A reversible hydrogen electrode
(RHE) is a 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...
, more specific a subtype of the standard hydrogen electrode
The standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
s for electrochemical processes and differs from the standard hydrogen electrode by the fact that the measured potential
*In linguistics, the potential mood*The mathematical study of potentials is known as potential theory; it is the study of harmonic functions on manifolds...
does not change with the pH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
so that they can be directly used in the electrolyte.
The name refers to the fact that the electrode is in the actual electrolyte solution and not separated by a salt bridge. The hydrogen ion concentration is therefore not 1, but corresponds to that of the electrolyte solution; in this way we can achieve a stable potential with a changing pH value.
The potential of the RHE correlates to the pH value:
In general, for hydrogen electrodes in which the reaction:
expires, the following dependence of the equilibrium
Equilibrium is the condition of a system in which competing influences are balanced. The word may refer to:-Biology:* Equilibrioception, the sense of a balance present in human beings and other animals...
, hydrogen pressure
and the activity of a
of the hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
is the standard reduction potential (this is by definition equal to zero), R is the universal gas constant, T the absolute temperature and F is the Faraday constant
Surges occur in the electrolysis of water
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through the water.-Principle:...
which means that the required cell voltage due to kinetic inhibition higher is than the equilibrium potential. The voltage increases with increasing current density at the electrodes. The measurement of equilibrium potentials is therefore possible without power.
The reversible hydrogen electrode is a fairly practical and reproducible electrode "standard." The term refers to a hydrogen electrode immersed in the electrolyte solution actually used.
The benefit of that electrode is that no 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...
- no contamination of the electrolyte by Cl- or SO42-
- no diffusion potentials at the electrolyte bridge (liquid junction potential
Liquid junction potential occurs when two solutions of different concentrations are in contact with each other. The more concentrated solution will have a tendency to diffuse into the comparatively less concentrated one. The rate of diffusion of each ion will be roughly proportional to its speed in...
). This is important at temperature different to 25°C.
- long time measurements possible (no electrolyte bridge means no maintenance of the bridge)