Electrode potential

The electrode potential (symbol: E) is defined by the electromotive force, which supplies an electrode of an electrochemical cell. For the purpose of dimensioning said electrode is placed in a test cell, in addition to a reference electrode, which is defined to be at zero potential. In common practice, this reference electrode is the standard hydrogen electrode. The electrode potential of the electrode to be measured is equal to their normally measured against the reference electrode voltage.

Further, it is capable of supplying an electrode in an electrolyte, which electric power or voltage which is required to - to obtain a certain stable state - for example, in an electrolysis. It is thus perhaps the most important parameter for describing the state of an electrode and a central concept of electrochemistry. Electrode potentials allow for the computation of the electric power that can supply or a battery or accumulator which is required for electrolysis.

Basics

The measurement of an electrical voltage is always between two points, for example between two electrodes. The voltage between the two poles is defined as the electrostatic energy is required to move a charge from a Coulomb pole to the other. This energy can be measured directly when moving charges in a vacuum, within a metal or between two metal poles. If, however, the energy required, a charge, for example an electron, bringing a metal electrode in an electrolyte solution, it is not only determined by electrostatic, but also by chemical interactions of the electrons with the metal or with the solution constituents. So we can not measure voltage between an electrode and the electrolyte, one always needs two electrodes for voltage measurement.

The electrode potential E is now the voltage of the electrode is measured against a reference electrode. Reference electrodes are electrodes of known potential, which means of known electro-chemical state. The potential between any two electrodes, voltages can be calculated on the basis of the electrode potentials: the voltage U is equal to the potential difference AE from the potentials of the electrodes E1 and E2, 1 and 2:

To make the concept clear potential, is sometimes the term " electron pressure " is used. An electrode having a large negative charge in the metal has a negative potential and a large " electron push ". She has the aspiration for these electrons, so they can have a reducing effect on the environment. Compounds which release their electrons easily, so be easily oxidized, can charge an electrode negative, ie generate a negative potential. On the basis of the potential can be specified so that processes can occur at an electrode. The potential must here, however, in its negativity always compared to the corresponding electrode can be seen.

When two oxidation states of a chemical element or a compound are in equilibrium in a galvanic half-cell each other, the potential of the cell is set: In the equilibrium is exchanged over the metal electrode electrically charged electrons between the different forms. The position of the equilibrium and hence the electrode potential will depend on the concentration conditions, and the temperature. This dependence is calculated by the Nernst equation.

Determination

The electrode potential being determined by a simple voltage measurement. The value is given in volts ( V). Since the potential of an electrode is always measured against a reference electrode, must be stated that the reference electrode was used, unless it was the normal hydrogen electrode is used: This is the most important point of reference, and in general, electrode potentials refer to this electrode.

A list of electrode potentials can be found at Electrochemical Series. The potentials given there refer to activities of 1 mol / l, ie about one molar solutions.

In the solution before the reference electrode is an ohmic voltage drop occurs when a current flows. Therefore need for accurate measurements of potential to be measured either completely currentless or at least high resistance value, or there is used a three-electrode setup, wherein the reference electrode for measurement of potential is carried out normally, and when a current flowing to the working electrode. The normally measured terminal voltage of a galvanic cell is also called electromotive force.

Normal potential

If the electrode potential of a standard electrode with the normal hydrogen electrode is determined as a reference, one speaks of the normal potential. Consequently, the normal hydrogen electrode itself has a standard potential of E0 = 0 volts.

The sign for the normal potential always refers to the reduction process at an electrode. Therefore, one often speaks of the reduction potential. The larger (positive ) is the electrode potential (or standard potential ) of a half-cell, the greater the oxidizing power of the oxidized form.

Example

The half- cell has a value of 2.85 V. This means you turn this cell to a normal hydrogen electrode, we can see an electric voltage of 2.85 V fixed. When current flows, the electrons flow through the electrical conductor of the half-cell to half-cell. Then the following reactions occur at the electrodes from:

And

Fluorine is the most elementary oxidant, it can be by chemical processes with elements thus reach no larger electrode potentials against the standard hydrogen electrode.

Absolute electrode potential

Electrode potentials can be measured as a voltage, for which one needs a second electrode. Therefore, the potential of an individual electrode is not directly measured, but must be specified in relation to a reference. A theoretical reference for a potential indication is - for an electrode for such charges in electrostatics - on the other hand, an electron at an infinite distance. Electrode potentials are given relative to a system without such a metal -electrolyte phase boundary called absolute electrode potentials. They can not be measured directly but can be calculated from measured values. For the normal hydrogen electrode is an absolute electrode potential of 4.44 V is given, according to other measurements but also a value of 4.7 V. The uncertainty in the specification of the absolute electrode potential is so much greater than the typical measurement accuracy at a potential measurement against a reference electrode. The conversion of a measured against a reference electrode potential in an absolute electrode potential is therefore not useful.