Mercury(II) chloride

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Mercury(II) chloride
Ball-and-stick model of the crystal structure
Space-filling model of the crystal structure
Mercury(II) chloride
Identifiers
CAS number 7487-94-7 YesY
PubChem 24085
ChemSpider 22517 N
EC number 231-299-8
UN number 1624
KEGG C13377 N
RTECS number OV9100000
ATC code D08AK03
Jmol-3D images Image 1
Properties
Molecular formula HgCl2
Molar mass 271.52 g/mol
Appearance colorless or white solid
Odor odorless
Density 5.43 g/cm3
Melting point 276 °C (529 °F; 549 K)
Boiling point 304 °C (579 °F; 577 K)
Solubility in water 3.6 g/100 mL (0 °C)
7.4 g/100 mL (20 °C)
48 g/100 mL (100 °C)
Solubility 4 g/100 mL (ether)
soluble in alcohol, acetone, ethyl acetate
slightly soluble in benzene, CS2, pyridine
Acidity (pKa) 3.2 (0.2M solution)
Refractive index (nD) 1.859
Structure
Crystal structure orthogonal
Coordination
geometry
linear
Molecular shape linear
Dipole moment zero
Thermochemistry
Std molar
entropy
So298
144 J·mol−1·K−1[1]
Std enthalpy of
formation
ΔfHo298
−230 kJ·mol−1[1]
Hazards
MSDS ICSC 0979
EU Index 080-010-00-X
EU classification Very toxic (T+)
Corrosive (C)
Dangerous for the environment (N)
R-phrases R28, R34, R48/24/25, R50/53
S-phrases (S1/2), S36/37/39, S45, S60, S61
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 4: Very short exposure could cause death or major residual injury. E.g., VX 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
Related compounds
Other anions Mercury(II) fluoride
Mercury(II) bromide
Mercury(II) iodide
Other cations Zinc chloride
Cadmium chloride
Mercury(I) chloride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Mercury(II) chloride or mercuric chloride (archaically, corrosive sublimate) is the chemical compound of mercury and chlorine with the formula HgCl2. This white crystalline solid is a laboratory reagent and a molecular compound. Once used as a treatment for syphilis, it is no longer used for medicinal purposes because of mercury toxicity and the availability of superior treatments.

Production and basic properties[edit]

Mercuric chloride is not a salt but a linear triatomic molecule, hence its tendency to sublime. In the crystal, each mercury atom is bonded to two close chloride ligands with Hg—Cl distance of 2.38 Å; six more chlorides are more distant at 3.38 Å.[2]

Mercuric chloride is obtained by the action of chlorine on mercury or mercury(I) chloride, by the addition of hydrochloric acid to a hot, concentrated solution of mercury(I) compounds such as the nitrate:

HgNO3 + 2 HCl → HgCl2 + H2O + NO2,

Heating a mixture of solid mercury(II) sulfate and sodium chloride also affords volatile HgCl2, which sublimes and condenses in the form of small rhombic crystals.

Its solubility increases from 6% at 20 °C (68 °F) to 36% in 100 °C (212 °F). In the presence of chloride ions, it dissolves to give the tetrahedral coordination complex [HgCl4]2−.

Applications[edit]

The main application of mercuric chloride is as a catalyst for the conversion of acetylene to vinyl chloride, the precursor to polyvinylchloride:

C2H2 + HCl → CH2=CHCl

For this application, the mercuric chloride is supported on carbon in concentrations of about 5 weight percent. This technology has been eclipsed by the thermal cracking of 1,2-dichloroethane. Other significant applications of mercuric chloride include its use as a depolarizer in batteries and as a reagent in organic synthesis and analytical chemistry (see below).[3] It is being used in plant tissue culture for surface sterilisation of explants such as leaf or stem nodes.

As a chemical reagent[edit]

Mercuric chloride is occasionally used to form an amalgam with metals, such as aluminium. Upon treatment with an aqueous solution of mercuric chloride, aluminium strips quickly become covered by a thin layer of the amalgam. Normally, aluminium is protected by a thin layer of oxide, thus making it inert. Once amalgamated, aluminium can undergo a variety of reactions. For example, it will dissolve in water (this can be dangerous, as hydrogen gas and heat are generated). Halocarbons react with amalgamated aluminium in the Barbier reaction. These alkylaluminium compounds are nucleophilic and can be used in a similar fashion to the Grignard reagent. Amalgamated aluminium is also used as a reducing agent in organic synthesis. Zinc is also commonly amalgamated using mercuric chloride.

Mercuric chloride is used to remove dithiane groups attached to a carbonyl in an umpolung reaction. This reaction exploits the high affinity of Hg2+ for anionic sulfur ligands.

