Mercury (element)
Mercury, also called quicksilver, is a chemical element in the periodic table that has the symbol Hg (from the Greek hydrargyrum, for watery (or liquid) silver) and atomic number 80. A heavy, silvery, transition metal, mercury is one of only two elements that are liquid at room temperature (the other is bromine). Mercury is used in thermometers, barometers and other scientific apparatuses. Mercury is mostly obtained by reduction from the mineral cinnabar.
Notable characteristics
Mercury is a relatively poor conductor of heat but is a good conductor of electricity.
Mercury easily forms alloys with almost all common metals, including gold, aluminium, and silver, but not iron. Tellurium forms an alloy also, but it reacts slowly to form mercury telluride. The reaction of mercury with sulfur is more easily noticed. Any of these alloys is called an amalgam.
This metal also has uniform volumetric thermal expansion, is less reactive than zinc and cadmium and does not displace hydrogen from acids. Common oxidation states of this element are +1 and +2. Rare instances of +3 mercury compounds exist.
The commercial unit for handling mercury is the “flask,” which weighs 76 lb (34.5 kg).
Applications
Mercury is used primarily for the manufacture of industrial chemicals or for electrical and electronic applications. It is used in some thermometers, especially ones which are used to measure high temperatures (Non-prescription sale of mercury fever thermometers was banned by the U.S. Senate in 2002). Other uses:
– Mercury sphygmomanometers (banned in some states and hospitals).
– Thimerosal, an organic compound used as a preservative in vaccines and tattoo inks (Thimerosal in vaccines (http://www.fda.gov/cber/vaccine/thimerosal.htm)).
– Mercury barometers, diffusion pumps, coulometers, and many other laboratory instruments.
– The triple point of mercury, -38.8344 °C, is a fixed point used as a temperature standard for the International Temperature Scale (ITS-90).
– In some gaseous electron tubes, mercury arc rectifier
– Gaseous mercury is used in mercury-vapor lamps and some “neon sign” type advertising signs and fluorescent lamps.
– Mercury was once used in the amalgamation process of refining gold and silver ores. The practice is continued by the garimpeiros (gold miners) of the Amazon basin in Brazil.
– Mercury is still used in some cultures for folk medicine and ceremonial purposes which may involve ingestion, injection, or the sprinkling of elemental mercury around the home.
Miscellaneous uses: mercury switches, mercury cells for sodium hydroxide and chlorine production, electrodes in some types of electrolysis, batteries (mercury cells), and catalysts, herbicides (discontinued in 1995), insecticides, and dental amalgams/preparations.
Historical uses: preserving wood, developing daguerreotypes, “silvering” mirrors, anti-fouling paints (discontinued in 1990), cleaning, and in road leveling devices in cars. Mercury compounds have been used in antiseptics, laxatives, antidepressants, and antisyphilitics.
Mercury was known to the ancient Chinese and Hindus and was found in Egyptian tombs that date from 1500 BCE. In China, India and Tibet, mercury use was thought to prolong life, heal fractures, and maintain generally good health. The ancient Greeks used mercury in ointments and the Romans used it in cosmetics. By 500 BCE mercury was used to make amalgams with other metals.
The Indian word for alchemy is “Rassayana” which means “˜the way of mercury.’ Alchemists thought of mercury as the first matter from which all metals were formed. Different metals could be produced by varying the quality and quantity of sulfur contained within the mercury. An ability to transform mercury into any metal resulted from the essentially mercurial quality of all metals. The purest of these was gold, and mercury was required for the transmutation of base (or impure) metals into gold. This was a primary goal of alchemy, either for material or spiritual gain.
Hg is the modern chemical symbol for mercury. It comes from hydrargyrum, a Latinised form of the Greek word hydrargyros, which is a compound word meaning ‘water’ and ‘silver’ “” since it is liquid, like water, and yet has a silvery metallic sheen. The element was named after the Roman god Mercury, known for speed and mobility. It is associated with the planet Mercury. The astrological symbol for the planet is also one of the alchemical symbols for the metal (left). Mercury is the only metal for which the alchemical planetary name became the common name.
