Calomel electrode - Calomel electrode

Also known as a calomel electrode. One of the most commonly used reference electrodes today. Usually, purified mercury is placed at the bottom of a glass container, on top of which is placed a paste-like substance made by grinding mercury(I) chloride (calomel) and mercury thoroughly in an agate mortar, and then this is filled with a potassium chloride solution of the required concentration. The electrode terminal is removed from the mercury, and the potassium chloride solution is in liquid junction with the electrode system to be measured. The electrode reaction in a calomel electrode is

Therefore, the equilibrium electrode potential E _eq is given by the following equation:

Here, E ^° is the standard electrode potential, F is the Faraday constant, R is the gas constant, T is the absolute temperature, and aCl- is the activity of chloride ions. Thus, at a constant temperature, this electrode shows a constant potential determined by the activity of chloride ions in the solution as long _^as there is a sufficient amount of _Hg2Cl2 on the mercury surface for the electrode reaction to proceed. Usually, 0.1 mol L ^-1 _, 1 mol L ^-1 , and saturated potassium chloride solutions are used, and these are called the 0.1 mol L ^-1 calomel electrode, the 1 mol L ^-1 calomel electrode (abbreviated as NCE), and the saturated calomel electrode (abbreviated as SCE), respectively. The equilibrium electrode potential values ​​(at 25 °C) of these calomel electrodes are given in the following table, based on the standard hydrogen electrode potential (abbreviated as NHE).

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

甘コウ電極ともいう．現在，もっともよく用いられている照合電極の一つ．通常，ガラス製容器の底に精製した水銀を入れ，その上に塩化水銀(Ⅰ)(甘コウ)と水銀とをめのう乳鉢でよくすり合わせた糊状物を置き，さらにその上に所要の濃度の塩化カリウム溶液を満たしたものである．電極の端子は水銀から取り出し，塩化カリウム溶液は測定しようとする電極系と液絡する．カロメル電極における電極反応は

で表されるので，その平衡電極電位E_eq は次式で与えられる．

ここで，E^° は標準電極電位，Fはファラデー定数，Rは気体定数，Tは絶対温度，a_Cl^－ は塩化物イオンの活量である．したがって，この電極は，一定温度においては，電極反応が進行するのに十分な量のHg₂Cl₂が水銀面上に存在する限り，溶液中の塩化物イオンの活量により定まる一定の電位を示す．通常，0.1 mol L^－1，1 mol L^－1 および飽和の塩化カリウム溶液が用いられ，それぞれ0.1 mol L^－1 カロメル電極，1 mol L^－1 カロメル電極(N.C.E.と略記)，および飽和カロメル電極(S.C.Eと略記)とよばれる．これらのカロメル電極の平衡電極電位の値(25 ℃)は，標準水素電極電位(N.H.E.と略記)を基準として，次表で与えられる．

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