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is the chemical compound
with the formula Hg2
. Also known as calomel
(a mineral form, rarely found in nature) or mercurous chloride
, this dense white or yellowish-white, odorless solid is the principal example of a mercury
(I) compound. It is a component of reference electrodes in electrochemistry
The name calomel is thought to come from the Greek
, and μελας black
. This name (somewhat surprising for a white compound) is probably due to its characteristic disproportionation
reaction with ammonia
, which gives a spectacular black coloration due to the finely dispersed metallic mercury
formed. It is also referred to as the mineral horn quicksilver
or horn mercury
. Calomel was taken internally and used as a laxative and disinfectant before the 20th century. It was also used by doctors in the 18th century in America, and during the revolution, to make patients regurgitate and release their body from "impurities".
Mercury is unique among the group 12 metals for its ability to form the M-M bond so readily. Hg2
is a linear molecule. The unit cell
of the crystal structure
is shown below:
|| distorted octahedral coordination of Hg |
The Hg-Hg bond length of 253 pm (Hg-Hg in the metal is 300 pm) and the Hg-Cl bond length in the linear Hg2Cl2 unit is 243 pm. The overall coordination of each Hg atom is octahedral as, in addition to the two nearest neighbours, there are four other Cl atoms at 321 pm.
Preparation and reactions
Mercurous chloride forms by the reaction of elemental mercury and mercuric chloride:
- Hg + HgCl2 → Hg2Cl2
It can be prepared via metathesis
reaction involving aqueous mercury(I) nitrate
using various chloride sources including NaCl or HCl.
- 2HCl + Hg2(NO3)2 → Hg2Cl2 + 2HNO3
- Hg2Cl2 + 2NH3 → Hg + Hg(NH2)Cl + NH4Cl
Mercurous chloride is employed extensively in electrochemistry
, taking advantage of the ease of its oxidation and reduction reactions. The calomel electrode is a reference electrode
, especially in older publications. Over the past 50 years, it has been superseded by the silver/silver chloride (Ag/AgCl) electrode. Although the mercury electrodes have been widely abandoned due to the dangerous nature of mercury
, many chemists believe they are still more accurate and are not dangerous as long as they are handled properly. The differences in experimental potentials vary little from literature values. Other electrodes can vary by 70 to 100 millivolts.
Mercurous chloride decomposes into mercury(II) chloride
and elemental mercury upon exposure to UV light.
- Hg2Cl2 → HgCl2 + Hg
The formation of Hg can be used to calculate the number of photons in the light beam, by the technique of actinometry
. By utilizing a light reaction in the presence of mercury(II) chloride
and ammonium oxalate
mercurous chloride is produced.
- 2HgCl2 + (NH4)2C2O4 + Light → Hg2Cl2(s) + 2[NH4+][Cl−] + 2CO2
This particular reaction was invented by J.M. Eder (hence the name Eder reaction) in 1880 and reinvestigated by W. E. Rosevaere in 1929
Related mercury(I) compounds
, a light yellow, whereas mercury(I) iodide
, is greenish in colour. Both are poorly soluble. Mercury(I) fluoride
is unstable in the absence of a strong acid.
Mercurous chloride is toxic, although due to its low solubility in water it is generally less dangerous than its mercuric chloride
counterpart. It was used in medicine as a diuretic
(laxative), e.g. from the early 1830s through the 1860s as a purgative in the U.S. These medicinal uses were discontinued.
It has also found uses in cosmetics as soaps and skin lightening creams, but these preparations are now illegal to manufacture or import in many countries including U.S., Canada, Japan and Europe. A study of workers involved in the production of these preparations, showed that the sodium salt of 2,3-dimercapto-1-propanesulfonic acid (DMPS) was effective in lowering the body burden of mercury and in decreasing the urinary mercury concentration to normal levels.