Saturated calomel electrode
3150639
193766645
2008-02-24T20:35:21Z
S Levchenkov
5037680
iwiki-ru
The '''Saturated calomel electrode''' (SCE) is a [[reference electrode]] based on the reaction between elemental [[Mercury (element)|mercury]] and mercury(I) [[chloride]]. The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg<sub>2</sub>Cl<sub>2</sub>, "[[calomel]]") is a saturated solution of [[potassium]] chloride in water. The electrode is normally linked ''via'' a porous frit to the solution in which the other electrode is immersed. This porous frit is a [[salt bridge]].
In [[cell notation]] the electrode is written as:
Cl<sup>-</sup> (4M)|Hg<sub>2</sub>Cl<sub>2</sub>(s)|Hg(l)|Pt
==Theory of operation==
The electrode is based on the redox reaction
:<math>\text{Hg}_2^{2+} + 2\text{e}^- \rightleftarrows 2\text{Hg(l)}</math>
The [[Nernst equation]] for this reaction is
:<math> E = E^0_{\text{Hg}_2^{2+}/\text{Hg}} - \frac{RT}{2F} \ln\frac{1}{a_{\text{Hg}_2^{2+}}} </math>
where ''E''<sup>0</sup> is the [[standard electrode potential]] for the reaction and ''a''<sub>Hg</sub> is the [[activity (chemistry)|activity]] for the mercury cation (the activity for a liquid is 1).
This activity can be found from the solubility product of the reaction
:<math>
\text{Hg}_2^{2+} + 2\text{Cl}^- \rightleftarrows \text{Hg}_2\text{Cl}_2\text{(s)},\qquad
K_{\text{sp}} = a_{\text{Hg}_2^{2+}} a_{\text{Cl}^-}^2
</math>
By replacing the activity in the Nernst equation with the value in the solubility equation, we get
:<math> E = E^0_{\text{Hg}_2^{2+}/\text{Hg}} + \frac{RT}{2F} \ln K_{\text{sp}} - \frac{RT}{2F} \ln a^2_{\text{Cl}^-} </math>
The only variable in this equation is the activity (or concentration) of the chloride anion. But since the inner solution is satured with potassium chloride, this activity is fixed by the solubility of potassium chloride. At standard conditions, the potential of the saturated calomel electrode should be +0.241 V versus the [[standard hydrogen electrode|SHE]].
==Application==
The SCE is used in [[PH meter|pH]] measurement, [[cyclic voltammetry]] and general aqueous [[electrochemistry]].
This electrode and the [[silver chloride electrode|silver/silver chloride reference electrode]] work in the same way. In both electrodes, the activity of the metal ion is fixed by the solubility of the metal salt.
The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in the Ag/AgCl electrode.
[[Category:Electrodes]]
[[Category:Electrochemistry]]
[[de:Elektroden zweiter Art]]
[[fr:Électrode au calomel saturée en KCl]]
[[ru:Каломельный электрод]]