Calomel

Mercury(I) chloride

Names
IUPAC name Dimercury dichloride
Other names Mercurous chloride Calomel
Identifiers
CAS Number	10112-91-1 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:33050 Y
ChemSpider	16740467 Y
ECHA InfoCard	100.030.266
EC Number	233-307-5
PubChem CID	24956
RTECS number	OV8750000
UNII	J2D46N657D N
UN number	3077
InChI InChI=1S/2ClH.2Hg/h2*1H;;/q;;2*+1/p-2 Y Key: ZOMNIUBKTOKEHS-UHFFFAOYSA-L Y
SMILES Cl[Hg][Hg]Cl
Properties
Chemical formula	Hg2Cl2
Molar mass	472.09 g/mol
Appearance	White 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)
Solubility in water	0.2 mg/100 mL
Solubility	insoluble in ethanol, ether
Magnetic susceptibility (χ)	−26.0·10−6 cm3/mol
Refractive index (nD)	1.973
Hazards
Safety data sheet	ICSC 0984
EU classification (DSD)	Harmful (Xn) Dangerous for the environment (N)
R-phrases	R22, R36/37/38, R50/53
S-phrases	(S2), S13, S24/25, S46, S60, S61
NFPA 704	0 3 0
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (median dose)	210 mg/kg (rat, oral)
Thermochemistry
Std molar entropy (So298)	196 J·mol−1·K−1
Std enthalpy of formation (ΔfHo298)	−265 kJ·mol−1
Related compounds
Other anions	Mercury(I) fluoride Mercury(I) bromide Mercury(I) iodide
Other cations	Mercury(II) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N  (what is YN ?)
Infobox references

Mercury(I) chloride is the chemical compound with the formula Hg₂Cl₂. 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 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 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, for example to treat George III in 1801, and disinfectant, as well as in the treatment of syphilis, until the early twentieth 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 amongst crops of the Brassicaceae family.

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 eighteenth 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. 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.