From the mid-18th to the mid-19th centuries, a process called “carroting” was used in the making of felt hats. Animal skins were rinsed in an orange solution of the mercury compound mercuric nitrate, Hg(NO3)2″¢2H2O. This process separated the fur from the pelt and matted it together. This solution and the vapors it produced were highly toxic. Its use resulted in widespread cases of mercury poisoning among hatters. Symptoms included tremors, emotional lability, insomnia, dementia and hallucinations. The United States Public Health Service banned the use of mercury in the felt industry in December 1941. The psychological symptoms associated with mercury poisoning may have inspired the simile “mad as a hatter”, and thereby the Mad Hatter of Alice in Wonderland fame.
Dentistry
Elemental mercury is the main ingredient in dental amalgams. Controversy over the health effects from the use of mercury amalgams began shortly after its introduction into the western world, nearly 200 years ago. In 1843, The American Society of Dental Surgeons, concerned about mercurial poisoning, required its members to sign a pledge that they would not use amalgam. In 1859, The American Dental Association was formed by dentists who believed amalgam was, “safe and effective.” The ADA, “continues to believe that amalgam is a valuable, viable and safe choice for dental patients,” as written in their statement on dental amalgam (http://www.ada.org/prof/resources/positions/statements/amalgam.asp) . In 1993, the United States Public Health Service reported that, “amalgam fillings release small amounts of mercury vapor,” but in such a small amount that it, “has not been shown to cause any “¦ adverse health effects.” In 2002, California became the first state to ban the future use of mercury fillings (effective 2006). As of 2005, the controversy continues.
Medicine
Mercury was used in the treatment of illnesses for centuries. Mercury(I) chloride and mercury(II) chloride were popular compounds. Mercury was included in the treatment of syphilis as early as the 16th century, before the advent of antibiotics. “Blue mass,” a small pill in which mercury is the main ingredient, was prescribed throughout the 1800s for numerous conditions including, constipation, depression, child-bearing and toothaches (National Geographic (http://news.nationalgeographic.com/news/2001/07/0717_lincoln.html)). In the early 20th century, mercury was administered to children yearly as a laxative and dewormer. It was a teething powder for infants and some vaccines have contained the preservative Thimerosal (partly ethyl mercury) since the 1930s (FDA report (http://www.fda.gov/cber/vaccine/thimerosal.htm)). Mercury(II) chloride was a disinfectant for doctors, patients and instruments.
Mercuric medicines and devices are inherently hazardous. Neither are used to the extent they were in the past. Thermometers and sphygmomanometers containing mercury were invented in the early 18th and late 19th centuries, respectively. In the early 21st century, their use is declining and has been banned in some countries, states and medical institutions. In 2002, the U.S. Senate passed legislation to phase out the sale of non-prescription mercury thermometers. In 2003, Washington and Maine became the first states to ban mercury blood pressure devices (HCWH News release (http://www.noharm.org/details.cfm?type=document&ID=782)). In 2005, mercury compounds are found in some OTC medications, including, topical antiseptics, stimulant laxatives, diaper rash ointment, eye drops and nose sprays. The FDA has “inadequate data to establish general recognition of the safety and effectiveness,” of the mercury ingredients in these products (Code of federal regulations (http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=310.545&SearchTerm=mercury)).
Mineral occurrence
A rare element in the earth’s crust, mercury is found either as a native metal (rare) or in cinnabar, corderoite, livingstonite, and other minerals with cinnabar (HgS) being the most common ore. Approximately 50% of the global supply comes from Spain and Italy, with much of the rest coming from Slovenia, Russia, and North America. The metal is extracted by heating cinnabar in a current of air and condensing the vapor
Compounds
The most important salts are:
– Mercury(I) chloride (AKA calomel and is sometimes still used in medicine)
– Mercury(II) chloride (which is very corrosive, sublimates and is a violent poison)
– Mercury fulminate, (a detonator widely used in explosives),
– Mercury(II) sulfide (AKA vermilion which is a high-grade paint pigment),
– Mercury(II) selenide a semi-metal,
– Mercury(II) telluride a semi-metal, and
– Mercury cadmium telluride an infrared detector material.
Organic mercury compounds are also important. Laboratory tests have found that an electrical discharge causes the noble gases to combine with mercury vapor. These compounds are held together with van der Waals forces and result in HgNe, HgAr, HgKr, and HgXe. Methylmercury is a dangerous compound that is widely found as a pollutant in water bodies and streams.
