The
standard hydrogen electrode (abbreviated
SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its
absolute electrode potentialAbsolute electrode potential, in electrochemistry, according to an IUPAC definition, is the electrode potential of a metal measured with respect to a universal reference system .-Definition:...
is estimated to be 4.44 ± 0.02 V at 25 °C, but to form a basis for comparison with all other electrode reactions, hydrogen's
standard electrode potentialIn electrochemistry, the standard electrode potential, abbreviated E° or E , is the measure of individual potential of a reversible electrode at standard state, which is with solutes at an effective concentration of 1 mol dm−3, and gases at a pressure of 1 atm...
(
E0) is declared to be zero at all temperatures. Potentials of any other electrodes are compared with that of the standard hydrogen electrode at the same temperature.
Hydrogen electrode is based on the redox
half cellA half-cell is a structure that contains a conductive electrode and a surrounding conductive electrolyte separated by a naturally occurring Helmholtz double layer. Chemical reactions within this layer momentarily pump electric charges between the electrode and the electrolyte, resulting in a...
:
- 2H+(aq) + 2e- → H2(g)
This redox reaction occurs at platinized
platinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
electrode.
The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it. The concentration of both the reduced form and oxidised form is maintained at unity. That implies that the pressure of hydrogen gas is 1 bar and the activity of hydrogen ions in the solution is 1 molar. The activity of hydrogen ions is their effective concentration, which is equal to the formal concentration times the activity coefficient. Activity coefficients are close to 1.00 for very dilute water solutions, but are usually lower for more concentrated solutions.
The
Nernst equationIn electrochemistry, the Nernst equation is an equation that can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell. It can also be used to determine the total voltage for a full electrochemical cell...
should be written as:
or
where:
- aH+ is the activity
In chemical thermodynamics, activity is a measure of the “effective concentration” of a species in a mixture, meaning that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.By convention, activity...
of the hydrogen ions, aH+=fH+ CH+ /C0
- pH2 is the partial pressure of the hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas, in pascalThe pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
s, Pa
- R is the universal gas constant
- T is the temperature, in kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s
- F is the Faraday constant (the charge per a mole of electrons), equal to 9.6485309*104 C mol−1
- p0 is the standard pressure 105 in Pa
Relationship between the Normal Hydrogen Electrode (NHE) and the Standard Hydrogen Electrode (SHE)
During the early development of electrochemistry, researchers used the normal hydrogen electrode as their standard for zero potential. This was convenient because is could
actually be constructed by having "[immersing] a platinum electrode into a
solution of 1N strong acid and [bubbling] hydrogen gas through the solution at about 1 atm pressure". However, this electrode/solution interface was not entirely reproducible, so the standard for zero potential was later changed. What replaced it was a theoretical electrode/solution interface, where the concentration of H+ was 1
m, but the H+ ions were assumed to have no interaction with other ion (a condition which is not physically attainable at those concentrations). To differentiate this new standard from the previous one it was given the name 'Standard Hydrogen Electrode'.
In summary,
NHE: potential of a platinum electrode in 1N acid solution (historical standard, no longer in use)
SHE: potential of a platinum electrode in a theoretical solution (the current
standard for zero potential)
Choice of platinum
The choice of platinum for the hydrogen electrode is due to several factors:
- inertness of platinum (it does not corrode).
- the capability of platinum to catalyze the reaction of proton reduction
- a high intrinsic exchange current density
In electrochemistry, exchange current density is a parameter used in the Tafel equation, Butler-Volmer equation and other expressions. The Tafel equation describes the dependence of current for an electrolytic process to overpotential....
for proton reduction on platinum
- excellent reproducibility of the potential (bias of less than 10 μV when two well-made hydrogen electrodes are compared with one another)
The surface of platinum is platinized (i.e., covered with
platinum blackPlatinum black is a fine powder of platinum with good catalytic properties. The name of platinum black is due to its black color....
) because of:
- necessity to employ electrode with large true surface area. The greater the electrode true area, the faster electrode kinetics
- necessity to use electrode material which can adsorb hydrogen at its interface. Platinization improves electrode kinetics
Nevertheless, other metals can be used for building electrodes with a similar function, for example,
palladium-hydrogen electrodeThe palladium-hydrogen electrode is one of the common reference electrodes used in electrochemical study. Most of its characteristics are similar to the standard hydrogen electrode...
.
Interference
Because of the high adsorption activity of the platinized platinum electrode, it's very important to protect electrode surface and solution from the presence of organic substances as well as from atmospheric oxygen. Inorganic ions that can reduce to a lower valency state at the electrode also have to be avoided (e.g., Fe
3+, CrO
42-). A number of organic substances are also reduced by hydrogen at a platinum surface, and these also have to be avoided.
Cations that can reduce and deposit on the platinum can be source of interference: silver, mercury, copper, lead, cadmium and thallium.
Substances that can inactivate ("poison") the catalytic sites include arsenic, sulfides and other sulfur compounds, colloidal substances, alkaloids, and material found in living systems.
Construction
The scheme of the standard hydrogen electrode:
- platinized
Platinum black is a fine powder of platinum with good catalytic properties. The name of platinum black is due to its black color....
platinum electrode
- hydrogen blow
- solution of the acid with activity of H+ = 1 mol dm−3
- hydroseal for prevention of the oxygen interference
- reservoir through which the second half-element of the galvanic cell should be attached. The connection can be direct, through a narrow tube to reduce mixing, or through a salt bridge
A salt bridge, in chemistry, is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell , a type of electrochemical cell...
, depending on the other electrode and solution. This creates an ionically conductive path to the working electrode of interest.
See also
- Table of standard electrode potentials
- Reference electrode
A reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant concentrations of each participants of the redox reaction.There are many ways reference...
- Dynamic hydrogen electrode
A dynamic hydrogen electrode is a reference electrode, more specific a subtype of the standard hydrogen electrodes for electrochemical processes by simulating a reversible hydrogen electrode with an approximately 20 to 40 mV more negative potential....
- Reversible hydrogen electrode
A reversible hydrogen electrode is a reference electrode, more specific a subtype of the standard hydrogen electrodes for electrochemical processes and differs from the standard hydrogen electrode by the fact that the measured potential does not change with the pH so that they can be directly used...
- Palladium-Hydrogen electrode
The palladium-hydrogen electrode is one of the common reference electrodes used in electrochemical study. Most of its characteristics are similar to the standard hydrogen electrode...
- Quinhydrone electrode
The quinhydrone electrode is a type of redox electrode which can be used to measure the hydrogen ion concentration of a solution in a chemical experiment. It provides an alternative to the commonly used glass electrode in a pH meter....