Mercuric chloride may be used as a stabilising agent for chemicals and analytical samples. Care must be taken to ensure that detected mercuric chloride does not eclipse the signals of other components in the sample, such as is possible in gas chromatography.[4]

Historic use in photography[edit]

Mercury(II) chloride was used as a photographic intensifier to produce positive pictures in the collodion process of the 1800s. When applied to a negative, the mercury(II) chloride whitens and thickens the image, thereby increasing the opacity of the shadows and creating the illusion of a positive image.[5]

Historic use in preservation[edit]

For the preservation of anthropological and biological specimens during the late 19th and early 20th centuries, objects were dipped in or were painted with a "mercuric solution".[why?] Objects in drawers were protected by scattering crystalline mercuric chloride over them.[6] It finds minor use in tanning, and wood was preserved by kyanizing (soaking in mercuric chloride).[7] Mercuric chloride was one of the three chemicals used for railroad tie wood treatment between 1830 and 1856 in Europe and the United States. Limited railroad ties were treated in the United States until there were concerns over lumber shortages in the 1890s.[8] The process was generally abandoned because mercuric chloride was water soluble and not effective for the long term, as well as poisonous. Furthermore, alternative treatment processes, such as copper sulfate, zinc chloride, and ultimately creosote; were found to be less toxic. Limited kyanizing was used for some railroad ties in the 1890s and early 1900s.[9]

Historic use in medicine[edit]

Mercuric chloride was used to disinfect wounds by Arab physicians in the Middle Ages.[10] It continued to be used by Arab doctors into the twentieth century, until modern medicine deemed it unsafe for use.

Syphilis was frequently treated with mercuric chloride before the advent of antibiotics. It was inhaled, ingested, injected, and applied topically. Poisoning was so common that its symptoms were confused with those of syphilis. This usage of "salts of white mercury" is referred to in the English folk-song, "The Unfortunate Rake".[11]

Toxicity[edit]

Main article: Mercury poisoning

Mercuric chloride is highly toxic, both acutely and as a cumulative poison.

References[edit]

  1. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A22. ISBN 0-618-94690-X. 
  2. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
  3. ^ Matthias Simon, Peter Jönk, Gabriele Wühl-Couturier, Stefan Halbach "Mercury, Mercury Alloys, and Mercury Compounds" in Ullmann's Encyclopedia of Industrial Chemistry 2006: Wiley-VCH, Weinheim. doi:10.1002/14356007.a16_269.pub2
  4. ^ Foreman, W. T.; Zaugg, S. D.; Faires, L. M.; Werner, M. G.; Leiker, T. J.; Rogerson, P. F. (1992). "Analytical interferences of mercuric chloride preservative in environmental water samples: Determination of organic compounds isolated by continuous liquid-liquid extraction or closed-loop stripping". Environmental Science & Technology 26 (7): 1307. doi:10.1021/es00031a004.  edit
  5. ^ Towler, J. (1864). Stereographic negatives and landscape photography. Chapter 28. In: The silver sunbeam: a practical and theoretical textbook of sun drawing and photographic printing. Retrieved on April 13, 2005.
  6. ^ Goldberg, L. (1996). A history of pest control measures in the anthropology collections, national museum of natural history, Smithsonian Institution. JAIC 35(1) 23–43. Retrieved on April 17, 2005.
  7. ^ Freeman, M.H. Shupe, T.F. Vlosky, R.P. Barnes, H.M. (2003). Past, present and future of the wood preservation industry. Forest Products Journal. 53(10) 8–15. Retrieved on April 17, 2005.
  8. ^ Pg. 19-75 "Date Nails and Railroad Tie Preservation" (3 vol.; 560 p.), published in 1999 by the Archeology and Forensics Laboratory, University of Indianapolis; Jeffrey A. Oaks
  9. ^ <History of Railroad Tie Preservation by Jeffrey A. Oaks, Univ. of Indiana, Pg. 20-30 and Pg. 64, Table I http://facstaff.uindy.edu/~%20oaks/Articles/History.pdf >
  10. ^ Maillard, Adam P. Fraise, Peter A. Lambert, Jean-Yves (2007). Principles and Practice of Disinfection, Preservation and Sterilization. Oxford: John Wiley & Sons. p. 4. ISBN 0470755067. 
  11. ^ Pimple, K.D. Pedroni, J.A. Berdon, V. (2002, July 09). Syphilis in history. Poynter Center for the Study of Ethics and American Institutions at Indiana University-Bloomington. Retrieved on April 20, 2008.

External links[edit]