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 | |
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| Names | |
|---|---|
| IUPAC name Dimercury dichloride | |
| Other names | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
|
| ECHA InfoCard | 100.030.266 |
| EC Number |
|
| 25976 | |
| RTECS number |
|
| UNII | |
| UN number | 3077 |
| |
| |
| Properties | |
| Hg2Cl2 | |
| Molar mass | 472.09 g/mol |
| Appearance | White solid |
| Density | 7.150 g/cm3 |
| Melting point | 383 °C (721 °F; 656 K) (sublimes) |
| 0.2 mg/100 mL | |
Solubility product (Ksp) | 1.43×10−18[1] |
| Solubility | insoluble inethanol,ether |
| −26.0·10−6 cm3/mol | |
Refractive index (nD) | 1.973 |
| Structure | |
| tetragonal | |
| Thermochemistry | |
Std molar entropy(S⦵298) | 196 J·mol−1·K−1[2] |
Std enthalpy of formation(ΔfH⦵298) | −265 kJ·mol−1[2] |
| Hazards | |
| GHS labelling: | |
  | |
| Warning | |
| H302,H315,H319,H335,H410 | |
| P261,P264,P270,P271,P273,P280,P301+P312,P302+P352,P304+P340,P305+P351+P338,P312,P321,P330,P332+P313,P337+P313,P362,P391,P403+P233,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 210 mg/kg (rat, oral)[3] |
| Safety data sheet (SDS) | ICSC 0984 |
| Related compounds | |
Otheranions | Mercury(I) fluoride Mercury(I) bromide Mercury(I) iodide |
Related compounds | Mercury(II) chloride |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Mercury(I) chloride is thechemical compound with the formula Hg2Cl2. Also known as themineralcalomel[4] (a rare mineral) ormercurous chloride, this dense white or yellowish-white, odorless solid is the principal example of amercury(I) compound. It is a component ofreference electrodes inelectrochemistry.[5][6]
The name calomel is thought to come from theGreekκαλός "beautiful", andμέλας "black"; orκαλός andμέλι "honey" from its sweet taste.[4] The "black" name (somewhat surprising for a white compound) is probably due to its characteristicdisproportionation reaction withammonia, which gives a spectacular black coloration due to the finely dispersed metallicmercury formed. It is also referred to as the mineralhorn quicksilver orhorn mercury.[4]
Calomel was taken internally and used as a laxative,[4] for example to treatGeorge III in 1801, and disinfectant, as well as in the treatment of syphilis, until the early 20th century. Until fairly recently,[when?] it was also used as a horticultural fungicide, most notably as a root dip to help prevent the occurrence ofclubroot amongst crops of the familyBrassicaceae.[7]
Mercury became a popular remedy for a variety of physical and mental ailments during the age of "heroic medicine". It was prescribed by doctors in America throughout the 18th century, and during the revolution, to make patients regurgitate and release their body from "impurities".Benjamin Rush was a well-known advocate of mercury in medicine and used calomel to treat sufferers ofyellow fever during its outbreak inPhiladelphia in 1793. Calomel was given to patients as apurgative orcathartic until they began to salivate and was often administered to patients in such great quantities that their hair and teeth fell out.[8]
Yellow fever was also treated with calomel.[9]
Lewis and Clark brought calomel on their expedition. Researchers used that same mercury, found deep inlatrine pits, to retrace the locations of their respective locations and campsites.[10]
Mercury is unique among the group 12 metals for its ability to form the M–M bond so readily. Hg2Cl2 is a linear molecule. The mineral calomel crystallizes in thetetragonal system, with space group I4/m 2/m 2/m. Theunit cell of thecrystal structure is shown below:
 |  |
| unit cell | 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.[11] 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. Longermercury polycations exist.
Mercurous chloride forms by the reaction of elementalmercury andmercuric chloride:
It can be prepared viametathesis reaction involving aqueousmercury(I) nitrate using various chloride sources including NaCl or HCl.
Ammonia causes Hg2Cl2 todisproportionate:
Mercurous chloride is employed extensively inelectrochemistry, taking advantage of the ease of its oxidation and reduction reactions. The calomel electrode is areference 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 ofmercury, 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.[citation needed]
Mercurous chloride decomposes intomercury(II) chloride and elemental mercury upon exposure to UV light.
The formation of Hg can be used to calculate the number of photons in the light beam, by the technique ofactinometry.
By utilizing a light reaction in the presence ofmercury(II) chloride andammonium oxalate, mercury(I) chloride,ammonium chloride andcarbon dioxide are produced.
This particular reaction was discovered by J. M. Eder (hence the nameEder reaction) in 1880 and reinvestigated by W. E. Rosevaere in 1929.[12]
Mercury(I) bromide, Hg2Br2, is light yellow, whereasmercury(I) iodide, Hg2I2, is greenish in colour. Both are poorly soluble.Mercury(I) fluoride is unstable in the absence of a strong acid.
Mercurous chloride istoxic, although due to its low solubility in water it is generally less dangerous than itsmercuric chloride counterpart. It was used in medicine as adiuretic andpurgative (laxative) in theUnited States from the late 1700s through the 1860s. Calomel was also a common ingredient inteething powders in Britain up until 1954, causing widespread mercury poisoning in the form ofpink disease, which at the time had a mortality rate of 1 in 10.[13] These medicinal uses were later discontinued when the compound's toxicity was discovered.
It has also found uses in cosmetics as soaps andskin lightening creams, but these preparations are now illegal to manufacture or import in many countries including the US, Canada, Japan and the European Union.[14] A study of workers involved in the production of these preparations showed that the sodium salt of2,3-dimercapto-1-propanesulfonic acid (DMPS) was effective in lowering thebody burden of mercury and in decreasing the urinary mercury concentration to normal levels.[15]
