Mercury(I) chloride

Last updated
Mercury(I) chloride
Mercury Chloride.jpg
IUPAC name
Dimercury dichloride
Other names
Mercury(I) chloride
Mercurous chloride
3D model (JSmol)
ECHA InfoCard 100.030.266
EC Number
  • 233-307-5
PubChem CID
RTECS number
  • OV8750000
UN number 3077
Molar mass 472.09 g/mol
AppearanceWhite solid
Density 7.150 g/cm3
Melting point 525 °C (977 °F; 798 K)(triple point)
Boiling point 383 °C (721 °F; 656 K)(sublimes)
0.2 mg/100 mL
Solubility insoluble in ethanol, ether
26.0·10−6 cm3/mol
Safety data sheet ICSC 0984
Toxic (T)
Harmful (Xn)
Dangerous for the environment (N)
R-phrases (outdated) R22, R36/37/38, R50/53
S-phrases (outdated) (S2), S13, S24/25, S46, S60, S61
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no codeMercury(I) chloride
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
210 mg/kg (rat, oral) [1]
196 J·mol−1·K−1 [2]
−265 kJ·mol−1 [2]
Related compounds
Other anions
Mercury(I) fluoride
Mercury(I) bromide
Mercury(I) iodide
Related compounds
Mercury(II) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Mercury(I) chloride is the chemical compound with the formula Hg2Cl2. Also known as the mineral calomel [3] (a rare mineral) 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. [4] [5]



The name calomel is thought to come from the Greek καλός beautiful, and μέλας black; or καλός and μέλι honey from its sweet taste. [3] The black 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. [3]

Calomel was taken internally and used as a laxative, [3] for example to treat George 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 of clubroot amongst crops of the family Brassicaceae. [6]

Mercury became a popular remedy for a variety of physical and mental ailments during the age of "heroic medicine". It was used 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 one particular well-known advocate of mercury in medicine and used calomel to treat sufferers of yellow fever during its outbreak in Philadelphia in 1793. Calomel was given to patients as a purgative or cathartic until they began to salivate and was often administered to patients in such great quantities that their hair and teeth fell out. [7]

Shortly after yellow fever struck Philadelphia, the disease broke out in Jamaica. A war of words erupted in the press concerning the best treatment for yellow fever: bleeding; or calomel. Anecdotal evidence indicates calomel was more effective than bleeding. [8]

Mormon prophet Joseph Smith's eldest brother Alvin Smith died in 1823 from mercury poisoning from calomel.[ citation needed ]

Lewis and Clark brought along the wonder drug of the day, mercury chloride (otherwise known as calomel), as a pill, a tincture, and an ointment. Modern researchers used that same mercury, found deep in latrine pits, to retrace the locations of their respective locations and campsites. [9]


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 the tetragonal system, with space group I4/m 2/m 2/m. The unit cell of the crystal 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. [10] 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. Longer mercury polycations exist.

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.

2 HCl + Hg2(NO3)2 → Hg2Cl2 + 2 HNO3

Ammonia causes Hg2Cl2 to disproportionate:

Hg2Cl2 + 2 NH3 → Hg + Hg(NH2)Cl + NH4Cl

Calomel electrode

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.[ citation needed ]


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, mercury(I) chloride, ammonium chloride and carbon dioxide are produced.

2 HgCl2 + (NH4)2C2O4Light Hg2Cl2(s) + 2 [NH+
][Cl] + 2 CO2

This particular reaction was discovered by J. M. Eder (hence the name Eder reaction) in 1880 and reinvestigated by W. E. Rosevaere in 1929. [11]

Mercury(I) bromide, Hg2Br2, is a light yellow, whereas mercury(I) iodide, Hg2I2, is greenish in colour. Both are poorly soluble. Mercury(I) fluoride is unstable in the absence of a strong acid.

Safety considerations

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 and purgative (laxative) in the United States from the late 1700s through the 1860s. Calomel was also a common ingredient in teething powders in Britain up until 1954, causing widespread mercury poisoning in the form of pink disease, which at the time had a mortality rate of 1 in 10. [12] These medicinal uses were later discontinued when the compound's toxicity was discovered.

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 the US, Canada, Japan and the European Union. [13] 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. [14]

Related Research Articles

Dimethylmercury chemical compound

Dimethylmercury ((CH3)2Hg) is an organomercury compound. A highly volatile, reactive, flammable, and colorless liquid, dimethylmercury is one of the strongest known neurotoxins, with a quantity of less than 0.1 mL capable of inducing severe mercury poisoning, and is easily absorbed through the skin. Dimethylmercury is capable of permeating many materials, including plastic and rubber compounds. It has a slightly sweet odor, although inhaling enough of the chemical to notice this would be hazardous.

Silver chloride chemical compound

Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water (this behavior being reminiscent of the chlorides of Tl+ and Pb2+). Upon illumination or heating, silver chloride converts to silver (and chlorine), which is signaled by grey to black or purplish coloration to some samples. AgCl occurs naturally as a mineral chlorargyrite.

Lead(II) chloride chemical compound

Lead(II) chloride (PbCl2) is an inorganic compound which is a white solid under ambient conditions. It is poorly soluble in water. Lead(II) chloride is one of the most important lead-based reagents. It also occurs naturally in the form of the mineral cotunnite.

