Standard hydrogen electrode

In the field of electrochemistry, scientists need a universal reference to measure electrical energy. They use a tool called the standard hydrogen electrode, often abbreviated as the SHE. This electrode is a redox electrode, meaning it handles reactions involving the transfer of electrons. It serves as the fundamental basis for the thermodynamic scale of oxidation-reduction potentials. While the absolute electrode potential of hydrogen is estimated at 25 °C, it is officially declared to be zero volts at any temperature. This zero-point allows scientists to compare the potentials of all other electrodes against a single, consistent standard.

The mechanism of the SHE relies on a specific redox half-reaction. This reaction involves the reduction of two hydrated protons into one molecule of gaseous hydrogen. The process follows the equation: 2H+(aq) + 2e- ⇌ H2(g). In this setup, a platinized platinum electrode is immersed in an acidic solution. Pure hydrogen gas is then bubbled over the surface of the platinum. For the electrode to function as a standard, the chemical activities of both the reduced and oxidized forms must be maintained at unity. This means the pressure of the hydrogen gas is kept at exactly 1 bar, which is 100 kilopascals. Simultaneously, the activity of the hydrogen ions in the solution must also be one.

To understand the math behind this, scientists use the Nernst equation. This equation describes how the electrode potential changes based on concentration and pressure. At equilibrium, the ratio of the reaction products to the reagents equals the equilibrium constant. For the SHE, the Nernst equation simplifies significantly because the activities are set to one. Under these specific standard conditions, the equation results in a potential of zero volts. However, the equation can also describe a straight line with a negative slope of −0.0591 volt per pH unit. This relationship helps define the stability regions of water in a Pourbaix diagram.

It is important to distinguish the SHE from other similar electrodes used in history. Early researchers utilized the normal hydrogen electrode, or NHE. The NHE was a practical device made by bubbling hydrogen gas through a 1 N strong acid solution at 1 atm of pressure. Eventually, scientists moved toward a more theoretical model to improve precision. The SHE replaced the NHE by assuming a theoretical interface where hydrogen ions have no interactions with other ions. This ideal state is not physically attainable in a real solution, but it provides a perfect mathematical standard. There is also the reversible hydrogen electrode, or RHE, which is a practical version where the potential depends on the pH of the solution.

The choice of platinum as the electrode material is not accidental. Platinum is an inert metal, which means it does not corrode during the reaction. It also acts as a catalyst, which helps speed up the reduction of protons. To improve performance, the surface is often "platinized." This means it is covered with a layer of fine platinum black powder. This powder increases the total surface area of the electrode. A larger surface area improves the reaction kinetics and allows for a higher maximum possible current. It also helps the surface adsorb hydrogen more effectively. Other metals, such as palladium, can sometimes be used for similar functions.

Despite its usefulness, the SHE is very sensitive to interference. Because the platinized platinum is so active, it can be easily "poisoned" by certain substances. Organic substances and atmospheric oxygen can block the catalytic sites on the metal surface. Certain inorganic ions can also interfere if they are reduced to a lower valency state. For example, ions like silver, mercury, copper, lead, cadmium, and thallium can deposit onto the platinum. Other substances, such as arsenic, sulfides, and various biological materials, can also inactivate the electrode. Keeping the electrode and the solution free from these contaminants is a major challenge in laboratory work.

The SHE is part of a much larger system of electrochemical study. It connects to concepts like thermodynamic activity and the study of gas dissolution via Henry's law. Even small changes in the type of hydrogen used can cause measurable differences. For instance, the standard redox potential of the deuterium couple is slightly different from the proton couple. This difference is approximately −0.0044 V. Scientists must carefully control every part of the setup, from the gas bubbler to the salt bridge, to ensure the connection to other electrodes remains stable and accurate.