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Tungsten,  74W
""Wolfram evaporated crystals and 1cm3 cube.jpg
General properties
Pronunciation "/ˈtʌŋstən/; "/ˈwʊlfrəm/
"TUNG-stən; "WUUL-frəm
Alternative name wolfram
Appearance grayish white, lustrous
"Standard atomic weight (Ar, std) 183.84(1)[1]
Tungsten in the "periodic table
"Hydrogen "Helium
"Lithium "Beryllium "Boron "Carbon "Nitrogen "Oxygen "Fluorine "Neon
"Sodium "Magnesium "Aluminium "Silicon "Phosphorus "Sulfur "Chlorine "Argon
"Potassium "Calcium "Scandium "Titanium "Vanadium "Chromium "Manganese "Iron "Cobalt "Nickel "Copper "Zinc "Gallium "Germanium "Arsenic "Selenium "Bromine "Krypton
"Rubidium "Strontium "Yttrium "Zirconium "Niobium "Molybdenum "Technetium "Ruthenium "Rhodium "Palladium "Silver "Cadmium "Indium "Tin "Antimony "Tellurium "Iodine "Xenon
"Caesium "Barium "Lanthanum "Cerium "Praseodymium "Neodymium "Promethium "Samarium "Europium "Gadolinium "Terbium "Dysprosium "Holmium "Erbium "Thulium "Ytterbium "Lutetium "Hafnium "Tantalum Tungsten "Rhenium "Osmium "Iridium "Platinum "Gold "Mercury (element) "Thallium "Lead "Bismuth "Polonium "Astatine "Radon
"Francium "Radium "Actinium "Thorium "Protactinium "Uranium "Neptunium "Plutonium "Americium "Curium "Berkelium "Californium "Einsteinium "Fermium "Mendelevium "Nobelium "Lawrencium "Rutherfordium "Dubnium "Seaborgium "Bohrium "Hassium "Meitnerium "Darmstadtium "Roentgenium "Copernicium "Nihonium "Flerovium "Moscovium "Livermorium "Tennessine "Oganesson
"Mo

W

"Sg
"tantalumtungsten → "rhenium
"Atomic number (Z) 74
"Group, "period "group 6, "period 6
"Block "d-block
"Element category   "transition metal
"Electron configuration ["Xe] 4f14 5d4 6s2[2]
Electrons per shell
2, 8, 18, 32, 12, 2
Physical properties
"Phase (at "STP) "solid
"Melting point 3695 "K ​(3422 °C, ​6192 °F)
"Boiling point 6203 K ​(5930 °C, ​10706 °F)
"Density (near r.t.) 19.25 g/cm3
when liquid (at m.p.) 17.6 g/cm3
"Heat of fusion 52.31 "kJ/mol[3][4]
"Heat of vaporization 774 kJ/mol
"Molar heat capacity 24.27 J/(mol·K)
"Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 3477 3773 4137 4579 5127 5823
Atomic properties
"Oxidation states 6, 5, 4, 3, 2, 1, 0, −1, −2, −4 ​(a mildly "acidic oxide)
"Electronegativity Pauling scale: 2.36
"Ionization energies
  • 1st: 770 kJ/mol
  • 2nd: 1700 kJ/mol
"Atomic radius empirical: 139 "pm
"Covalent radius 162±7 pm
""Color lines in a spectral range
Miscellanea
"Crystal structure ​"body-centered cubic (bcc)
""Body-centered cubic crystal structure for tungsten
"Speed of sound thin rod 4620 m/s (at r.t.) (annealed)
"Thermal expansion 4.5 µm/(m·K) (at 25 °C)
"Thermal conductivity 173 W/(m·K)
"Electrical resistivity 52.8 nΩ·m (at 20 °C)
"Magnetic ordering "paramagnetic[5]
"Magnetic susceptibility +59.0·10−6 cm3/mol (298 K)[6]
"Young's modulus 411 GPa
"Shear modulus 161 GPa
"Bulk modulus 310 GPa
"Poisson ratio 0.28
"Mohs hardness 7.5
"Vickers hardness 3430–4600 MPa
"Brinell hardness 2000–4000 MPa
"CAS Number 7440-33-7
History
"Discovery "Carl Wilhelm Scheele (1781)
First isolation "Juan José Elhuyar and "Fausto Elhuyar (1783)
Named by "Torbern Bergman (1781)
Main "isotopes of tungsten
"Iso­tope "Abun­dance "Half-life (t1/2) "Decay mode "Pro­duct
180W 0.12% 1.8×1018 y "α "176Hf
181W "syn 121.2 d "ε "181Ta
182W 26.50% "stable
183W 14.31% stable
184W 30.64% stable
185W syn 75.1 d "β "185Re
186W 28.43% stable
| "references | in Wikidata

