Reference electrode

A reference electrode, often referred to as a reference electrode or a less frequent comparison electrode is an electrode ( half-cell ) at a constant equilibrium potential which sets rapidly and reproducibly. It is used as a reference point for measuring the relative potential of other electrodes. The absolute potential of an individual electrode is not in principle be determined experimentally, but the potential difference between two electrodes. At electrode potentials should therefore always be stated to what reference electrode, it is related.

Potential measurements with reference electrodes

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

If there are electrode and the reference electrode in different ( separate for example, through a membrane ) electrolyte, the measurement result is affected by the adjusting itself to the phase boundary between the two electrolyte potential difference, called the diffusion potential. The numerical value of the diffusion potential is usually of the order of 5-50 mV and is therefore often consciously or unconsciously neglected. For precision measurements the diffusion potential can be reduced with a suitable experimental setup to values ​​< 1 mV. One possibility is the use of a salt bridge. That is, the different electrolytes are not directly, but via a with a concentrated electrolytes same ion mobility -filled U-tube. In this case there is instead a phase Genze between the electrolyte with a high diffusion potential two phase boundaries 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 it was measured, with the results indicated.

Types and selection of reference electrodes

The different hydrogen electrodes are among the most well-known reference electrodes. The standard potential of the standard hydrogen electrode (abbreviated NHE; are the standard potential of, here is c (H ) = 1 M) or the standard hydrogen electrode (abbreviated SHE, is the standard potential of, here's a ( H ) = 1 M), is the reference point for the electrochemical series and arbitrary than 0.00 V defined.

• Standard hydrogen electrode ( SHE): E0 = 0.000 V a (H ) = 1 M
• Normal hydrogen electrode ( NHE): E0 ≈ 0.000 V c (H ) = 1 M

In practice, mainly electrodes of the second kind are used, which are simpler in construction and their equilibrium potential also adjusts quickly and reproducibly. These are metal ion electrodes, in which the metal ions are in solution in equilibrium with a sparingly soluble salt of the metal ( the bottom of the body). In addition, the solution containing the anion of the sparingly soluble salt. The potential determining concentration of metal ions in solution is then determined by the concentration of the anions and the solubility product of the sparingly soluble salt. In this manner, the electrode potential will be dependent on the concentration of the anions in the solution. Examples of reference electrodes of the second kind are:

• Silver -silver chloride electrode ( Ag/AgCl/Cl- )
• Calomel or mercury - mercury (I ) chloride electrode ( Hg/Hg2Cl2/Cl- )
• Mercury - mercury sulfate electrode ( Hg/Hg2SO4/SO42- )
• Mercury mercury oxide electrode ( Hg/HgO/OH- )

In some cases is used as the reference electrode an electrode of the first kind. It is the electrode potential of the metal ions concentration of the metal ions in solution is dependent. At constant concentration of metal ions, a stable and reproducible potential electrode sets so that the electrode can be used as a reference electrode. Examples of reference electrodes of the first kind are:

• Copper / copper sulphate electrode ( Cu/CuSO4 )

The selection of the reference electrode, depends on the conditions. In acidic solution, usually the calomel or silver -silver chloride electrode may be used. Today, the silver-silver chloride electrode is often preferred to avoid the use of toxic mercury and mercury (I ) chloride ( calomel ). In ( strong) alkaline solution, however, these are of limited use, since hydroxide ions diffuse into the reference electrode and can distort the potential. Therefore, in alkaline solutions, the mercury - mercury oxide electrode is commonly used. In cases where must be measured free of chloride, is usually the mercury - mercury sulfate electrode or, more rarely, the reversible hydrogen electrode ( RHE abbreviation ) used.

For certain applications, for example in the study of corrosion processes, so-called pseudo - reference electrodes are often used. In most cases these are metal wires that are directly immersed in the electrolyte solution. Namely, on such an electrode and a constant potential, this is unknown and depends on the composition of the electrolyte solution. An advantage of such pseudo - reference electrode is that with a suitable choice of any additional traces of impurities ( for example, chloride ions) are introduced.