Mercury(I) chloride is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with the formula Hg₂Cl₂. Also known as the

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

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 is a component of

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 ...

s in

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 out ...

History

The name calomel is thought to come from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

''καλός'' "beautiful", and ''μέλας'' "black"; or ''καλός'' and ''μέλι'' "honey" from its sweet taste. The "black" name (somewhat surprising for a white compound) is probably due to its characteristic

disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...

reaction with

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

, 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, for example to treat

George III George III (George William Frederick; 4 June 173829 January 1820) was King of Great Britain and of Ireland from 25 October 1760 until the union of the two kingdoms on 1 January 1801, after which he was King of the United Kingdom of Great Br ...

in 1801, and disinfectant, as well as in the treatment of syphilis, until the early 20th century. Until fairly recently, it was also used as a horticultural fungicide, most notably as a root dip to help prevent the occurrence of

clubroot Clubroot is a common disease of cabbages, broccoli, cauliflower, Brussels sprouts, radishes, turnips, stocks, wallflowers and other plants of the family Brassicaceae (Cruciferae). It is caused by ''Plasmodiophora brassicae'', which was once cons ...

amongst crops of the family

Brassicaceae Brassicaceae () or (the older) Cruciferae () is a medium-sized and economically important family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous plants, while some are shrubs. The leav ...

. Mercury became a popular remedy for a variety of physical and mental ailments during the age of "

heroic medicine Heroic medicine, also referred to as heroic depletion theory, was a therapeutic method advocating for rigorous treatment of bloodletting, purging, and sweating to shock the body back to health after an illness caused by a humoral imbalance. Risi ...

". 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 Benjamin Rush (April 19, 1813) was a Founding Father of the United States who signed the United States Declaration of Independence, and a civic leader in Philadelphia, where he was a physician, politician, social reformer, humanitarian, educa ...

was one particular well-known advocate of mercury in medicine and used calomel to treat sufferers of

yellow fever Yellow fever is a viral disease of typically short duration. In most cases, symptoms include fever, chills, loss of appetite, nausea, muscle pains – particularly in the back – and headaches. Symptoms typically improve within five days. ...

during its outbreak in

Philadelphia Philadelphia, often called Philly, is the largest city in the Commonwealth of Pennsylvania, the sixth-largest city in the U.S., the second-largest city in both the Northeast megalopolis and Mid-Atlantic regions after New York City. Since ...

in 1793. Calomel was given to patients as a

purgative Laxatives, purgatives, or aperients are substances that loosen stools and increase bowel movements. They are used to treat and prevent constipation. Laxatives vary as to how they work and the side effects they may have. Certain stimulant, lubri ...

cathartic In medicine, a cathartic is a substance that ''accelerates'' defecation. This is similar to a laxative, which is a substance that ''eases'' defecation, usually by softening feces. It is possible for a substance to be both a laxative and a catha ...

until they began to salivate and was often administered to patients in such great quantities that their hair and teeth fell out. 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. pp. 82-83. Mormon prophet

Joseph Smith Joseph Smith Jr. (December 23, 1805June 27, 1844) was an American religious leader and founder of Mormonism and the Latter Day Saint movement. When he was 24, Smith published the Book of Mormon. By the time of his death, 14 years later, h ...

's eldest brother Alvin Smith died in 1823 from mercury poisoning from calomel.

Lewis and Clark Lewis may refer to: Names * Lewis (given name), including a list of people with the given name * Lewis (surname), including a list of people with the surname Music * Lewis (musician), Canadian singer * "Lewis (Mistreated)", a song by Radiohead ...

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 A latrine is a toilet or an even simpler facility that is used as a toilet within a sanitation system. For example, it can be a communal trench in the earth in a camp to be used as emergency sanitation, a hole in the ground ( pit latrine), or ...

pits, to retrace the locations of their respective locations and campsites.

Properties

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 In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...

system, with space group I4/m 2/m 2/m. The

unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...

of the

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...

is shown below: 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.Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications 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 Mercury polycations are polyatomic cations that contain only mercury atoms. The best known example is the ion, found in mercury(I) (mercurous) compounds. The existence of the metal–metal bond in Hg(I) compounds was established using X-ray studi ...

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 Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

causes Hg₂Cl₂ to

disproportionate In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...

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

Calomel electrode

Mercurous chloride is employed extensively in

, taking advantage of the ease of its oxidation and reduction reactions. The calomel electrode is a

, 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.

Photochemistry

Mercurous chloride decomposes into

mercury(II) chloride Mercury(II) chloride (or mercury bichloride, mercury dichloride), historically also known as sulema or corrosive sublimate, is the inorganic chemical compound of mercury and chlorine with the formula HgCl2. It is white crystalline solid and is ...

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

and

ammonium oxalate The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary amm ...

, mercury(I) chloride, ammonium chloride and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

are produced. :2 HgCl₂ + (NH₄)₂C₂O₄ Hg₂Cl_2(s) + 2 [][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.

Related mercury(I) compounds

Mercury(I) bromide 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 a ...

, Hg₂Br₂, is light yellow, whereas

mercury(I) iodide Mercury(I) iodide is a chemical compound of mercury and iodine. The chemical formula is Hg2I2. It is photosensitive and decomposes easily to mercury and HgI2. Synthesis Mercury(I) iodide can be prepared by directly reacting mercury and iodine. ...

, Hg₂I₂, is greenish in colour. Both are poorly soluble.

Mercury(I) fluoride 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. Synthesis Mercury(I) fluoride is pre ...

is unstable in the absence of a strong acid.

Safety considerations

Mercurous chloride is

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a sub ...

, although due to its low solubility in water it is generally less dangerous than its

mercuric chloride Mercury(II) chloride (or mercury bichloride, mercury dichloride), historically also known as sulema or corrosive sublimate, is the inorganic chemical compound of mercury and chlorine with the formula HgCl2. It is white crystalline solid and is a ...

counterpart. It was used in medicine as a

diuretic A diuretic () is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics i ...

and

(laxative) in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

from the late 1700s through the 1860s. Calomel was also a common ingredient in

teething Teething is the process by which an infant's first teeth (the deciduous teeth, often called "baby teeth" or "milk teeth") appear by emerging through the gums, typically arriving in pairs. The mandibular central incisors are the first primary te ...

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. 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. A study of workers involved in the production of these preparations showed that the sodium salt of

2,3-dimercapto-1-propanesulfonic acid 2,3-Dimercapto-1-propanesulfonic acid (abbreviated DMPS) and its sodium salt (known as Unithiol) are chelating agents that form complexes with various heavy metals. They are related to dimercaprol, which is another chelating agent. The synthesis ...

(DMPS) was effective in lowering the body burden of mercury and in decreasing the urinary mercury concentration to normal levels.

References

External links

International Chemical Safety Card 0984

* ttps://www.cdc.gov/niosh/npg/npgd0383.html NIOSH Pocket Guide to Chemical Hazards {{DEFAULTSORT:Mercury(I) Chloride Mercury(I) compounds Chlorides Metal halides Alchemical substances Obsolete pesticides Laxatives Diuretics Luminescent minerals