Tungsten, also known as wolfram, is a "chemical element with symbol W and "atomic number 74. The name tungsten comes from the former Swedish name for the "tungstate "mineral "scheelite, from tung sten "heavy stone".[7] Its name in Swedish is volfram, in order to distinguish it from scheelite. Tungsten is a "rare metal found naturally on Earth almost exclusively in chemical compounds. It was identified as a new element in 1781, and first isolated as a metal in 1783. Its important "ores include "wolframite and "scheelite.

The "free element is remarkable for its robustness, especially the fact that it has the highest "melting point of all the elements discovered, melting at 3422 °C (6192 °F, 3695 K). It also has the second highest "boiling point, at 5930 °C (10706 °F, 6203 K). Its density is 19.3 times that of water, comparable to that of "uranium and "gold, and much higher (about 1.7 times) than that of "lead.[8] Polycrystalline tungsten is an intrinsically "brittle[9][10] and "hard material (under standard conditions, when uncombined), making it difficult to "work. However, pure single-crystalline tungsten is more "ductile, and can be cut with a hard-steel "hacksaw.[11]

Tungsten's many alloys have numerous applications, including incandescent "light bulb filaments, "X-ray tubes (as both the filament and target), electrodes in "TIG welding, "superalloys, and "radiation shielding. Tungsten's hardness and high "density give it military applications in penetrating "projectiles. Tungsten compounds are also often used as industrial "catalysts.

Tungsten is the only metal from the third "transition series that is known to occur in "biomolecules, where it is used in a few species of bacteria and "archaea. It is the heaviest element known to be essential to any living organism.[12] Tungsten interferes with "molybdenum and "copper metabolism and is somewhat toxic to animal life.[13][14]

Contents

Characteristics[edit]

Physical properties[edit]

In its raw form, tungsten is a hard steel-grey "metal that is often "brittle and hard to "work. If made very pure, tungsten retains its "hardness (which exceeds that of many steels), and becomes "malleable enough that it can be worked easily.[11] It is worked by "forging, "drawing, or "extruding. Tungsten objects are also commonly formed by "sintering.

Of all metals in pure form, tungsten has the highest "melting point (3422 °"C, 6192 °"F), lowest "vapor pressure (at temperatures above 1650 °C, 3000 °F), and the highest "tensile strength.[15] Although "carbon remains solid at higher temperatures than tungsten, carbon "sublimes at "atmospheric pressure instead of melting, so it has no melting point. Tungsten has the lowest "coefficient of thermal expansion of any pure metal. The low thermal expansion and high melting point and "tensile strength of tungsten originate from strong "covalent bonds formed between tungsten atoms by the 5d electrons.[16] Alloying small quantities of tungsten with "steel greatly increases its "toughness.[8]

Tungsten exists in two major "crystalline forms: α and β. The former has a "body-centered cubic structure and is the more stable form. The structure of the β phase is called "A15 cubic; it is "metastable, but can coexist with the α phase at ambient conditions owing to non-equilibrium synthesis or stabilization by impurities. Contrary to the α phase which crystallizes in isometric grains, the β form exhibits a columnar "habit. The α phase has one third of the "electrical resistivity[17] and a much lower "superconducting transition temperature TC relative to the β phase: ca. 0.015 K vs. 1–4 K; mixing the two phases allows obtaining intermediate TC values.[18][19] The TC value can also be raised by "alloying tungsten with another metal (e.g. 7.9 K for W-"Tc).[20] Such tungsten alloys are sometimes used in low-temperature superconducting circuits.[21][22][23]

