Electrodes of reference in electrochemistry

Need for two electrodes…

One of the important parameters in electrochemistry is the potential difference between two elements of a system. When one of these elements is a liquid, one cannot be satisfied “to connect” the liquid with the voltmeter. It is necessarily necessary to introduce into the solution one second electrode or electrode of reference . This second electrode will be used as probe of potential.
Mais these two electrodes have a potential difference (DDP) with the electrolytic solution: it is the interfacial DDP. This interfacial DDP depends on the electrolytic solution and the nature of the electrode: there is thus no reason so that it is identical for our two electrodes. Only the difference of the internal potentials of the two electrodes is thus accessible. The expression of a tension of electrode then has direction only compared to one reference. In experiments, a priori almost any “fast” redox cell can be used to produce an electrode of reference.

Some traditional electrodes of reference

The electrode of reference par excellence is the standard electrode with hydrogen or ESH. When one gives a value of tension of electrode compared to this electrode of reference, one will note our result in the form: E =… V_/ESH. However, the ESH is only one ideal reference, i.e. impossible to implement in practice. One thus carries out generally a normal electrode with hydrogen or ENH for the calibration of the other electrodes of reference while generally regarding as negligible the existing differences between the ENH and the ESH.

Other electrodes of references very much used in practice are for example the saturated calomel electrode or ECS and the electrode with mercureux sulfate or ESS.

Various types of electrodes

One can classify the electrodes of reference according to balances likely to be realized on their surface:

One speaks about electrode redox when the electrode does not intervene in the Réaction of oxydoreduction being held on its surface.

Example: reduction of the Ion ferric Fe³⁺ in ferrous ion Fe²⁺ on the surface of an unattackable electrode

One speaks about metal electrode of first species when the metal of the electrode is in balance with its ion. Example: an electrode of Copper plunged in a solution of ions Cu²⁺

One speaks about metal electrode of second species to indicate a metal balances some with one of its not very soluble compounds. Example: the calomel electrode saturated or ECS.

As well as the electrode copper-sulfate of copper or Cu - CuSO₄ used in Cathodic protection for the measurements taken on buried structures.

One speaks about electrode to gas so at least one of the électroactives species is in gas form.

Example: ESH

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