corrosive sublimate

corrosive sublimate: see mercuric chloride.

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Mercury(II) chloride or mercuric chloride (formerly corrosive sublimate), is the chemical compound with the formula HgCl₂. This white crystalline solid is a laboratory reagent. It was formerly used more widely, however it is one of the most toxic forms of mercury because it is more soluble than most other forms in water.

Production and basic properties

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; four more chlorides are more distant at 3.38 Å.

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 to 36% in boiling water. In the presence of chloride ions, it dissolves to give the tetrahedral coordination complex [HgCl₄]^2-.

Applications

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).

As a chemical reagent

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 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.

Historic use in photography

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.

Historic use in preservation

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." Objects in drawers were protected by scattering crystalline mercuric chloride over them. It finds minor use in tanning, and wood was preserved by kyanizing (soaking in mercuric chloride) beginning in 1848.

Historic use in medicine

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.

Toxicity

Mercuric chloride is highly toxic, not only acutely but as a cumulative poison.

References

External links

• Agency for toxic substances and disease registry. (2001, May 25). Toxicological profile for Mercury Retrieved on April 17, 2005.
• National institutes of health. (2002, October 31). Hazardous substances data bank: Mercuric chloride Retrieved on April 17, 2005.
• Young, R.(2004, October 6). Toxicity summary for mercury The risk assessment information system. Retrieved on April 17, 2005.
• ATSDR - ToxFAQs: Mercury
• ATSDR - Public Health Statement: Mercury
• ATSDR - Medical Management Guidelines (MMGs) for Mercury (Hg)
• ATSDR - Toxicological Profile: Mercury
• National Pollutant Inventory - Mercury and compounds Fact Sheet
• NIOSH Pocket Guide to Chemical Hazards
• Mercury chloride toxicity - includes excerpts from research reports.