Mercury (element)

Mercury, also called quicksilver, is a chemical element in the periodic table that has the symbol Hg (from the Latinized Greek hydrargyrum, for watery [or liquid] silver) and atomic number 80. A heavy, silvery, transition metal, mercury is one of five elements that are liquid at or near room temperature (the others are the metals caesium, francium, and gallium, and the nonmetal bromine). Mercury is used in thermometers, barometers and other scientific apparatus, although the use of mercury in thermometers has been largely phased out in clinical and scientific environments (in favor of alcohol-filled, digital or thermistor-based replacements) in the interests of health and safety due to the toxicity of the element. Mercury is mostly obtained by reduction from the mineral cinnabar. Its high density allows things such as billiard balls to float with less than 20% of their volume submerged.

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 (In the United States, non-prescription sale of mercury fever thermometers was banned by a number of different states and localities). Other uses:

Mercury sphygmomanometers.
Thimerosal, an organic compound used as a preservative in vaccines and tattoo inks (Thimerosal in vaccines).
Mercury barometers, diffusion pumps, coulometers, and many other laboratory instruments. As an opaque liquid with a very high density, it is ideal for this role
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.
Liquid mercury was sometimes used a coolant for nuclear reactors. However sodium is proposed for reactors cooled with liquid metal, because the high density of mercury requires much energy for circulating the coolant.
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.
Alexander Calder built a mercury fountain for the Spanish Pavilion at the 1937 World's Fair in Paris.
Used in electrochemistry as part of a secondary reference electrode called the calomel electrode as an alternative to the Standard Hydrogen Electrode. This is used to work out the electrode potential of half cells.

Miscellaneous uses: mercury switches, mercury cells for sodium hydroxide and chlorine production, electrodes in some types of electrolysis, batteries (mercury cells), and catalysts, insecticides, dental amalgams/preparations and liquid mirror telescopes.

Historical uses: preserving wood, developing daguerreotypes, silvering mirrors, anti-fouling paints (discontinued in 1990), herbicides (discontinued in 1995), cleaning, and in road leveling devices in cars. Mercury compounds have been used in antiseptics, laxatives, antidepressants, and antisyphilitics. It was also allegedly used by allied spies to sabotage German planes. A mercury paste was applied to bare aluminium, causing the metal to rapidly corrode. This would cause mysterious structural failures.

In Islamic Spain it was used for filling decorative pools and for fountains [1] [2].

In some applications, mercury can be replaced with less toxic but considerably more expensive galinstan alloy.

History

Mercury was known to the ancient Chinese and Hindus and was found in Egyptian tombs that date from 1500 BC. 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 BC mercury was used to make amalgams with other metals. The Indian word for alchemy is Rassayana which means ‘the way of mercury.’ Alchemists often 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 Latinized form of the Greek word `Υδραργυρος (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 (above 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 phrase "mad as a hatter"; see the hatter article on the origin of the phrase.

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 mercury 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. 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."

Medicine

Mercury has been 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). 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). Mercury(II) chloride was a disinfectant for doctors, patients and instruments.

Mercury in the form of cinnabar remains an important component of Chinese, Tibetan, and Ayurvedic medicine. As problems may arise when these medicines are exported to countries that prohibit the use of mercury in medicines, in recent times, less toxic substitutes have been devised.

Today, mercuric medicines and devices are generally considered 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). 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).

In the European Union, RoHS legislation being introduced will ban mercury from certain products, and limit the amount of mercury in other products to less than 1000 ppm (except for certain exemptions).

Mineral occurrence

Mercury is an extremely rare element in the earth's crust, having an average crustal abundance by mass of only 0.08 parts per million. However, because it does not blend geochemically with those elements that comprise the majority of the crustal mass, mercury ores can be extraordinarily concentrated considering the element's abundance in ordinary rock. The richest mercury ores contain up to 2.5% mercury by mass, and even the leanest concentrated deposits are at least 0.1% mercury (12,000 times average crustal abundance). This makes mercury ore the most easily depleted of all metal ores. Depletion of mercury ores have been a major concern since the 1960s and it is now almost certain that the last mineable deposits were discovered in Algeria in the mid-1970s. Since the early 1970s, total world production of mercury has fallen from 9,000 tonnes to 1,600 tonnes due to depletion of reserves.

It is found either as a native metal (rare) or in cinnabar, corderoite, livingstonite, and other minerals with cinnabar (HgS) being the most common ore. Most present-day production occurs in Spain, Kyrgyzstan, China and Tajikistan. Over 100,000 tons of mercury were mined from the region of Huancavelica, Peru, over the course of three centuries following the discovery of deposits there in 1563; mercury from Huancavelica was crucial in the production of silver in colonial Spanish America. Many former ores in Italy, Slovenia, the United States and Mexico which once produced a large proportion of the world's supply have now been completely mined out. The metal is extracted by heating cinnabar in a current of air and condensing the vapor. The equation for this extraction is

HgS + O₂ → Hg + SO₂

Compounds

The most important salts are:

Mercury(I) chloride (AKA calomel) is sometimes still used in medicine and acousto-optical filters
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 cinnabar mercuric ore still used in oriental medicine, or vermilion which is a high-grade paint pigment),
Mercury(II) selenide a semi-metal,
Mercury(II) telluride a semi-metal, and
Mercury cadmium telluride and mercury zinc telluride, infrared detector materials.

