Calomel is a mercury chloride mineral with the chemical formula Hg₂Cl₂, also known as mercury(I) chloride or mercurous chloride. It is a dense, white or yellowish-white, odorless and tasteless solid that occurs naturally as a rare halide mineral and can also be produced synthetically.
Etymology and Nomenclature
The name "calomel" is thought to derive from the Greek kalos ("beautiful") and melas ("black"), referring to its characteristic reaction with ammonia, which produces a black coloration due to the formation of finely dispersed metallic mercury. The name was first used in 1655 by Théodore de Mayerne. Alternative names include horn mercury, horn quicksilver, and mercurous chloride.
Chemical and Physical Properties
- Chemical formula: Hg₂Cl₂
- Molecular weight: 472.09 g/mol
- Crystal system: Tetragonal (ditetragonal dipyramidal)
- Mohs hardness: 1.5
- Density: 7.15–7.27 g/cm³
- Luster: Adamantine
- Color: Colorless, white, grayish, yellowish-white, brown
- Melting point: 302 °C (sublimes at 384 °C)
- Solubility: Insoluble in water, ethanol, ether, and cold dilute acids
- Fluorescence: Brick-red under UV light
The compound features a linear Hg–Hg–Cl molecular structure, with a mercury–mercury bond length of approximately 253 pm.
History and Medicinal Use
Calomel was first documented in ancient Persia by the physician and alchemist Rhazes around 850 CE. It entered Western medical literature in 1608 through Oswald Croll's Tyroncium Chemicum.
From the 16th through the early 20th century, calomel was widely used as a medicine despite its toxicity. It was regarded as a panacea and prescribed for numerous conditions including syphilis, bronchitis, cholera, tuberculosis, influenza, teething, gout, and cancer. During the 18th and 19th centuries, it was commonly administered as a purgative and laxative. Benjamin Rush, a prominent American physician, advocated "heroic doses" of calomel—approximately 20 grains (1.3 g) taken four times daily.
The compound was a component of "blue pill" and "blue mass" formulations. It was also used in teething powders in Britain until 1954, causing widespread mercury poisoning in the form of acrodynia (pink disease), which had a mortality rate of approximately 1 in 10.
Toxicity
Calomel is toxic due to its mercury content. Oral ingestion primarily damages the gastrointestinal tract, while inhalation of powdered forms allows mercury to enter the bloodstream, cross the blood–brain barrier, and accumulate in the brain. Mercury can also pass through the placenta, causing harm to unborn children. Symptoms of mercury poisoning from calomel included gangrene of the mouth, loss of teeth, facial deformities, extreme cramping, vomiting, and bloody diarrhea.
By the mid-19th century, some physicians began questioning its safety. In 1863, U.S. Surgeon General William A. Hammond banned calomel from U.S. Army medical supplies, a decision that contributed to his removal from office. Its medicinal use gradually declined through the late 19th and early 20th centuries as safer alternatives were developed and its toxicity became better understood.
Electrochemical Applications
Calomel is used in the construction of the saturated calomel electrode (SCE), a reference electrode employed in electrochemistry for measuring pH and electrical potentials in solutions. The SCE consists of mercury in contact with a saturated solution of potassium chloride and calomel (Hg₂Cl₂). Although largely superseded by the silver/silver chloride electrode in modern practice, the calomel electrode remains in use in some applications.
Other Uses
- Fungicide and insecticide: Used as a horticultural fungicide, particularly as a root dip to prevent clubroot in Brassicaceae crops, until the 1970s.
- Pigment: Used as a white pigment in 17th-century South American art and in European medieval manuscripts.
- Cosmetics: Formerly used in soaps and skin-lightening creams, now illegal in many countries including the U.S., Canada, Japan, and the European Union.
Occurrence
As a mineral, calomel is found in oxidized mercury deposits, often associated with native mercury, cinnabar, calcite, limonite, and clay. Notable localities include Moschellandsberg, Germany; Zimapán, Mexico; and Terlingua, Brewster County, Texas, USA.
