Mercury(I) chloride

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Mercury(I) chloride
Mercury Chloride.jpg
Calomel-2D.png
Calomel-xtal-3D-vdW.png
Names
IUPAC name
Dimercury dichloride
Other names
Mercury(I) chloride
Mercurous chloride
Calomel
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.030.266 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 233-307-5
25976
PubChem CID
RTECS number
  • OV8750000
UNII
UN number 3077
  • InChI=1S/2ClH.2Hg/h2*1H;;/q;;2*+1/p-2 Yes check.svgY
    Key: ZOMNIUBKTOKEHS-UHFFFAOYSA-L Yes check.svgY
  • Cl[Hg][Hg]Cl
Properties
Hg2Cl2
Molar mass 472.09 g/mol
AppearanceWhite solid
Density 7.150 g/cm3
Melting point 383 °C (721 °F; 656 K) (sublimes)
0.2 mg/100 mL
1.43×10−18 [1]
Solubility insoluble in ethanol, ether
−26.0·10−6 cm3/mol
1.973
Structure
tetragonal
Thermochemistry
Std molar
entropy (S⦵298)
196 J·mol−1·K−1 [2]
−265 kJ·mol−1 [2]
Hazards
GHS labelling:
GHS-pictogram-exclam.svg GHS-pictogram-pollu.svg
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)
3
0
0
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
210 mg/kg (rat, oral) [3]
Safety data sheet (SDS) ICSC 0984
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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Mercury(I) chloride is the chemical compound with the formula Hg2Cl2. Also known as the mineral calomel [4] (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. [5] [6]

Contents

History

The name calomel is thought to come from the Greek καλός "beautiful", and μέλας "black"; or καλός and μέλι "honey" from its sweet taste. [4] 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. [4]

Calomel was taken internally and used as a laxative, [4] 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. [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 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. [8]

Yellow fever was also treated with calomel. [9]

Lewis and Clark brought calomel on their expedition. Researchers used that same mercury, found deep in latrine pits, to retrace the locations of their respective locations and campsites. [10]

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 system, with space group I4/m 2/m 2/m. The unit cell of the crystal structure is shown below:

Calomel-unit-cell-3D-balls.png Calomel-Hg-coordination-3D-balls.png
unit celldistorted 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. 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 ]

Photochemistry

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+
4][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. [12]

Mercury(I) bromide, Hg2Br2, is 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. [13] 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. [14] 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. [15]

Related Research Articles

The term chloride refers either to a chloride ion, which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond. Many inorganic chlorides are salts. Many organic compounds are chlorides. The pronunciation of the word "chloride" is.

<span class="mw-page-title-main">Silver chloride</span> Chemical compound with the formula AgCl

Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water and its sensitivity to light. Upon illumination or heating, silver chloride converts to silver, which is signaled by grey to black or purplish coloration in some samples. AgCl occurs naturally as a mineral chlorargyrite.

<span class="mw-page-title-main">Calomel</span> 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 melas (black) because it turns black on reaction with ammonia. This was known to alchemists.

<span class="mw-page-title-main">Phosphorus trichloride</span> Chemical compound

Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxic and reacts readily with water to release hydrogen chloride.

<span class="mw-page-title-main">Mercury sulfide</span> Chemical compound

Mercury sulfide, 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.

<span class="mw-page-title-main">Phosphorous acid</span> Chemical compound

Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula RPO3H2, are called phosphonic acids.

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.

<span class="mw-page-title-main">Mercury(II) iodide</span> 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).

<span class="mw-page-title-main">Mercury(II) acetate</span> Chemical compound

Mercury(II) acetate is the chemical compound with the formula Hg(O2CCH3)2. Commonly abbreviated Hg(OAc)2, this compound is employed as a reagent to generate organomercury compounds from unsaturated organic precursors. It is a white water-soluble solid, but samples appear yellowish with time owing to decomposition.

Mercury(I) sulfide or mercurous sulfide is a hypothetical chemical compound of mercury and sulfur, with elemental formula Hg
2S. 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.