Isotopes[edit]

Naturally occurring tungsten consists of five "isotopes whose "half-lives are so long that they can be considered "stable. Theoretically, all five can decay into isotopes of element 72 ("hafnium) by "alpha emission, but only 180W has been observed[24][25] to do so with a half-life of (1.8 ± 0.2)×1018 years; on average, this yields about two alpha decays of 180W in one gram of natural tungsten per year.[26] The other naturally occurring isotopes have not been observed to decay, constraining their half-lives to be at least 4×1021 years.

Another 30 artificial "radioisotopes of tungsten have been characterized, the most stable of which are 181W with a half-life of 121.2 days, 185W with a half-life of 75.1 days, 188W with a half-life of 69.4 days, 178W with a half-life of 21.6 days, and 187W with a half-life of 23.72 h.[26] All of the remaining "radioactive isotopes have half-lives of less than 3 hours, and most of these have half-lives below 8 minutes.[26] Tungsten also has 4 "meta states, the most stable being 179mW (t1/2 6.4 minutes).

Chemical properties[edit]

Elemental tungsten resists "attack by oxygen, "acids, and "alkalis.[27]

The most common formal "oxidation state of tungsten is +6, but it exhibits all oxidation states from −2 to +6.[27][28] Tungsten typically combines with oxygen to form the yellow "tungstic oxide, WO3, which dissolves in aqueous alkaline solutions to form tungstate ions, WO2−
4
.

"Tungsten carbides (W2C and WC) are produced by heating powdered tungsten with "carbon. W2C is resistant to chemical attack, although it reacts strongly with "chlorine to form "tungsten hexachloride (WCl6).[8]

In aqueous solution, tungstate gives the "heteropoly acids and "polyoxometalate "anions under neutral and acidic conditions. As "tungstate is progressively treated with acid, it first yields the soluble, "metastable "paratungstate A" "anion, W
7
O6–
24
, which over time converts to the less soluble "paratungstate B" anion, H
2
W
12
O10–
42
.[29] Further acidification produces the very soluble metatungstate anion, H
2
W
12
O6–
40
, after which equilibrium is reached. The metatungstate ion exists as a symmetric cluster of twelve tungsten-"oxygen "octahedra known as the "Keggin anion. Many other polyoxometalate anions exist as metastable species. The inclusion of a different atom such as "phosphorus in place of the two central "hydrogens in metatungstate produces a wide variety of heteropoly acids, such as "phosphotungstic acid H3PW12O40.

Tungsten trioxide can form "intercalation compounds with alkali metals. These are known as bronzes; an example is "sodium tungsten bronze.

History[edit]

In 1781, "Carl Wilhelm Scheele discovered that a new "acid, "tungstic acid, could be made from "scheelite (at the time named tungsten).[30] Scheele and "Torbern Bergman suggested that it might be possible to obtain a new metal by reducing this acid.[31] In 1783, "José and "Fausto Elhuyar found an acid made from "wolframite that was identical to tungstic acid. Later that year, at the "Royal Basque Society in the town of "Bergara, Spain, the brothers succeeded in isolating tungsten by reduction of this acid with "charcoal, and they are credited with the discovery of the element.[32][33][34]

In "World War II, tungsten played a significant role in background political dealings. Portugal, as the main European source of the element, was put under pressure from both sides, because of its deposits of wolframite ore at "Panasqueira. Tungsten's desirable properties such as resistance to high temperatures, its hardness and density, and its strengthening of alloys made it an important raw material for the arms industry,[35][36] both as a constituent of weapons and equipment and employed in production itself, e.g., in "tungsten carbide cutting tools for machining steel.

