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Saturated calomel electrode : ウィキペディア英語版
Saturated calomel electrode

The Saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental mercury and mercury(I) chloride. The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg2Cl2, "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:
:\text^-\big(4M\big) \big| \text_2\text_2 \big(\text\big) \big| \text\big(\text\big) \big| \text
==Theory of operation==

The electrode is based on the redox reaction
:\text_2^ + 2\text^- \rightleftarrows 2\text
The Nernst equation for this reaction is
: E = E^0_/\text} - \frac \ln\frac}}
where ''E''0 is the standard electrode potential for the reaction and ''a''Hg is the activity for the mercury cation (the activity for a liquid of 1 Molar is 1).
This activity can be found from the solubility product of the reaction
:
\text_2^ + 2\text^- \rightleftarrows \text_2\text_2\text,\qquad
K__2^} a_^2

By replacing the activity in the Nernst equation with the value in the solubility equation, we get
: E = E^0_/\text} + \frac \ln K_ \ln a^2_
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. 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.〔http://www.cem.msu.edu/~cem333/Week11.pdf〕

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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