27/03/2015 Mercury

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Mercury

The chemical element mercury is a shiny metallic liquid. Its chemical symbol, Hg, is derived from theGreek word hydrargyrum, meaning "liquid silver," or "quick silver." Although now obsolete, the wordquicksilver has long been used as a synonym for mercury. The element shares group IIB of theperiodic table with zinc and cadmium. The atomic number of mercury is 80; its atomic weight is200.59. Mercury is very heavy, weighing 13.6 times as much as an equal volume of water. Stone,iron, and even lead can float on its surface. Mercury occurs in only trace amounts in igneous rocks;sedimentary rocks are slightly richer.

The element constitutes only 0.5 ppm of the earth's crust, making it scarcer than uranium but moreplentiful than gold or silver. Mercury is found principally in the form of the ore cinnabar (mercurysulfide) and can also be found in the uncombined state. The preparation of mercury from its ores isfairly simple. The ore is ground up and heated to about 580 deg C in the presence of oxygen. Mercuryvapor escapes from the ores and sulfur dioxide is removed. The metal is condensed and purified bywashing with nitric acid, followed by distillation.

Mercury was among the first metals known, and its compounds have been used throughout history.Archaeologists found mercury in an Egyptian tomb dating from 1500 BC. The Egyptians and theChinese may have been using cinnabar as a red pigment for centuries before the birth of Christ. Inmany civilizations mercury was used to placate or chase away evil spirits. The alchemists thought thatmercury, which they associated with the planet Mercury, had mystical properties and used it in theirattempts to transmute base metals into gold. The Greeks knew of mercury and used it as a medicine.Mercury and mercury compounds were used from about the 15th century to the mid 20th century tocure syphilis. Because mercury is extremely toxic and its curative effect is unproven, other syphilismedicines are now used. The usefulness of mercury is limited by its poisonous nature and scarcity.

Mercury is used in electrical switches; these consist of a small tube with two contacts at one end. Ifthe tube is held in such a way that the mercury collects at this end, then contact is made and the circuitis completed. If the tube is tilted, contact is broken. Mercury switches are used in thermostats andsome doze alarm type alarm clocks. Mercury is highly suitable for use in thermometers because itdoes not moisten glass and its thermal expansion is uniform. Although many liquids could be used inpressure measuring devices, mercury is used because its high density requires less space. Mercury willdissolve numerous metals to form amalgams and is thus used to extract gold dust from rocks bydissolving the gold and then boiling off the mercury. The amalgam used in dental fillings contains tinand silver (and sometimes gold) dissolved in mercury. Mercury vapor lamps are widely used becausethey are powerful and economical sources of ultraviolet and visible light.

Mercury is also used in a number of industrial applications, such as fluid bearing and fluid clutchesthat require a heavy liquid. Mercury is a fairly unreactive metal and is highly resistant to corrosion.When heated to near its boiling point (346.72 deg C/675 deg F), mercury oxidizes in air, and mercuricoxide is formed. At 500 deg C, mercuric oxide decomposes into mercury and oxygen, a phenomenonthat led to the discovery of oxygen by Joseph Priestley and Karl Scheele. Mercuric oxide is aconstituent of mercury batteries, which have been invaluable as compact, efficient power sources inexploration of outer space. The most useful mercury salts are the two mercury chlorides and mercurysulfide. Mercurous chloride, or calomel, is a white, relatively insoluble salt. It is used in calomelelectrodes, which are commonly used in electrochemistry, and in medicine as a cathartic and diuretic.When calomel is used as a teething powder for young children, it can poison them. Mercuric chloride,or corrosive sublimate, is highly poisonous because it is so soluble. It was used for deliberatepoisonings as early as the 14th century. It is now used as a disinfectant, in preparing other mercurycompounds, and in antifungal skin ointments.

Mercuric sulfide occurs in a red form and an amorphous black form. The red form (vermilion) is used

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as a coloring material. Cinnabar is sometimes used to color tattoos red, but it causes significant skinirritations and obstructions of the lymphatic system. Mercuric fulminate is an explosive that issensitive to impact and is used in percussion caps for munitions. Mercurochrome is an organicmercury compound that is used on wounds as an antibacterial agent. There are two types of mercurypoisoning, acute and chronic. Acute mercury poisoning results from the ingestion of soluble mercurysalts, which violently corrode skin and mucous membranes. Although cases have occurred in whichpersons have ingested elemental mercury without suffering permanent damage, mercury vaporaspirated into the lungs can cause severe pneumonia and death. Chronic mercury poisoning occursthrough the regular absorption of small amounts of mercury. This condition is often a disease ofworkers in mercury mines, laboratories, and industries that use mercury. The most toxic mercurycompounds are those that are fat soluble, because this property assists in their distribution throughoutthe body. Methyl mercury compounds, such as dimethyl mercury, are among the most dangerous.Mercury salts released into the environment may frequently be converted by anaerobic bacteria intosuch compounds, which can then be carried through the food chain to humans as in the disaster atMinamata Bay, Japan Other micro organisms can convert methyl mercury compounds into theinsoluble, and therefore harmless, mercury sulfide.