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= Tungsten =
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Introduction
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Tungsten (also called wolfram) is a chemical element; it has symbol W
and atomic number 74. It is a metal found naturally on Earth almost
exclusively in compounds with other elements. It was identified as a
distinct element in 1781 and first isolated as a metal in 1783. Its
important ores include scheelite and wolframite, the latter lending
the element its alternative name.
The free element is remarkable for its robustness, especially the fact
that it has the highest melting point of all known elements, melting
at 3422 C. It also has the highest boiling point, at . Its density is
19.254 g/cm3, comparable with that of uranium and gold, and much
higher (about 1.7 times) than that of lead. Polycrystalline tungsten
is an intrinsically brittle and hard material (under standard
conditions, when uncombined), making it difficult to work into metal.
However, pure single-crystalline tungsten is more ductile and can be
cut with a hard-steel hacksaw.
Tungsten occurs in many alloys, which have numerous applications,
including incandescent light bulb filaments, X-ray tubes, electrodes
in gas tungsten arc welding, superalloys, and radiation shielding.
Tungsten's hardness and high density make it suitable for military
applications in penetrating projectiles. Tungsten compounds are often
used as industrial catalysts. Its largest use is in tungsten carbide,
a wear-resistant material used in metalworking, mining, and
construction. About 50% of tungsten is used in tungsten carbide, with
the remaining major use being alloys and steels: less than 10% is used
in other compounds.
Tungsten is the only metal in the third transition series that is
known to occur in biomolecules, being found in a few species of
bacteria and archaea. However, tungsten interferes with molybdenum
and copper metabolism and is somewhat toxic to most forms of animal
life.
Physical properties
=====================
In its raw form, tungsten is a hard steel-grey metal that is often
brittle and hard to work. Purified, monocrystalline tungsten retains
its hardness (which exceeds that of many steels), and becomes
malleable enough that it can be worked easily. It is worked by
forging, drawing, or extruding but it is more commonly formed by
sintering. Sintering is often used due to the very high melting point
of tungsten.
Of all metals in pure form, tungsten has the highest melting point (),
lowest vapor pressure (at temperatures above ), and the highest
tensile strength. Although carbon remains solid at higher temperatures
than tungsten, carbon sublimes at atmospheric pressure instead of
melting, so it has no melting point. Moreover, tungsten's most stable
crystal phase does not exhibit any high-pressure-induced structural
transformations for pressures up to at least 364 gigapascals. 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.
Alloying small quantities of tungsten with steel greatly increases its
toughness.
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 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. The TC value can also be
raised by alloying tungsten with another metal (e.g. 7.9 K for W-Tc).
Such tungsten alloys are sometimes used in low-temperature
superconducting circuits.{{Cite journal
| doi = 10.1103/PhysRevB.35.8850 | pmid = 9941272| volume = 35| issue
= 16| pages = 8850-8852| last = Kirk| first = M. D.| author2 = D. P.
E. Smith| author3 = D. B. Mitzi| author4 = J. Z. Sun| author5 = D. J.
Webb| author6 = K. Char| author7 = M. R. Hahn| author8 = M. Naito|
author9 = B. Oh| author10 = M. R. Beasley| author11 = T. H. Geballe|
author12 = R. H. Hammond| author13 = A. Kapitulnik| author14 = C. F.
Quate| title = Point-contact electron tunneling into the high-T_{c}
superconductor Y-Ba-Cu-O| journal = Physical Review B| date=
1987|bibcode = 1987PhRvB..35.8850K }}
Isotopes
==========
Naturally occurring tungsten consists of four stable isotopes (182W,
183W, 184W, and 186W) and one very long-lived radioisotope, 180W.
Theoretically, all five can decay into isotopes of element 72
(hafnium) by alpha emission, but only 180W has been observed to do so,
with a half-life of years; on average, this yields about two alpha
decays of 180W per gram of natural tungsten per year. This rate is
equivalent to a specific activity of roughly 63 micro-becquerel per
kilogram. This rate of decay is orders of magnitude lower than that
observed in carbon or potassium as found on earth, which likewise
contain small amounts of long-lived radioactive isotopes. Bismuth was
long thought to be non-radioactive, but (its longest lived isotope)
actually decays with a half-life of years or about a factor 10 slower
than . However, due to naturally occurring bismuth being 100% , its
specific activity is actually higher than that of natural tungsten at
3 milli-becquerel per kilogram. The other naturally occurring isotopes
of tungsten have not been observed to decay, constraining their
half-lives to be at least .
