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| [[File:ECS.PNG|thumb|200px|Saturated calomel electrode]]
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| 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>, "[[Mercury(I) chloride|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]].
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| In [[cell notation]] the electrode is written as:
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| :<math>Cl^-\big(4M\big) \big| Hg_2Cl_2 \big(s\big) \big| Hg\big(l\big) \big| Pt</math>
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| ==Theory of operation==
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| The electrode is based on the redox reaction
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| :<math>\text{Hg}_2^{2+} + 2\text{e}^- \rightleftarrows 2\text{Hg(l)}</math>
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| The [[Nernst equation]] for this reaction is
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| :<math> E = E^0_{\text{Hg}_2^{2+}/\text{Hg}} - \frac{RT}{2F} \ln\frac{1}{a_{\text{Hg}_2^{2+}}} </math>
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| 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 of 1 Molar is 1). | |
| This activity can be found from the [[Solubility equilibrium|solubility product]] of the reaction
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| :<math> | |
| \text{Hg}_2^{2+} + 2\text{Cl}^- \rightleftarrows \text{Hg}_2\text{Cl}_2\text{(s)},\qquad
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| K_{\text{sp}} = a_{\text{Hg}_2^{2+}} a_{\text{Cl}^-}^2
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| </math>
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| By replacing the activity in the Nernst equation with the value in the solubility equation, we get
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| :<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 saturated with potassium chloride, this activity is fixed by the solubility of potassium chloride. When saturated the [[redox potential]] of the calomel electrode is +0.2444 V vs. [[standard hydrogen electrode|SHE]] at 25 °C, but slightly higher when the chloride solution is less than saturated. For example, a 3.5M KCl electrolyte solution increases the reference potential to +0.250 V vs. SHE at 25 °C, and a 0.1 M solution to +0.3356 V at the same temperature.<ref>http://www.cem.msu.edu/~cem333/Week11.pdf</ref>
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| ==Application==
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| The SCE is used in [[PH meter|pH]] measurement, [[cyclic voltammetry]] and general aqueous [[electrochemistry]].
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| 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.
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| The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in the Ag/AgCl electrode.
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| ==See also==
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| *[[Cyclic voltammetry]]
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| *[[Standard Hydrogen Electrode]]
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| *[[Table of standard electrode potentials]]
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| *[[Reference electrode]]
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| ==References==
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| {{Reflist}}
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| *{{cite journal |author=Banus MG |title=A DESIGN FOR A SATURATED CALOMEL ELECTRODE |journal=Science |volume=93 |issue=2425 |pages=601–602 |date=June 1941 |pmid=17795970 |doi=10.1126/science.93.2425.601-a |url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=17795970}}
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| [[Category:Electrodes]]
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| [[ja:基準電極#カロメル電極]]
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Myrtle Benny is how I'm known as and I really feel comfy when individuals use the full name. To collect coins is 1 of the things I love most. Hiring is my profession. California is where I've usually been residing and I love each day residing right here.
My homepage :: std testing at home