Mercury(II) chloride (mercury bichloride, mercury dichloride, mercuric chloride), historically also sulema or corrosive sublimate, is the inorganic chemical compound of mercury and chlorine with the formula HgCl2, used as a laboratory reagent. It is a white crystalline solid and a molecular compound that is very toxic to humans. Once used as a first line treatment for syphilis, it has been replaced by the more effective and less toxic procaine penicillin since at least 1948.
Synthesis Mercuric chloride is obtained by the action of chlorine on mercury or on mercury(I) chloride. It can also be produced by the addition of hydrochloric acid to a hot, concentrated solution of mercury(I) compounds such as the nitrate:
Hg2(NO3)2 + 4 HCl → 2 HgCl2 + 2 H2O + 2 NO2
Heating a mixture of solid mercury(II) sulfate and sodium chloride also affords volatile HgCl2, which can be separated by sublimation.
Properties Mercuric chloride is not a salt composed of discrete ions, but it is made of linear triatomic molecules, hence its tendency to sublime. In the crystal, each mercury atom is bonded to two chloride ligands with Hg–Cl distance of 2.38 Å; six other chlorides are more distant at 3.38 Å.Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. .
Its solubility in water increases from 6% at to 36% at .
Applications The main application of mercuric chloride is as a catalyst for the conversion of acetylene to vinyl chloride, the precursor to polyvinyl chloride: 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).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. It is being used in plant tissue culture for surface sterilisation of explants such as leaf or stem nodes.
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, thus making it inert. Amalgamated aluminium exhibits a variety of reactions not observed for aluminium itself. For example, amalgamated aluminum reacts with water generating Al(OH)3 and hydrogen gas. 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.
History Discovery of the mineral acids Around 900, the authors of the Arabic writings attributed to Jabir ibn Hayyan (Latin: Geber) and the Persian physician and alchemist Abu Bakr al-Razi (Latin: Rhazes) were experimenting with sal ammoniac (ammonium chloride), which when it was distilled together with vitriol (hydrated sulfates of various metals) produced hydrogen chloride. vol. II, pp. 41–42; pp. 141-142. It is possible that in one of his experiments, al-Razi stumbled upon a primitive method to produce hydrochloric acid. p. 333. The relevant recipe reads as follows: "Take equal parts of sweet salt, Bitter salt, salt, salt, Indian salt, salt of , and salt of Urine. After adding an equal weight of good crystallised Sal-ammoniac, dissolve by moisture, and distil (the mixture). There will distil over a strong water, which will cleave stone () instantly." (p. 333) For a glossary of the terms used in this recipe, see p. 322. German translation of the same passage in p. 182, §5. An English translation of Ruska 1937's translation can be found in pp. 139–140. However, it appears that in most of these early experiments with chloride salts, the gaseous products were discarded, and hydrogen chloride may have been produced many times before it was discovered that it can be put to chemical use..
One of the first such uses of hydrogen chloride was in the synthesis of mercury(II) chloride (corrosive sublimate), whose production from the heating of mercury either with alum and ammonium chloride or with vitriol and sodium chloride was first described in the ("On Alums and Salts").. On the , see further , See also more briefly pp. 40–43, and the sources cited in . See also pp. 106–107. This eleventh- or twelfth-century Arabic alchemical text is anonymous in most manuscripts, though some manuscripts attribute it to Hermes Trismegistus, and a few falsely attribute it to Abu Bakr al-Razi.. On the false attribution to al-Razi, see and the sources cited there. Moureau 2020, p. 117 stresses that the only Latin work which in the current state of research is known to be a translation of an authentic Arabic work by al-Razi is the , an interpolated paraphrase of al-Razi's . It was translated into Hebrew and two times into Latin, with one Latin translation by ..
In the process described in the , hydrochloric acid started to form, but it immediately reacted with the mercury to produce mercury(II) chloride. Thirteenth-century Latin alchemists, for whom the was one of the main reference works, were fascinated by the chlorinating properties of mercury(II) chloride, and they eventually discovered that when the metals are eliminated from the process of heating vitriols, alums, and salts, strong mineral acids can directly be distilled..
Historical 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.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.
Historical 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". This was done to prevent the specimens' destruction by moths, mites and mold. 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).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. 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.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 The process was generally abandoned because mercuric chloride was water-soluble and not effective for the long term, as well as being highly 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.
Historic use in medicine Mercuric chloride was a common over-the-counter disinfectant in the early twentieth century, recommended for everything from fighting measles germs to protecting fur coats and exterminating red ants. A New York physician, Carlin Philips, wrote in 1913 that "it is one of our most popular and effective household antiseptics", but so corrosive and poisonous that it should only be available by prescription. A group of physicians in Chicago made the same demand later the same month. The product frequently caused accidental poisonings and was used as a suicide method.
It was used by Arab physicians in the Middle Ages to disinfect wounds. It continued to be used by Arab physicians 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. Both mercuric-chloride treatment for syphilis and poisoning during the course of treatment were so common that the latter's symptoms were often confused with those of syphilis. This use of "salts of white mercury" is referred to in the English-language folk song "The Unfortunate Rake".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.
Yaws was treated with mercuric chloride (labeled as Corrosive Sublimate) before the advent of antibiotics. It was applied topically to alleviate ulcerative symptoms. Evidence of this is found in Jack London's book The Cruise of the Snark in the chapter entitled "The Amateur M.D."
Between 1901 and 1904 the US Marine Hospital Service quarantined and engaged in an extensive disinfection program of San Francisco's Chinatown in response to an epidemic of bubonic plague. This program forced the closure of over 14,000 rooms and the eviction of thousands of Chinese residents whose dwellings were rendered toxic and uninhabitable from the disinfection program. Long-term mercury pollution is a concern for construction workers in Chinatown to this day.
Historic use in crime and accidental poisonings In 1613, whilst imprisoned in the Tower of London, Thomas Overbury was poisoned with an enema of mercury sublimate. The following trial saw the downfall of the murderers, Robert Carr and his wife, Frances. In Volume V of Alexandre Dumas' Celebrated Crimes, he recounts the history of Antoine François Desrues, who killed noblewoman Madame de Lamotte with "corrosive sublimate." In 1906 in New York, Richard Tilghman died after mistaking bichloride of mercury tablets for lithium citrate. Actor Lon Chaney's estranged wife Cleva attempted suicide by swallowing mercuric chloride in 1913. Although the attempt failed, the toxic effects ruined her singing career. In a highly publicized case in 1920, mercury bichloride was reported to have caused the death of 25-year-old American silent film star Olive Thomas. While vacationing in France, she accidentally ingested the compound, which had been prescribed to her husband Jack Pickford in liquid topical form to treat his syphilis. Contemporary press speculation was that she drank the poison intentionally, but there is no good evidence for this. Thomas died five days later.Foster, Charles (2000). Stardust and Shadows: Canadians in Early Hollywood, page 257. Toronto, Canada: Dundurn Press, 2000. .