Etymology[edit]

The name "tungsten" (from the "Swedish tung sten, "heavy stone") is used in English, French, and many other languages as the name of the element, but not in the "Nordic countries. Tungsten was the old Swedish name for the mineral "scheelite. The other name "wolfram" (or "volfram") is used in most European (especially Germanic and Slavic) languages, and is derived from the mineral "wolframite, which is the origin of its chemical symbol, W.[11] The name "wolframite" is derived from German "wolf rahm" ("wolf soot" or "wolf cream"), the name given to tungsten by "Johan Gottschalk Wallerius in 1747. This, in turn, derives from "lupi spuma", the name "Georg Agricola used for the element in 1546, which translates into English as "wolf's froth", and is a reference to the large amounts of "tin consumed by the mineral during its extraction.[37]

Occurrence[edit]

""
""
Wolframite mineral, with a scale in cm.

Tungsten is found mainly in the mineral "wolframite ("iron–"manganese tungstate (Fe,Mn)WO4, which is a solid solution of the two minerals "ferberite FeWO4, and "hübnerite MnWO4) and "scheelite ("calcium tungstate (CaWO4). Other tungsten minerals range in their level of abundance from moderate to very rare, and have almost no economical value.

Production[edit]

""
""
Tungsten mined in 2012

About 61,300 tonnes of tungsten concentrates were produced in the year 2009,[38] and in 2010, world production of tungsten was about 68,000 "tonnes.[39] The main producers were as follows (data in tonnes):[40]

Major producers of Tungsten[39]
Country Production (tonnes)
2009 2010 2011 2012
 "China 51,000 59,000 61,800 64,000
 "Russia 2,665 2,785 3,314 3,537
 "Canada 1,964 420 1,966 2,194
 "Bolivia 1,023 1,204 1,124 1,247
 "Vietnam 725 1,150 1,635 1,050
 "Portugal 823 799 819 763
 "Austria 887 977 861 706
 "Rwanda 380 330 520 700
 "Spain 225 240 497 542
 "Brazil 192 166 244 381
 "Australia 33 18 15 290
 "Peru 502 571 439 276
 "Burundi 110 100 165 190
 "Myanmar 874 163 140 140
 "North Korea 100 110 110 100
 "Congo 200 25 70 95
 "Thailand 190 300 160 80
 "Mongolia 39 20 13 66
 "Uganda 7 44 8 21
Total 61,200 68,400 73,900 76,400
""
""
Tungsten mining in Rwanda forms an important part of the country's economy.

There is additional production in the U.S., but the amount is proprietary company information. U.S. reserves are 140,000 tonnes.[40] US industrial use of wolfram is 20,000 tones: 15,000 tones are imported and the remaining 5,000 tones come from domestic recycling.[41]

Tungsten is considered to be a "conflict mineral due to the unethical mining practices observed in the "Democratic Republic of the Congo.[42][43]

There is a large deposit of tungsten ore on the edge of "Dartmoor in the "United Kingdom, which was exploited during "World War I and "World War II as the "Hemerdon Mine. With recent increases in tungsten prices, as of 2014 this mine has been reactivated.[44]

Tungsten is extracted from its ores in several stages. The ore is eventually converted to "tungsten(VI) oxide (WO3), which is heated with "hydrogen or "carbon to produce powdered tungsten.[31] Because of tungsten's high melting point, it is not commercially feasible to cast tungsten "ingots. Instead, powdered tungsten is mixed with small amounts of powdered nickel or other metals, and "sintered. During the sintering process, the nickel diffuses into the tungsten, producing an alloy.

Tungsten can also be extracted by hydrogen reduction of "WF6:

WF6 + 3 H2 → W + 6 HF

or "pyrolytic decomposition:[45]

WF6 → W + 3 F2 ("ΔHr = +)

Tungsten is not traded as a futures contract and cannot be tracked on exchanges like the "London Metal Exchange. The prices are usually quoted for tungsten concentrate or WO3. If converted to the metal equivalent, they were about US$19 per kilogram in 2009.[38]

Applications[edit]

""
""
Close-up of a tungsten filament inside a "halogen lamp
""
""
"Tungsten carbide ring (jewelry)
""
""
1 kilogram tungsten cylinder (scale shown below)

Approximately half of the tungsten is consumed for the production of hard materials – namely "tungsten carbide – with the remaining major use being in alloys and steels. Less than 10% is used in other "chemical compounds.[46]