In analytical chemistry, 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 electric 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.

<span class="mw-page-title-main">Mercury(I) sulfate</span> Chemical compound

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.

<span class="mw-page-title-main">Mercuric amidochloride</span> Chemical compound

Mercuric amidochloride is an inorganic compound with the formula Hg(NH2)Cl.

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.

<span class="mw-page-title-main">Mercury(I) fluoride</span> 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.

<span class="mw-page-title-main">Mercury (element)</span> Chemical element, symbol Hg and atomic number 80

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver and was formerly named hydrargyrum from the Greek words hydro (water) and argyros (silver). A heavy, silvery d-block element, mercury is the only metallic element that is known to be liquid at standard 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+
2 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.

<span class="mw-page-title-main">Mercury(I) nitrate</span> Chemical compound

Mercury(I) nitrate is a inorganic compound, a salt of mercury and nitric acid with the formula Hg2(NO3)2. A yellow solid, the compound is used as a precursor to other complexes of Hg22+ complexes.The structure of the hydrate has been determined by X-ray crystallography. It consists of a [H2O-Hg-Hg-OH2]2+ center, with a Hg-Hg distance of 254 pm.

Mercury(I) oxide, also known as mercurous oxide, is an inorganic metal oxide with the chemical formula Hg2O.

References

  1. ↑ John Rumble (June 18, 2018). CRC Handbook of Chemistry and Physics (99 ed.). CRC Press. pp. 5–188. ISBN   978-1138561632.
  2. 1 2 Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A22. ISBN   978-0-618-94690-7.
  3. ↑ "Mercury compounds [except (organo) alkyls] (as Hg)". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  4. 1 2 3 4 Chisholm, Hugh, ed. (1911). "Calomel"  . Encyclopædia Britannica (11th ed.). Cambridge University Press.
  5. ↑ Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. pp. 696–697. ISBN   978-0-13-039913-7.
  6. ↑ Skoog, Douglas A.; Holler, F. James; Nieman, Timothy A. (1998). Principles of Instrumental Analysis (5th ed.). Saunders College Pub. pp. 253–271. ISBN   978-0-03-002078-0.
  7. ↑ Buczacki, S., Pests, Diseases and Disorders of Garden Plants, Collins, 1998, pp 449-50. ISBN   0-00-220063-5
  8. ↑ Koehler, Christopher S. W. (January 2001). "Heavy Metal Medicine". Today's Chemist at Work. 10 (1): 61–65. ISSN   1062-094X . Retrieved 2009-02-02.
  9. ↑ Johnston, Elizabeth Lichtenstein (1901). Recollections of a Georgia Loyalist...written in 1836. New York: Mansfield & Company. p.  82. pp. 82-83.
  10. ↑ Inglis-Arkell, Esther. "Archaeologists Tracked Lewis and Clark by Following Their Trail of Laxatives". io9. Retrieved 2018-11-09.
  11. ↑ Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications ISBN   0-19-855370-6
  12. ↑ Roseveare, W. E. (1930). "The X-Ray Photochemical Reaction between Potassium Oxalate and Mercuric Chloride". J. Am. Chem. Soc. 52 (7): 2612–2619. doi:10.1021/ja01370a005.
  13. ↑ Sneader, Walter (2005). Drug Discovery: A History. John Wiley and Sons. pp. 45–46. ISBN   978-0-471-89980-8 . Retrieved 2009-02-02.
  14. ↑ "Commission Directive 86/199/EEC, OJ L 149, p. 38 of 3.6.1986".
  15. ↑ D. Gonzalez-Ramirez; M. Zuniga-Charles; A. Narro-Juarez; Y. Molina-Recio; K. M. Hurlbut; R. C. Dart; H. V. Aposhian (1 October 1998). "DMPS (2,3-Dimercaptopropane-1-sulfonate, Dimaval) Decreases the Body Burden of Mercury in Humans Exposed to Mercurous Chloride" (free full text). The Journal of Pharmacology and Experimental Therapeutics. 287 (1): 8–12. PMID   9765315.
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