The
Saturated calomel electrode (SCE) is a
reference electrodeA 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 concentrations of each participants of the redox reaction.There are many ways reference...
based on the reaction between elemental
mercuryMercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
and
mercury(I) chlorideMercury chloride is the chemical compound with the formula Hg2Cl2. Also known as calomel or mercurous chloride, this dense white or yellowish-white, odorless solid is the principal example of a mercury compound...
. The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg
2Cl
2, "
calomelMercury chloride is the chemical compound with the formula Hg2Cl2. Also known as calomel or mercurous chloride, this dense white or yellowish-white, odorless solid is the principal example of a mercury compound...
") is a saturated solution of
potassiumPotassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
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 bridgeA salt bridge, in chemistry, is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell , a type of electrochemical cell...
.
In
cell notationCell notation in chemistry is a shorthand way of expressing a certain reaction in an electrochemical cell. The cell anode and cathode are separated by two bars or slashes representing a salt bridge, with the anode on the left and cathode on the right. Individual solid, liquid or aqueous phases...
the electrode is written as:
Theory of operation
The electrode is based on the redox reaction
The
Nernst equationIn electrochemistry, the Nernst equation is an equation that can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell. It can also be used to determine the total voltage for a full electrochemical cell...
for this reaction is
where
E0 is the
standard electrode potentialIn electrochemistry, the standard electrode potential, abbreviated E° or E , is the measure of individual potential of a reversible electrode at standard state, which is with solutes at an effective concentration of 1 mol dm−3, and gases at a pressure of 1 atm...
for the reaction and
aHg is the
activityIn chemical thermodynamics, activity is a measure of the “effective concentration” of a species in a mixture, meaning that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.By convention, activity...
for the mercury cation (the activity for a liquid of 1 Molar is 1).
This activity can be found from the
solubility productSolubility equilibrium is a type of dynamic equilibrium. It exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound. The solid may dissolve unchanged, with dissociation or with chemical reaction with another constituent of the solvent, such as...
of the reaction
By replacing the activity in the Nernst equation with the value in the solubility equation, we get
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.
SHEThe standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
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.
Application
The SCE is used in
pHA pH meter is an electronic instrument used for measuring the pH of a liquid...
measurement,
cyclic voltammetryCyclic voltammetry or CV is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time like linear sweep voltammetry. Cyclic voltammetry takes the experiment a step further than linear sweep voltammetry...
and general aqueous
electrochemistryElectrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
.
This electrode and the
silver/silver chloride reference electrodeA silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For example, it is usually the internal reference electrode in pH meters...
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.
See also
- Cyclic voltammetry
Cyclic voltammetry or CV is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time like linear sweep voltammetry. Cyclic voltammetry takes the experiment a step further than linear sweep voltammetry...
- Standard Hydrogen Electrode
The standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
- Table of standard electrode potentials
- Reference electrode
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 concentrations of each participants of the redox reaction.There are many ways reference...