Mercury(I) chloride

From Wikipedia, the free encyclopedia
  (Redirected from Calomel)
Jump to: navigation, search
Mercury(I) chloride
Mercury Chloride.jpg
Calomel-2D.png
Calomel-xtal-3D-vdW.png
Identifiers
CAS number 10112-91-1 YesY
PubChem 24956
ChemSpider 16740467 YesY
EC number 233-307-5
UN number 3077 keshav
ChEBI CHEBI:33050 YesY
RTECS number OV8750000
Jmol-3D images Image 1
Properties
Molecular 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
Refractive index (nD) 1.973
Hazards
MSDS ICSC 0984
EU Index 080-003-00-1
EU classification 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
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
Thermochemistry
Std molar
entropy
So298
196 J·mol−1·K−1[1]
Std enthalpy of
formation
ΔfHo298
−265 kJ·mol−1[1]
Related compounds
Other anions Mercury(I) fluoride
Mercury(I) bromide
Mercury(I) iodide
Other cations Mercury(II) chloride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
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.[2][3]

History[edit]

The name calomel is thought to come from the Greek καλός beautiful, and μέλας black. This 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 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 amongst crops of the Brassicaceae family.[4]

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.[5] 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.[6]

In popular culture[edit]

Calomel is mentioned in Gilbert & Sullivan's operetta Patience (1881), during a poem recitation in the first act, by the character Bunthorne, entitled "Oh, Hollow! Hollow! Hollow!":

What time the poet hath hymned
The writhing maid, lithe-limbed,
Quivering on amaranthine asphodel,
How can he paint her woes,
Knowing, as well he knows,
That all can be set right with calomel?

When from the poet's plinth
The amorous colocynth
Yearns for the aloe, faint with rapturous thrills,
How can he hymn their throes
Knowing, as well he knows,
That they are only uncompounded pills?

Is it, and can it be,
Nature hath this decree,
Nothing poetic in the world shall dwell?
Or that in all her works
Something poetic lurks,
Even in colocynth and calomel?
I cannot tell.

Properties[edit]

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:

Ball-and-stick model of calomel's unit cell
Ball-and-stick model of the distorted octahedral coordination of mercury in calomel
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.[7] 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[edit]

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.

2HCl + Hg2(NO3)2 → Hg2Cl2 + 2HNO3

Ammonia causes Hg2Cl2 to disproportionate:

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

Calomel electrode[edit]

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[edit]

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 is produced.

2HgCl2 + (NH4)2C2O4 + Light → Hg2Cl2(s) + 2[NH4+][Cl] + 2CO2

This particular reaction was discovered by J.M. Eder (hence the name Eder reaction) in 1880 and reinvestigated by W. E. Rosevaere in 1929[8]

Related mercury(I) compounds[edit]

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[edit]

Main article: Mercury poisoning

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 (as used by Revolutionary War physician Dr. Benjamin Rush, vide supra) 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.[9] 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 U.S., Canada, Japan and the European Union.[10] 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.[11]

References[edit]

  1. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A22. ISBN 0-618-94690-X. 
  2. ^ Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. pp. 696–697. ISBN 978-0130399137. 
  3. ^ Skoog, Douglas A., F. James Holler and Timothy A. Nieman (1998). Principles of Instrumental Analysis (5th ed.). Saunders College Pub. pp. 253–271. ISBN 0-03-002078-6. 
  4. ^ Buczacki, S., Pests, Diseases and Disorders of Garden Plants, Collins, 1998, pp 449-50. ISBN 0-00-220063-5
  5. ^ Koehler, Christopher S. W. (January 2001). "Heavy Metal Medicine". Today's Chemist at Work (American Chemical Society) 10 (1): 61–65. ISSN 1062-094X. Retrieved 2009-02-02. 
  6. ^ Johnston, Elizabeth Lichtenstein (1901). Recollections of a Georgia Loyalist. The Bankside Press. pp. 82–83. 
  7. ^ Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications ISBN 0-19-855370-6
  8. ^ W. E. Roseveare (1930). "The X-Ray Photochemical Reaction between Potassium Oxalate and Mercuric Chloride". J. Am. Chem. Soc. 52 (7): 2612–2619. doi:10.1021/ja01370a005. 
  9. ^ Sneader, Walter (2005). Drug Discovery: A History. John Wiley and Sons. pp. 45–46. ISBN 0-471-89980-1. Retrieved 2009-02-02. 
  10. ^ "Commission Directive 86/199/EEC, OJ L 149, p. 38 of 3.6.1986". 
  11. ^ D. Gonzalez-Ramirez, M. Zuniga-Charles, A. Narro-Juarez, Y. Molina-Recio, K. M. Hurlbut, R. C. Dart and 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). Journal of Pharmacology and Experimental Therapy 287 (1): 8–12. PMID 9765315. 

External links[edit]