Mercury(II) chloride (or mercury bichloride[citation needed], mercury dichloride), historically also known as sulema or corrosive sublimate, [2] 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 treatment for syphilis, it is no longer used for medicinal purposes because of mercury toxicity and the availability of superior treatments. 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:[2] 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.[2] Properties Mercuric chloride exists not as a salt composed of discrete ions, but rather is composed 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 more chlorides are more distant at 3.38 Å.[3] Its solubility increases from 6% at 20 °C (68 °F) to 36% at 100 °C (212 °F). 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).[4] 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.[5] 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.[6]