A Reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each participants of the redox reaction. There are many ways reference electrodes are used. The simplest is when the reference electrode is used as a half cell to build an electrochemical cell. This allows the potential of the other half cell to be determined. An accurate and practical method to measure an electrode's potential in isolation (absolute electrode potential) has yet to be developed.
Aqueous Reference Electrodes
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:
 Theory of operation
The electrode is based on the redox reaction
The Nernst equation for this reaction is
where E0 is the standard electrode potential for the reaction and aHg is the activity for the mercury cation (the activity for a liquid is 1). This activity can be found from the solubility product of the reaction [pic]
By replacing the activity in the Nernst equation with the value in the solubility equation, we get [pic]
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. At standard conditions, the potential of the saturated...