Isotopes
There are seven stable isotopes of mercury with Hg-202 being the most abundant (29.86%). The longest-lived radioisotopes are Hg-194 with a half-life of 444 years, and Hg-203 with a half-life of 46.612 days. Most of the remaining radioisotopes have half-lifes that are less than a day.
Occurrence in the environment
Preindustrial deposition rates of mercury from the atmosphere may be in the range of 4 ng/L in the western USA. Although that can be considered a natural level of exposure, regional or global sources have significant effects. Volcanic eruptions can increase the atmospheric source by 4″“6 times. [1] (http://toxics.usgs.gov/pubs/FS-051-02/)
Mercury enters the environment as a pollutant from various different industries:
1. coal-fired power plants are the largest source (40% of USA emissions in 1999, which have since declined by 85%). [2] (http://www.epa.gov/mercury/faq.htm#14)
2. industrial processes
– chlorine, steel, phosphate & gold production
– metal smelting
– manufacture & repair of weather and electronic devices
3. medical applications, including vaccinations
– dentistry
– cosmetic industries
4. laboratory work involving mercury or sulfur compounds
The World Health Organization, OSHA, and NIOSH, all agree that mercury is an environmental poison and have established specific occupational exposure limits.
Inorganic mercury is less toxic than organic compounds (molecules containing carbon). Watersheds tend to concentrate mercury through erosion of mineral deposits and atmospheric deposition. Plants absorb mercury when wet but may emit it in dry air. [3] (http://www.ornl.gov/info/ornlreview/rev29-12/text/environ.htm#trees) Plant and sedimentary deposits in coal contain various levels of mercury.
Most modern governments claim that certain fish in excess quantities are unsafe due to methylmercury levels (death has been known to occur from mercury contaminated fish). The warnings especially target pregnant women. Larger species of fish, such as tuna or swordfish, are usually of greater concern than smaller species, since the mercury accumulates up the food chain.
Precautions
Elemental, liquid mercury is slightly toxic, while its vapor, compounds and salts are highly toxic and have been implicated as causing brain and liver damage when ingested, inhaled or contacted. For this reason (along with exaggeration of the actual risk by the media), most thermometers now use pigmented alcohol instead of mercury. Mercury thermometers are still occasionally used in the medical field because they are more accurate than alcohol thermometers, though both types are falling into disuse with the wide availability of electronic thermometers. Mercury thermometers are still widely used for certain scientific applications because of their greater accuracy and working range.
The main dangers associated with elemental mercury are that at STP, mercury tends to oxidize forming mercury oxide, and that if dropped or disturbed, mercury will form microscopic drops, increasing its surface area dramatically.
Even though it is far less toxic than its compounds, elemental mercury still poses significant environmental pollution and remediation problems due to the fact that mercury forms organic compounds inside of living organisms. Methylmercury works its way up the food chain, reaching high concentrations among populations of some species such as tuna. Mercury poisoning in humans will result from persistent consumption of tainted foodstuffs.
One of the most dangerous mercury compounds, dimethylmercury, is so toxic that even a few microliters spilled on the skin can cause death. One of the chief targets of the toxin is the enzyme pyruvate dehydrogenase (PDH). The enzyme is irreversibly inhibited by several mercury compounds, the lipoic acid component of the multienzyme complex binds mercury compounds tightly and thus inhibits PDH.
Mercury is a bioaccumulative toxin that is easily absorbed through the skin, respiratory and gastrointestinal tissues. Minamata disease is a form of mercury poisoning. Mercury attacks the central nervous system and endocrine system and adversely affects the mouth, gums, and teeth. High exposure over long periods of time will result in brain damage and ultimately death. It can pose a major health risk to the unborn fetus. Air saturated with mercury vapor at room temperature is at a concentration many times the toxic level, despite the high boiling point (the danger is increased at higher temperatures).
Mercury should therefore be handled with great care. Containers of mercury need to be covered securely to avoid spillage and evaporation. Heating of mercury or mercury compounds should always be done under a well-ventilated, filtered hood. Additionally, some oxides can decompose into elemental mercury, which immediately evaporates and may not be apparent.
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