Electrode potential

The electrode potential (symbol: E ) is defined by the electromotive force supplied by an electrode of an electrochemical cell . For the purpose of measurement, this electrode is placed in a test cell next to a reference electrode, which by definition is at zero potential. In general practice, this reference electrode is the standard hydrogen electrode . The electrode potential of the electrode to be measured is equal to its voltage measured without current against the reference electrode .

Furthermore, the electrode potential indicates which electrical voltage an electrode in an electrolyte can deliver or which voltage is required in order to maintain a certain state in a stable manner , for example in the case of electrolysis . It is thus perhaps the most important variable for describing the condition of an electrode and a central term in electrochemistry . Electrode potentials allow the calculation of the electrical voltage that batteries or accumulators can supply or that is required for electrolysis.

Basics

An electrical voltage is always measured between two points, for example between two electrodes. The voltage between two poles is defined as the electrostatic energy it takes to move a Coulomb charge from one pole to the other. This energy can be measured directly by moving charges in a vacuum , within a metal or between two metal poles. If, however, a charge, for example an electron, is brought from a metal electrode into an electrolyte solution, the energy required for this is determined not only by electrostatic, but also by chemical interactions of the electron with the metal or with the components of the solution. This is why you cannot measure a voltage between an electrode and the electrolyte; you always need two electrodes to measure the voltage.

The electrode potential E is now the voltage of the electrode, which is measured against a reference electrode . Reference electrodes are electrodes with a known potential, i.e. with a known electrochemical state. The possible voltages between any two electrodes can then be calculated using the electrode potentials: The voltage U is equal to the potential difference Δ E from the potentials E ₁ and E ₂ of electrodes 1 and 2:

{\ displaystyle U = \ Delta E = E_ {2} -E_ {1} \ mid E_ {1} = {\ text {anode}}, E_ {2} = {\ text {cathode}}}

.

To make the term potential clear, the term “electron pressure” is sometimes used. An electrode with a large negative charge in the metal has a negative potential and a large “electron pressure”. It strives to release these electrons, so it can have a reducing effect on the environment. Compounds that give off their electrons easily, i.e. are easily oxidized , can charge an electrode negatively, i.e. generate a negative potential. The potential can be used to indicate which processes can take place on an electrode. However, the potential must always be seen in its negativity in comparison to the corresponding electrode.

When two oxidation states of a chemical element or a compound are in equilibrium in a galvanic half-cell , the potential of the cell is fixed: In equilibrium, electrons are exchanged between the different electrically charged forms via the metal electrode . The position of equilibrium and thus the electrode potential depend on the concentration ratios and the temperature . This dependency is calculated using the Nernst equation .

determination

The electrode potential is 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 , it must be stated which reference electrode was used, unless the standard hydrogen electrode was used: This is the most important reference point , and electrode potentials usually refer to this electrode.

Electrochemical three-electrode measuring arrangement

A list of electrode potentials can be found under Electrochemical series . The potentials given there relate to activities of 1 mol / l, i.e. to approximately one-molar solutions.

In the solution in front of the reference electrode, an ohmic voltage drop occurs when a current flows. Therefore, for accurate potential measurements, measurements must either be completely currentless or at least as high-resistance as possible, or a three-electrode arrangement is used in which the potential measurement to the reference electrode takes place without current, even if a current flows to the working electrode . The terminal voltage of a galvanic cell , measured without current, is also called electromotive force .

Normal potential

If the electrode potential of a standard electrode is determined with the normal hydrogen electrode as reference, it is referred to as normal potential . The normal hydrogen electrode itself consequently has a normal potential of E ⁰ = 0 volts.

The sign for the normal potential always relates to the reduction process at an electrode. That is why one often speaks of the reduction potential . The greater (more positive) the electrode potential (or normal potential) of a half-cell, the stronger the oxidizing power of the oxidized form.

The normal potential is a parameter of a chemical element in the periodic table . The element with the highest normal potential difference is lithium with -3.04 volts, which explains its good suitability for storing electrical energy in lithium-ion batteries .

example

The half-cell has a value of +2.85 V. This means that if you switch this cell against a normal hydrogen electrode, an electrical voltage of 2.85 V is determined. When current flows, the electrons flow through the electrical conductor from the half-cell to the half-cell . The following reactions then take place at the electrodes: ${\ displaystyle 2 \ mathrm {F} ^ {-} | \ mathrm {F} _ {2}}$ ${\ displaystyle E ^ {0}}$ ${\ displaystyle \ mathrm {H} | \ mathrm {H} ^ {+}}$ ${\ displaystyle 2 \ mathrm {F} ^ {-} | \ mathrm {F} _ {2}}$

{\ displaystyle \ mathrm {H} _ {2} \ to 2 \ mathrm {H} ^ {+} + 2e ^ {-}}

( Oxidation )

and

{\ displaystyle \ mathrm {F} _ {2} + 2e ^ {-} \ to 2 \ mathrm {F} ^ {-}}

( Reduction )

Fluorine is the strongest elementary oxidizing agent , so chemical processes with elements cannot achieve higher electrode potentials than the normal hydrogen electrode.

Absolute electrode potential

Electrode potentials can only be measured as voltage, for which a second electrode is required. Therefore, the potential of an individual electrode cannot be measured directly, but must always be specified in relation to a reference. A theoretical reference point for a potential specification is - for an electrode as well as for charges in electrostatics - an electron at an infinite distance. Electrode potentials that are specified relative to such a system without a metal-electrolyte phase boundary are called absolute electrode potentials. Although they can not be measured directly , they can be calculated using measured values. An absolute electrode potential of 4.44 V is given for the standard hydrogen electrode , but also a value of 4.7 V according to other measurements. The uncertainty in the specification of the absolute electrode potential is therefore much greater than the typical measurement accuracy for a potential measurement against a reference electrode. The conversion of a potential measured against a reference electrode into an absolute electrode potential is therefore not useful.

literature

Entry for absolute electrode potential . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.A00022 Version: 2.3.1.
Entry for standard hydrogen electrode . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.S05917 Version: 2.3.1.
Sergio Trasatti: The “absolute” electrode potential: an explanatory note. In: Pure and Applied chemistry. Vol. 58, No. 7, 1986, ISSN 0033-4545 , pp. 955-966, online (PDF; 381 kB) .

Individual evidence

↑ BMBF batterieforum-deutschland.de: Lithium-ion batteries , accessed February 22, 2020

[1] BMBF batterieforum-deutschland.de: Lithium-ion batteries , accessed February 22, 2020