Hard materials[edit]

Tungsten is mainly used in the production of hard materials based on tungsten carbide, one of the hardest "carbides, with a melting point of 2770 °C. WC is an efficient "electrical conductor, but W2C is less so. WC is used to make wear-resistant "abrasives, and "carbide" cutting tools such as knives, drills, "circular saws, "milling and "turning tools used by the metalworking, woodworking, "mining, "petroleum and construction industries.[8] Carbide tooling is actually a ceramic/metal composite, where metallic cobalt acts as a binding (matrix) material to hold the WC particles in place. This type of industrial use accounts for about 60% of current tungsten consumption.[47]

The "jewelry industry makes rings of sintered tungsten carbide, tungsten carbide/metal composites, and also metallic tungsten.[48] WC/metal composite rings use nickel as the metal matrix in place of cobalt because it takes a higher luster when polished. Sometimes manufacturers or retailers refer to tungsten carbide as a metal, but it is a ceramic.[49] Because of tungsten carbide's hardness, rings made of this material are extremely abrasion resistant, and will hold a burnished finish longer than rings made of metallic tungsten. Tungsten carbide rings are brittle, however, and may crack under a sharp blow.[50]

Alloys[edit]

The hardness and density of tungsten are applied in obtaining "heavy metal "alloys. A good example is "high speed steel, which can contain as much as 18% tungsten.[51] Tungsten's high melting point makes tungsten a good material for applications like "rocket nozzles, for example in the "UGM-27 Polaris "submarine-launched ballistic missile.[52] Tungsten alloys are used in a wide range of different applications, including the aerospace and automotive industries and radiation shielding.[53] "Superalloys containing tungsten, such as "Hastelloy and "Stellite, are used in "turbine blades and wear-resistant parts and coatings. Quenched (martensitic) tungsten steel (approx. 5.5% to 7.0% W with 0.5% to 0.7% C) was used for making hard permanent magnets, due to its high "remanence and "coercivity, as noted by "John Hopkinson (1849 - 1898) as early as 1886. The magnetic properties of a metal or an alloy are very sensitive to microstructure. For example, while the element tungsten is not ferromagnetic (but "iron, of course, is), when present in steel in these proportions, it stabilizes the "martensite phase, which has an enhanced ferromagnetism, as compared to the "ferrite (iron) phase, due to its greater resistance to magnetic domain wall motion.

Tungsten's heat resistance makes it useful in "arc welding applications when combined with another highly-conductive metal such as silver or copper. The silver or copper provides the necessary conductivity and the tungsten allows the welding rod to withstand the high-temperatures of the arc welding environment.

Armaments[edit]

Tungsten, usually alloyed with "nickel and "iron or "cobalt to form heavy alloys, is used in "kinetic energy penetrators as an alternative to "depleted uranium, in applications where uranium's "radioactivity is problematic even in depleted form, or where uranium's additional "pyrophoric properties are not required (for example, in ordinary small arms bullets designed to penetrate body armor). Similarly, tungsten alloys have also been used in cannon shells, "grenades and "missiles, to create supersonic shrapnel. Germany used tungsten during World War II to produce shells for anti-tank gun designs using the Gerlich "squeeze" principle to achieve very high muzzle velocity and enhanced armor penetration from comparatively small caliber and light weight field artillery. The weapons were highly effective but a shortage of tungsten used in the shell core limited that effectiveness.

Tungsten has also been used in "Dense Inert Metal Explosives, which use it as dense powder to reduce collateral damage while increasing the lethality of explosives within a small radius.[54]

Chemical applications[edit]

"Tungsten(IV) sulfide is a high temperature "lubricant and is a component of catalysts for "hydrodesulfurization.[55] MoS2 is more commonly used for such applications.[56]

Tungsten "oxides are used in "ceramic glazes and "calcium/"magnesium tungstates are used widely in "fluorescent lighting. Crystal "tungstates are used as "scintillation detectors in "nuclear physics and "nuclear medicine. Other salts that contain tungsten are used in the chemical and "tanning industries.[15]