Reference electrode electrode with a stable, well-known electrode potential

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 participant of the redox reaction.

Calomel mineral form of the mercury(I) chloride

Calomel is a mercury chloride mineral with formula Hg2Cl2 (see mercury(I) chloride). The name derives from Greek kalos (beautiful) and melos (black) because it turns black on reaction with ammonia. This was known to alchemists.

Phosphorus trichloride chemical compound

Phosphorus trichloride is a chemical compound of phosphorus and chlorine, having the chemical formula PCl3. It is a toxic and volatile liquid which reacts violently with water to release HCl gas. It has a trigonal pyramidal shape, owing to the lone pairs on the phosphorus. It is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds for a wide variety of applications. It has a 31P NMR signal at around +220 ppm with reference to a phosphoric acid standard.

Mercury sulfide chemical compound

Mercury sulfide, mercuric sulfide, mercury sulphide, or mercury(II) sulfide is a chemical compound composed of the chemical elements mercury and sulfur. It is represented by the chemical formula HgS. It is virtually insoluble in water.

The saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental mercury and mercury(I) chloride. It has been widely replaced by the silver chloride electrode, 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") 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.

Mercury(II) iodide chemical compound

Mercury(II) iodide is a chemical compound with the molecular formula HgI2. It is typically produced synthetically but can also be found in nature as the extremely rare mineral coccinite. Unlike the related mercury(II) chloride it is hardly soluble in water (<100 ppm).

Mercury(I) sulfide or mercurous sulfide is a hypothetical chemical compound of mercury and sulfur, with elemental formula Hg
. Its existence has been disputed; it may be stable below 0 °C or in suitable environments, but is unstable at room temperature, decomposing into metallic mercury and mercury(II) sulfide.

Potentiometric titration is a technique similar to direct titration of a redox reaction. It is a useful means of characterizing an acid. No indicator is used; instead the potential is measured across the analyte, typically an electrolyte solution. To do this, two electrodes are used, an indicator electrode and a reference electrode. Reference electrodes generally used are hydrogen electrodes, calomel electrodes, and silver chloride electrodes. The indicator electrode forms an electrochemical half cell with the interested ions in the test solution. The reference electrode forms the other half cell.

Mercury(I) sulfate, commonly called mercurous sulphate (UK) or mercurous sulfate (US) is the chemical compound Hg2SO4. Mercury(I) sulfate is a metallic compound that is a white, pale yellow or beige powder. It is a metallic salt of sulfuric acid formed by replacing both hydrogen atoms with mercury(I). It is highly toxic; it could be fatal if inhaled, ingested, or absorbed by skin.

Mercuric amidochloride chemical compound

Mercuric amidochloride is an inorganic compound with the formula HgNH2Cl. It consists of a zig-zag 1-dimensional polymer (HgNH2)n with chloride counterions. It arises from the reaction of ammonia and mercuric chloride. Addition of base converts it into "Millon's base", which has the formula [Hg2N]OH·(H2O)x. A variety of related amido and nitrido materials with chloride, bromide, and hydroxide are known.

Mercury(I) bromide chemical compound

Mercury(I) bromide or mercurous bromide is the chemical compound composed of mercury and bromine with the formula Hg2Br2. It changes color from white to yellow when heated and fluoresces a salmon color when exposed to ultraviolet light. It has applications in acousto-optical devices.

Mercury(I) fluoride chemical compound

Mercury(I) fluoride or mercurous fluoride is the chemical compound composed of mercury and fluorine with the formula Hg2F2. It consists of small yellow cubic crystals, which turn black when exposed to light.

Cadmium(I) tetrachloroaluminate chemical compound

Cadmium(I) tetrachloroaluminate is the inorganic compound with the formula Cd2(AlCl4)2, a tetrachloroaluminate of cadmium(I). It was the first compound reported (1961) that contained cadmium in the +1 oxidation state. Subsequent studies of the Raman vibrational spectrum indicated the presence of a cadmium–cadmium bond, which was confirmed by two separate X-Ray diffraction studies of single crystals. The compound can therefore be compared to mercury(I) (mercurous) compounds (such as mercury(I) chloride), which contain Hg2+

Mercury (element) Chemical element with atomic number 80

Mercury is a chemical element with the symbol Hg and atomic number 80. It is commonly known as quicksilver and was formerly named hydrargyrum. A heavy, silvery d-block element, mercury is the only metallic element that is liquid at standard conditions for temperature and pressure; the only other element that is liquid under these conditions is the halogen bromine, though metals such as caesium, gallium, and rubidium melt just above room temperature.

Mercury polycations are polyatomic cations that contain only mercury atoms. The best known example is the Hg2+
ion, found in mercury(I) (mercurous) compounds. The existence of the metal–metal bond in Hg(I) compounds was established using X-ray studies in 1927 and Raman spectroscopy in 1934 making it one of the earliest, if not the first, metal–metal covalent bonds to be characterised.

Mercury(I) nitrate is a chemical compound with the formula Hg2(NO3)2. It is used in the preparation of other mercury(I) compounds, and is toxic.


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