Mercury(II) chloride

Mercury(II) chloride (also called mercuric chloride and corrosive sublimate) is a poisonous white soluble crystalline salt of mercury. It was formerly used in insecticides, batteries; as an antiseptic, disinfectant, preservative, in metallurgy and as a photographic fixitive. Mercuric chloride is one of the most toxic forms of mercury because it easily forms organomercury complexes with proteins. Unlike other mercury halogenides it is soluble in water (~6%).

Production
Mercury(II) 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) nitrate,
 * HgNO3 + 2HCl → HgCl2 + H2O + NO2,

or by heating a mixture of mercury(II) sulfate and sodium chloride. The mercuric chloride then sublimes and condenses in the form of small rhombic crystals.

Applications
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.

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.

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.

Mercury(II) chloride is used in the manufacture of calomel, chemical reagents, metallurgy, tanning, as a catalyst for vinyl chloride and in electroplating. Wood was preserved by kyanizing (soaking in mercuric chloride) beginning in 1848.

Mercuric chloride is often used to form an amalgam with mercury soluble metals, such as aluminum. When aluminum strips are soaked in mercuric chloride solution, they quickly become covered by a thin layer of mercury. Normally, aluminum is protected by a thin layer of oxide making it inert, but amalgamated aluminum can undergo a variety of reactions - it will dissolve in water (this can be dangerous, as hydrogen gas and heat are generated.) When added to a halocarbon, amalgamated aluminum can insert in the carbon-halogen bond (the Barbier reaction). These alkylaluminum compounds are nucleophilic and can be used in a similar fashion to the Grignard reagent. Amalgamated aluminum is also used to reduce carbonyls.

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

Toxicity
Mercury(II) chloride is highly toxic and corrosive. Once absorbed into the bloodstream, inorganic mercury 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. 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 accumlates 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 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.