Calomel electrode

The calomel electrode (GKE, Saturated Calomel Electrode , SCE) is an electrode of the second kind .

construction

Schematic structure of a calomel electrode with a salt bridge

A calomel electrode consists of mercury which is coated with poorly soluble calomel (Hg ₂ Cl ₂ ) and immersed in a saturated potassium chloride solution. Contact can be established with the analysis solution via a diaphragm ( semipermeable membrane) or a liquid film in the ground-glass stopper.

The cell notation of the saturated calomel electrode is:

${\displaystyle \mathrm {Cl} ^{-}\,{\big (}4M{\big )}{\big |}\mathrm {Hg_{2}Cl_{2}} \,{\big (}s{\big )}{\big |}\mathrm {Hg} \,{\big (}l{\big )}{\big |}\mathrm {Pt} }$

Working principle

The electrode reaction that determines the potential is the equilibrium between metal in the electrode and metal ions in the KCl solution:

${\displaystyle \mathrm {Hg_{2}^{2+}+2\ e^{-}\leftrightharpoons 2\ Hg} }$

The potential of the electrode, E, is described by the Nernst equation , where E ° is the standard potential of the electrode reaction, R the general gas constant, T the thermodynamic temperature, and F the Faraday constant:

${\displaystyle E=E^{\circ }(\mathrm {Hg} /\mathrm {Hg} _{2}^{2+})+{\frac {RT}{2F}}\ln[\mathrm {Hg} _{2}^{2+}]}$

The following applies to the solubility equilibrium:

${\displaystyle \mathrm {Hg_{2}^{2+}+2\ Cl^{-}\leftrightharpoons Hg_{2}Cl_{2}} }$

The solubility product , K _L , is therefore:

${\displaystyle K_{L}=[\mathrm {Hg_{2}^{2+}} ]\ [\mathrm {Cl} ^{-}]^{2}}$

The equation of the solubility product can be used for the mercury concentration to get the potential as a function of the chloride concentration alone:

${\displaystyle E=E^{\circ }(\mathrm {Hg} /\mathrm {Hg} _{2}^{2+})+{\frac {RT}{2F}}\ln {\frac {K_{L}}{[\mathrm {Cl} ^{-}]^{2}}}=E^{\circ }(\mathrm {Hg} /\mathrm {Hg} _{2}^{2+})+{\frac {RT}{2F}}(\ln(K_{L})-2\ln {[\mathrm {Cl} ^{-}])}}$

Like the standard potential, the K _L term only depends on the temperature. So it makes sense to include it in a newly defined standard potential, that of the calomel electrode:

${\displaystyle E=E^{\circ }(\mathrm {Hg} /\mathrm {Hg} _{2}^{2+}/\mathrm {Cl} ^{-})-{\frac {RT}{F}}\ln[\mathrm {Cl} ^{-}]}$

literature

• Matthias Otto: Analytical Chemistry. 3rd, completely revised and expanded edition. Wiley-VCH, Weinheim 2006, ISBN 3-527-31416-4 , p. 366.