""
""
Tungsten trioxide powder

Tungsten oxide (WO3) is incorporated into "selective catalytic reduction (SCR) catalysts found in coal-fired power plants. These catalysts convert "nitrogen oxides ("NOx) to nitrogen (N2) and water (H2O) using ammonia (NH3). The tungsten oxide helps with the physical strength of the catalyst and extends catalyst life.[57]

Niche uses[edit]

Applications requiring its high density include weights, "counterweights, ballast keels for yachts, tail ballast for commercial aircraft, and as ballast in race cars for "NASCAR and "Formula One; "depleted uranium is also used for these purposes, due to similarly high density. Seventy-five-kg blocks of tungsten were used as "cruise balance mass devices" on the entry vehicle portion of the 2012 "Mars Science Laboratory spacecraft. It is an ideal material to use as a "dolly for "riveting, where the mass necessary for good results can be achieved in a compact bar. High-density alloys of tungsten with nickel, copper or iron are used in high-quality "darts[58] (to allow for a smaller diameter and thus tighter groupings) or for "fishing lures (tungsten beads allow the fly to sink rapidly). Some "cello C strings are wound with tungsten. The extra density gives this string more projection and often cellists will buy just this string and use it with three strings from a different set.[59] Tungsten is used as an absorber on the electron telescope on the "Cosmic Ray System of the two "Voyager spacecraft.[60]

"Sodium tungstate is used in "Folin-Ciocalteu's reagent, a mixture of different chemicals used in the "Lowry Assay" for protein content analysis.

Gold substitution[edit]

Its density, similar to that of gold, allows tungsten to be used in jewelry as an alternative to "gold or "platinum.[11][61] Metallic tungsten is "hypoallergenic, and is harder than gold alloys (though not as hard as tungsten carbide), making it useful for "rings that will resist scratching, especially in designs with a "brushed finish.

Because the density is so similar to that of gold (tungsten is only 0.36% less dense), tungsten can also be used in "counterfeiting of "gold bars, such as by plating a tungsten bar with gold,[62][63][64] which has been observed since the 1980s,[65] or taking an existing gold bar, drilling holes, and replacing the removed gold with tungsten rods.[66] The densities are not exactly the same, and other properties of gold and tungsten differ, but gold-plated tungsten will pass superficial tests.[62]

Gold-plated tungsten is available commercially from China (the main source of tungsten), both in jewelry and as bars.[67]

Electronics[edit]

Because it retains its strength at high temperatures and has a high "melting point, elemental tungsten is used in many high-temperature applications,[68] such as "light bulb, "cathode-ray tube, and "vacuum tube filaments, "heating elements, and "rocket engine nozzles.[11] Its high melting point also makes tungsten suitable for aerospace and high-temperature uses such as electrical, heating, and welding applications, notably in the "gas tungsten arc welding process (also called tungsten inert gas (TIG) welding).

""
""
Tungsten electrode used in a "Gas tungsten arc welding torch

Because of its conductive properties and relative chemical inertness, tungsten is also used in "electrodes, and in the emitter tips in electron-beam instruments that use "field emission guns, such as "electron microscopes. In electronics, tungsten is used as an interconnect material in "integrated circuits, between the "silicon dioxide "dielectric material and the transistors. It is used in metallic films, which replace the wiring used in conventional electronics with a coat of tungsten (or "molybdenum) on "silicon.[45]

The electronic structure of tungsten makes it one of the main sources for "X-ray targets,[69][70] and also for shielding from high-energy radiations (such as in the "radiopharmaceutical industry for shielding radioactive samples of "FDG). It is also used in gamma imaging as a material from which coded apertures are made, due to its excellent shielding properties. Tungsten powder is used as a filler material in "plastic composites, which are used as a nontoxic substitute for "lead in "bullets, "shot, and radiation shields. Since this element's thermal expansion is similar to "borosilicate glass, it is used for making glass-to-metal seals.[15] In addition to its high melting point, when tungsten is doped with potassium, it leads to an increased shape stability (compared to non-doped tungsten). This ensures that the filament does not sag, and no undesired changes occur.[71]

