential is assigned.
For the potential to be exactly 0.00 V, the system must meet strict thermodynamic conditions. The temperature must be 298 Kelvin (25 degrees Celsius). Hydrogen ion (\(\text{H}^+\)) concentration in the aqueous solution must be precisely 1 molar (M). The partial pressure of the hydrogen gas (\(\text{H}_2\)) interacting with the electrode must be exactly 1 bar (100 kilopascals).
These strict conditions ensure that the electrode potential is perfectly reproducible across different laboratories worldwide. If any of these parameters deviates from its standard value, the resulting potential of the hydrogen electrode will shift away from zero.
Essential Components and Reaction Mechanism
The SHE construction involves a specific assembly designed to facilitate a reversible chemical reaction. The core is a platinum electrode, often coated with finely powdered platinum black. This platinum surface is not a reactant, but functions as an inert, conductive platform for electron transfer. The platinum black coating significantly increases surface area, catalyzing the reaction and ensuring rapid equilibrium.
The platinum electrode is immersed in an aqueous solution containing hydrogen ions at the standard concentration of 1 M. Pure hydrogen gas is continuously bubbled over the platinum surface at the defined pressure of 1 bar. This setup creates a dynamic equilibrium between the gaseous hydrogen and the hydrogen ions dissolved in the solution.
The chemical process is the reversible half-reaction: \(\text{2H}^+ (\text{aq}) + \text{2e}^- \rightleftharpoons \text{H}_2 (\text{g})\). Hydrogen ions can gain electrons (reduction) to form hydrogen gas, or hydrogen gas can lose electrons (oxidation) to form hydrogen ions. The platinum acts as the electrical connection, allowing electrons to be introduced or removed to maintain equilibrium.
Using the SHE as a Reference Baseline
The utility of the SHE lies in its function as the universal reference for measuring the potential of any other half-cell. Since a potential difference must always be measured between two half-cells, the unknown half-cell is connected to the SHE to form a complete galvanic cell and determine its standard reduction potential (\(E^\circ\)).
The resulting potential difference, or cell voltage, is then measured using a voltmeter. Since the SHE’s potential is defined as zero, the measured voltage of the complete cell directly represents the standard potential of the unknown half-cell. For instance, if the SHE is connected to a copper half-cell under standard conditions and the voltmeter reads +0.34 V, the standard reduction potential for the copper half-cell is \(+0.34\) V.
This process allows electrochemists to create the electrochemical series, a comprehensive list ranking the relative oxidizing and reducing strengths of various chemical species. Although the physical SHE apparatus is cumbersome for routine laboratory use, its conceptual definition underpins all modern electrochemical measurements. More practical reference electrodes, such as the silver/silver chloride or saturated calomel electrodes, are routinely used, but their potentials are calibrated relative to the SHE’s theoretical zero point.