Reference electrode

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A reference electrode , often referred to as a reference electrode or, more rarely, a comparison electrode , is an electrode (half-cell) with a constant equilibrium potential that is quickly and reproducibly established. It is used as a reference point for measuring the relative potentials of other electrodes. In principle, the absolute potential of an individual electrode cannot be determined experimentally, only the potential difference between two electrodes. In the case of electrode potentials, it should therefore always be stated which reference electrode it is related to.

Potential measurements with reference electrodes

In order to be able to measure the potential difference between an electrode and the reference electrode, they must be connected via an ion conductor ( electrolyte ) and an electrical conductor to a suitable measuring device, for example a high-resistance voltmeter or a potentiometer circuit ( Wheatstone bridge circuit ).

If the electrode and reference electrode are in different electrolytes (for example, separated by a membrane), the measurement result is influenced by the potential difference between the two electrolytes, known as the diffusion potential, at the phase boundary . The numerical value of the diffusion potential is usually in the range of 5–50 mV and is therefore often consciously or unconsciously neglected. In the case of precision measurements, the diffusion potential can be reduced to values ​​<1 mV with a suitable test setup. One option is to use a salt bridge . This means that the various electrolytes are not connected directly, but rather via a U-tube filled with a concentrated electrolyte of the same ion mobility. In this case, instead of a phase boundary between the electrolytes with a high diffusion potential, there are two phase boundaries with a low diffusion potential.

Electrode potentials are temperature-dependent (see Nernst equation ). Therefore, potential measurements should be carried out at constant temperature and the temperature at which the measurement was carried out should be given with the result.

Types and selection of reference electrodes

The various hydrogen electrodes are among the best-known reference electrodes. The standard potential of the normal hydrogen electrode (abbreviation NHE; indicates the standard potential, here c (H + ) = 1 M) or of the standard hydrogen electrode (abbreviation SHE; indicates the standard potential , here a (H + ) = 1 M ), is the reference point for the electrochemical series and is arbitrarily defined as 0.00 V.

In practice, mainly electrodes of the second type are used, which are more simply constructed and whose equilibrium potential is also set quickly and reproducibly. These are metal ion electrodes in which the metal ions in the solution are in equilibrium with a poorly soluble salt of the metal (the soil body). The solution also contains the anion of the sparingly soluble salt. The potential-determining concentration of the metal ions in solution is then determined by the concentration of the anions and the solubility product of the sparingly soluble salt. In this way the electrode potential then becomes dependent on the concentration of the anions in the solution. Examples of reference electrodes of the second type are:

In some cases, an electrode of the first type is also used as the reference electrode . These are metal ion electrodes whose potential depends on the concentration of metal ions in solution. With a constant metal ion concentration, a stable and reproducible electrode potential is established so that the electrode can be used as a reference electrode. Examples of reference electrodes of the first type are:

The selection of the reference electrode depends on the conditions. The calomel or silver-silver chloride electrode is usually used in acidic solutions. Today, the silver-silver chloride electrode is often preferred in order to avoid handling the toxic mercury and mercury (I) chloride (calomel). In (strongly) alkaline solution , however, these can only be used to a limited extent, since hydroxide ions diffuse into the reference electrode and can falsify the potential. Therefore, the mercury-mercury oxide electrode is usually used in alkaline solutions. In cases where chloride-free measurements are required, the mercury-mercury sulfate electrode or, less often, the reversible hydrogen electrode (abbreviation RHE) is used. Furthermore, so-called double junction reference electrodes can be used, which have two electrolyte chambers. The inner chamber sets the potential (Ag / AgCl / KCl reference), the second chamber creates the bridge between sample and reference. For samples that z. B. react with chloride, a chloride-free bridge electrolyte can be selected.

So-called pseudo-reference electrodes are often used for certain applications, for example when examining corrosion processes. These are mostly metal wires that are dipped directly into the electrolyte solution. A constant potential is established at such electrodes, but this is unknown and depends on the composition of the electrolyte solution. One advantage of such pseudo-reference electrodes is that, with a suitable selection, no additional traces of impurities (for example chloride ions) are introduced.

See also

literature

  • David JG Ives, George J. Janz (Eds.): Reference Electrodes. Theory and Practice. Academic Press, New York et al. 1961. (1983, ISBN 0-12-376856-X ) (English-language standard work)

Web links

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