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. Organic mercury compounds are also important. 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

Abundance

Crustal ~7×10^-2 mg/kg
Oceans ~3×10^-5 mg/L

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. [3]

Mercury enters the environment as a pollutant from various industries:

coal-fired power plants are the largest source (40% of USA emissions in 1999). [4]
industrial processes
- chlorine, steel, phosphate & gold production
- metal smelting
- manufacture & repair of weather and electronic devices
- incineration of municipal waste streams
medical applications, including vaccinations
- dentistry
- cosmetic industries
laboratory work involving mercury or sulfur compounds

Mercury also enters into the environment through the disposal (e.g., landfilling, incineration) of certain products. Products containing mercury include: auto parts, batteries, fluorescent bulbs, medical products, thermometers, and thermostats.[5] Due to health concerns (see below), toxics use reduction efforts are cutting back or eliminating mercury in such products. For example, 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 are being replaced by electronic thermometers. Mercury thermometers are still widely used for certain scientific applications because of their greater accuracy and working range.

One of the worst industrial disasters in history was caused by the dumping of mercury compounds into Minamata Bay, Japan. The Chisso Corporation, a fertilizer and later petrochemical company, was found responsible for polluting the bay from 1932–1968. It is estimated that over 3,000 people suffered various deformities, severe mercury poisoning symptoms or death from what became known as Minamata disease.

Health and Environmental Effects

See mercury poisoning

Precautions and regulation

Mercury should be handled with care. Containers of mercury should be securely sealed to avoid spills and evaporation. Heating of mercury, or compounds of mercury that may decompose when heated, should always be carried out with adequate ventilation in order to avoid human exposure to mercury vapor.

Occupational exposure

Due to the health effects of mercury exposure, industrial and commercial uses are regulated in many countries. The World Health Organization, OSHA, and NIOSH all treat mercury as an occupational hazard, and have established specific occupational exposure limits. Environmental releases and disposal of mercury are regulated in the U.S. primarily by the Environmental Protection Agency.

Mercury in fish

Fish and shellfish have a natural tendency to concentrate mercury in their bodies, often in the form of methylmercury, a highly toxic organic compound of mercury. Some species of large fish (notably shark, swordfish, king mackerel, and tilefish) concentrate mercury more readily than others, and because methylmercury or high levels of elemental mercury can be particularly toxic to unborn or young children, organizations such as the U.S. EPA and FDA recommend that pregnant women and young children avoid eating these species in large amounts.

Species with characteristically low levels of mercury include shrimp, tilapia, salmon, pollock, and catfish (FDA March 2004). The FDA characterizes shrimp, catfish, pollock, salmon, and canned light tuna as low-mercury seafood, although recent tests have indicated that up to 6 percent of canned light tuna may contain high levels. (Chicago Tribune)

Release of mercury into the environment

The United States Clean Air Act, passed in 1990, put mercury on a list of toxic pollutants which need to be controlled to the greatest possible extent. Thus, certain industries that emit mercury into the environment must install maximum achievable control technologies (MACT). However, a March 2005 EPA rule[6] took power plants off the list of sources which must reduce mercury to the maximum extent. Instead, a cap and trade rule was issued, with most of the reductions in mercury pollution from power plants beginning in the year 2018. The rule was being subjected to legal challenges filed by several States in 2005.

Mercury and aluminium

Mercury readily combines with aluminium to form an amalgam when the two pure metals come into contact. However, when the amalgam is exposed to air, the aluminium oxidizes, leaving behind mercury. The oxide flakes away, exposing more mercury amalgam, which repeats the process. This process continues until the supply of amalgam is exhausted, and since it releases mercury, a small amount of mercury can “eat through” a large amount of aluminium over time, by progressively forming amalgam and relinquishing the aluminium as oxide.

Aluminium in air is ordinarily protected by a molecule-thin layer of its own oxide (which is not porous to oxygen). Mercury coming into contact with this oxide does no harm. However, if any elemental aluminium is exposed (even by a recent scratch), the mercury may combine with it, starting the process described above, and potentially damaging a large part of the aluminium before it finally ends (Ornitz 1998).

For this reason, restrictions are placed on the use and handling of mercury in proximity with aluminium. In particular, mercury is not allowed aboard aircraft under most circumstances because of the risk of it forming amalgam with exposed aluminium parts in the aircraft.

References

Calvert J.B. (29 May 2004). Mercury: The lore of mercury, especially its uses in science and engineering. Retrieved April 5 2005.
FDA (March 2004) What you need to know about mercury in fish and shellfish, FDA, Washington D.C., March 2004. Retrieved 29 January 2006.
Kolev, S.T. Bates, N. Mercury (UK PID). National Poisons Information Service: Medical Toxicology Unit (London Centre).
Ornitz, Barry L. (1998), “Re: Aluminium Alloys and Mercury and FEATHERS,” USENET sci.engr.chem, <gymd2.437$VR5.4589@news12.ispnews.com>, 15 December 1998. Retrieved 29 January 2006.

History

American Dental Association. (January 9 2004). ADA statement on dental amalgam. Retrieved April 10 2005.
Brown, R.H. (December 19 2003). Mercury’s fall from medicine to toxin. Georgia Public Policy Foundation. Retrieved April 3 2005. "Permission to reprint in whole or in part is hereby granted, provided the author and his affiliations are cited."
Goldwater, L.J. (1955). Hat Industry. In: Mercury; a History of Quicksilver. York Press. Retrieved April 9 2005.
Kelly, E. (1676). The stone of the philosophers. Transcribed by: L. Roberts. Retrieved April 3 2005.
Mercury in Schools. (August 20 2004). Mercury through the Ages. Retrieved April 5 2005.
U.S. Food and Drug Administration. (April 1 2004). Drugs for human use: New drugs. In: Food and drugs. Code of Federal Regulations. Retrieved April 3 2005.

External links

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