Nanowires[edit]

Through top-down "nanofabrication processes, tungsten "nanowires have been fabricated and studied since 2002.[72] Due to a particularly high surface to volume ratio, the formation of a surface oxide layer and the single crystal nature of such material, the mechanical properties differ fundamentally from those of bulk tungsten.[73] Such tungsten nanowires have potential applications in "nanoelectronics and importantly as pH probes and gas sensors.[74] In similarity to "silicon nanowires, tungsten nanowires are frequently produced from a bulk tungsten precursor followed by a "thermal oxidation step to control morphology in terms of length and aspect ratio.[75] Using the "Deal–Grove model it is possible to predict the oxidation kinetics of nanowires fabricated through such thermal oxidation processing.[76]

Biological role[edit]

Tungsten, at atomic number 74, is the heaviest element known to be biologically functional, with the next heaviest being "iodine (Z = 53). It is used by some bacteria, but not in "eukaryotes. For example, "enzymes called "oxidoreductases use tungsten similarly to "molybdenum by using it in a tungsten-"pterin complex with "molybdopterin (molybdopterin, despite its name, does not contain molybdenum, but may complex with either molybdenum or tungsten in use by living organisms). Tungsten-using enzymes typically reduce carboxylic acids to aldehydes.[77] The tungsten "oxidoreductases may also catalyse oxidations. The first tungsten-requiring enzyme to be discovered also requires selenium, and in this case the tungsten-selenium pair may function analogously to the molybdenum-sulfur pairing of some "molybdenum cofactor-requiring enzymes.[78] One of the enzymes in the oxidoreductase family which sometimes employ tungsten (bacterial "formate dehydrogenase H) is known to use a selenium-molybdenum version of molybdopterin.[79] "Acetylene hydratase is an unusual "metalloenzyme in that it catalyzes a hydration reaction. Two reaction mechanisms have been proposed, in one of which there is a direct interaction between the tungsten atom and the C≡C triple bond.[80] Although a tungsten-containing xanthine dehydrogenase from bacteria has been found to contain tungsten-molydopterin and also non-protein bound selenium, a tungsten-selenium molybdopterin complex has not been definitively described.[81]

In soil, tungsten metal oxidizes to the "tungstate anion. It can be selectively or non-selectively imported by some "prokaryotic organisms and may substitute for "molybdate in certain "enzymes. Its effect on the action of these enzymes is in some cases inhibitory and in others positive.[82] The soil's chemistry determines how the tungsten polymerizes; "alkaline soils cause monomeric tungstates; "acidic soils cause polymeric tungstates.[83]

"Sodium tungstate and "lead have been studied for their effect on "earthworms. Lead was found to be lethal at low levels and sodium tungstate was much less toxic, but the tungstate completely inhibited their "reproductive ability.[84]

Tungsten has been studied as a biological copper metabolic "antagonist, in a role similar to the action of molybdenum. It has been found that tetrathiotungstates may be used as biological copper "chelation chemicals, similar to the "tetrathiomolybdates.[85]

Health factors[edit]

Because tungsten is rare["clarification needed] and its compounds are generally inert, the effects of tungsten on the environment are limited.[86] The "median lethal dose LD50 depends strongly on the animal and the method of administration and varies between 59 mg/kg (intravenous, rabbits)[87][88] and 5000 mg/kg (tungsten metal powder, "intraperitoneal, rats).[89][90]

People can be exposed to tungsten in the workplace by breathing it in, swallowing it, skin contact, and eye contact. The "National Institute for Occupational Safety and Health (NIOSH) has set a "recommended exposure limit (REL) of 5 mg/m3 over an 8-hour workday and a short term limit of 10 mg/m3.[91]

Patent claim[edit]

Tungsten is unique amongst the elements in that it has been the subject of patent proceedings. In 1928, a US court rejected "General Electric's attempt to patent it, overturning U.S. Patent 1,082,933 granted in 1913 to "William D. Coolidge.[92][93]

See also[edit]

References[edit]

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External links[edit]

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