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The saturated calomel electrode (SCE) is a
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 (buffered or saturated) concentrations of ea ...
based on the reaction between elemental mercury and
mercury(I) chloride Mercury(I) chloride is the chemical compound with the formula Hg2Cl2. Also known as the mineral calomel (a rare mineral) or mercurous chloride, this dense white or yellowish-white, odorless solid is the principal example of a mercury(I) compound. ...
. It has been widely replaced by the
silver chloride electrode A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For environmental reasons it has widely replaced the saturated calomel electrode. For example, it is usually the internal reference elect ...
, however the calomel electrode has a reputation of being more robust. The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg2Cl2, "
calomel Calomel is a mercury chloride mineral with formula Hg2Cl2 (see mercury(I) chloride). The name derives from Greek ''kalos'' (beautiful) and ''melas'' (black) because it turns black on reaction with ammonia. This was known to alchemists. 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 Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
frit is a
salt bridge In electrochemistry, a salt bridge or ion bridge is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell (voltaic cell), a type of electrochemical cell. It maintains electrical neutrality within the in ...
. In
cell notation Cell notation or cell representation in chemistry is a shorthand method of expressing a reaction in an electrochemical cell. In cell notation, the two half-cells are described by writing the formula of each individual chemical species involved ...
the electrode is written as: :(4M) , , , Pt


Theory of electrolysis


Solubility product

The electrode is based on the redox reactions :\ce, \qquad \ce \quad E^0_\ce = +0.80\ \ce :\ce, \qquad \ce \quad E^0_\ce = +0.27\ \ce The half reactions can be balanced to the following reaction :\ce, \qquad \ce \quad E^0_\ce = +0.53\ \ce. Which can be simplified to the precipitation reaction, with the equilibrium constant of the
solubility product Solubility equilibrium is a type of dynamic equilibrium that 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 reac ...
. : \ce, \qquad K_ = a_ a_^2 = ce\cdot ce2 The
Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction (half-cell or full cell reaction) from the standard electrode potential, absolute tempera ...
s for these half reactions are: : \begin E_ &= E^0_\ce - \frac \ln\frac \qquad &\text \quad E^0_\ce = +0.80\ \ce.\\ E_ &= E^0_\ce - \frac \ln a_^2 \qquad &\text \quad E^0_\ce = +0.27\ \ce.\\ \end The
Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction (half-cell or full cell reaction) from the standard electrode potential, absolute tempera ...
for the balanced reaction is: : \begin E_\ce &= E_ - E_ \\ &= E^0_\ce - \frac \ln\frac \\ &= E^0_\ce - \frac \ln\frac \qquad \text \quad E^0_\ce = +0.53\ \ce \end where ''E''0 is the
standard electrode potential In electrochemistry, standard electrode potential E^\ominus, or E^\ominus_, is a measure of the reducing power of any element or compound. The IUPAC "Gold Book" defines it as: ''"the value of the standard emf (electromotive force) of a cell in wh ...
for the reaction and ''a''Hg is the activity for the mercury cation (the activity for a liquid of 1 Molar is 1). At equilibrium, : \Delta G = - n F E = 0 \mathrm, or equivalently E_\text = 0\ \mathrm. This equality allows us to find the solubility product. : E_\text = E^0_\ce - \frac \ln\frac = +0.53 + \frac \ln = 0\ \ce : \begin \ln &= -0.53 \cdot \frac\\ K_ &= e^\\ & = ce\cdot ce2 = 1.184 \times 10^ \end Due to the high concentration of chloride ions, the concentration of mercury ions ( g2^2+/chem>) is low. This reduces risk of mercury poisoning for users and other mercury problems.


SCE potential

: \ce, \qquad \ce \quad E^0_\ce = +0.27\ \ce : \begin E_ &= E^0_\ce - \frac \ln a_^2 \\ &= +0.27 - \frac \ln ce \end 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, which is: . This gives the SCE a potential of +0.248 V vs.
SHE She most commonly refers to: *She (pronoun), the third person singular, feminine, nominative case pronoun in modern English. She or S.H.E. may also refer to: Literature and films *'' She: A History of Adventure'', an 1887 novel by H. Rider Hagga ...
at 20 °C and +0.244 V vs.
SHE She most commonly refers to: *She (pronoun), the third person singular, feminine, nominative case pronoun in modern English. She or S.H.E. may also refer to: Literature and films *'' She: A History of Adventure'', an 1887 novel by H. Rider Hagga ...
at 25 °C, but slightly higher when the chloride solution is less than saturated. For example, a 3.5M KCl electrolyte solution has an increased reference potential of +0.250 V vs.
SHE She most commonly refers to: *She (pronoun), the third person singular, feminine, nominative case pronoun in modern English. She or S.H.E. may also refer to: Literature and films *'' She: A History of Adventure'', an 1887 novel by H. Rider Hagga ...
at 25°C while a 1 M solution has a +0.283 V potential at the same temperature.


Application

The SCE is used in pH measurement, cyclic voltammetry and general aqueous
electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
. This electrode and the 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. 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 * Standard hydrogen electrode * 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 (buffered or saturated) concentrations of ea ...


References

*{{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 , s2cid=39905013 Electrodes ja:基準電極#カロメル電極