Mercury(II) chloride

Mercury(II) chloride, more commonly called mercuric chloride, 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 in water.

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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 chloride are more distance at 3.38 Å.^[1]

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 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 applcation, the mercuric chloride is supported on carbon in concentrations of ca 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).^[2]

As a chemical reagent

Mercuric chloride is often used to form an amalgam with metals, such as aluminium. When aluminium strips are soaked in mercuric chloride solution, they quickly become covered by a thin layer of mercury. 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). Halocarbon 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 aluminum is also used as a reducing agent in organic synthesis. Another metal commonly amalgated using mercuric chloride include zinc.

Mercuric chloride is used to remove dithiane groups attached to a carbonyl in an umpolung reaction.

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.^[3]

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.^[4] It finds minor use in tanning, and wood was preserved by kyanizing (soaking in mercuric chloride) beginning in 1848.^[5]

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.^[6]

Toxicity

Mercury(II) chloride is highly toxic and corrosive. Once absorbed into the bloodstream, Hg²⁺ combines with proteins in the plasma or enters the red blood cells. It does not readily pass into the brain or fetus but may enter into other body organs.^{[citation needed]} The liver is a major site of metabolism for mercury, and all mercury absorbed from the stomach and intestine is carried in blood directly to the liver. It accumulates in the kidneys, and may cause severe damage. Poisoning can result from inhalation, ingestion, or absorption through the skin.

Inhalation may result in corrosive bronchitis, interstitial pneumonitis, and death. Systemic effects following inhalation exposure may include shock, renal disorders, and central nervous system effects characterized by lethargy and neurobehavioral effects (insomnia, loss of memory, excitability, etc). Chronic exposure to low levels of vapor may result in central nervous system effects including fatigue, tremors, and gingivitis. As exposure increases, the frequency and magnitude of muscle tremors increase and are accompanied by personality and behavioral changes (memory loss, excitability, depression, and hallucinations).

Ingestion may cause severe gastrointestinal irritation, renal failure, and death with acute lethal doses in humans ranging from 1 to 4 g. The toxic effects are usually evident within 10-15 minutes of ingestion. Death can occur within 24 hours, resulting from shock, renal damage, severe gastrointestinal damage or kidney failure. Chronic symptoms include increased salivation, bleeding gums and loosening of the teeth.

Dermal contact with mercuric chloride may cause dermatitis and neurological effects. Acrodynia occurs in children and is characterised by a generalised body rash. Other symptoms include swelling and irritation of the hands, feet, cheeks and nose, hair loss, irritability, insomnia, and profuse perspiration which may lead to dehydration. Chronic exposure through absorption is usually the result of regular applications of topical ointments containing mercuric chloride.

In popular culture

Humbert Humbert, the protagonist in Vladimir Nabakov's novel Lolita, contemplates killing his child lover's mother with "five bichloride-of-mercury tablets in her preprandial sherry.", and newspaper headlines in the early part of the 20th century suggest that this compound was popular method of suicide and attempted suicide. Perhaps the most famous^{[citation needed]} person to succumb to what the papers called "mercury bichloride" or "bi-chloride of mercury" was silent film star Olive Thomas whose 1920 death in Paris was ruled accidental.

In Patrick O'Brian's novel The Wine-Dark Sea, set in the early 19th century, the character Nathaniel Martin is a physician's assistant. He nearly poisons himself with "The Vienna Treatment", a preparation of the "corrosive sublimate" which is considered extremely dangerous in the wrong hands.

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