Another 34 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. All of the remaining radioactive isotopes have
half-lives of less than 3 hours, and most of these have half-lives
below 8 minutes. Tungsten also has 12 meta states, with the most
stable being 179mW ('t'1/2 6.4 minutes).
Chemical properties
=====================
Tungsten is a mostly non-reactive element: it does not react with
water, is immune to attack by most acids and bases, and does not react
with oxygen or air at room temperature. At elevated temperatures
(i.e., when red-hot) it reacts with oxygen to form the trioxide
compound tungsten(VI), WO3. It will, however, react directly with
fluorine (F2) at room temperature to form tungsten(VI) fluoride (WF6),
a colorless gas. At around 250 °C it will react with chlorine or
bromine, and under certain hot conditions will react with iodine.
Finely divided tungsten is pyrophoric.
The most common formal oxidation state of tungsten is +6, but it
exhibits all oxidation states from −2 to +6. Tungsten typically
combines with oxygen to form the yellow tungstic oxide, WO3, which
dissolves in aqueous alkaline solutions to form tungstate ions, .
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).
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, , which over time
converts to the less soluble "paratungstate B" anion, . Further
acidification produces the very soluble metatungstate anion, , 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.
In gaseous form, tungsten forms the diatomic species W2. These
molecules feature a sextuple bond between tungsten atoms — the highest
known bond order among stable atoms.
History
======================================================================
In 1781, Carl Wilhelm Scheele discovered that a new acid, tungstic
acid, could be made from scheelite (at the time called tungsten).
Scheele and Torbern Bergman suggested that it might be possible to
obtain a new metal by reducing this acid. 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 (they called it "wolfram" or "volfram").
The strategic value of tungsten came to notice in the early 20th
century. British authorities acted in 1912 to free the Carrock mine
from the German owned Cumbrian Mining Company and, during World War I,
restrict German access elsewhere. In World War II, tungsten played a
more 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, both
as a constituent of weapons and equipment and employed in production
itself, e.g., in tungsten carbide cutting tools for machining steel.
Now tungsten is used in many more applications such as aircraft and
motorsport ballast weights, darts, anti-vibration tooling, and
sporting equipment.
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 granted in 1913 to
William D. Coolidge.
It is suggested that remnants of wolfram have been found in what may
have been the garden of the astronomer and alchemist Tycho Brahe.
Etymology
===========
The name 'tungsten' (which means in Swedish and was the old Swedish
name for the mineral scheelite and other minerals of similar density)
is used in English, French, and many other languages as the name of
the element, but '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 the chemical symbol W. The
name 'wolframite' is derived from German (), the name given to
tungsten by Johan Gottschalk Wallerius in 1747. This, in turn, derives
from Latin , the name Georg Agricola used for the mineral in 1546,
which translates into English as and is a reference to the large
amounts of tin consumed by the mineral during its extraction, as
though the mineral devoured it like a wolf. This naming follows a
tradition of colorful names miners from the Ore Mountains would give
various minerals, out of a superstition that certain ones that looked
as if they contained then-known valuable metals but when extracted
were somehow "hexed". Cobalt (cf. Kobold), pitchblende (cf. German
for ) and nickel (cf. "Old Nick") derive their names from the same
miners' idiom.
Occurrence
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Tungsten has thus far not been found in nature in its pure form.
Instead, tungsten is found mainly in the minerals wolframite and
scheelite. Wolframite is iron-manganese tungstate , a solid solution
of the two minerals ferberite (FeWO4) and hübnerite (MnWO4), while
scheelite is calcium tungstate (CaWO4). Other tungsten minerals range
in their level of abundance from moderate to very rare, and have
almost no economic value.
Chemical compounds
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Structure of W6Cl18 ("tungsten trichloride")
Tungsten forms chemical compounds in oxidation states from −2 to +6.
Higher oxidation states, always as oxides, are relevant to its
terrestrial occurrence and its biological roles, mid-level oxidation
states are often associated with metal clusters, and very low
oxidation states are typically associated with CO complexes. The
chemistries of tungsten and molybdenum show strong similarities to
each other, as well as contrasts with their lighter congener,
chromium. The relative rarity of tungsten(III), for example, contrasts
with the pervasiveness of the chromium(III) compounds. The highest
oxidation state is seen in tungsten(VI) oxide (WO3). Tungsten(VI)
oxide is soluble in aqueous base, forming tungstate (WO42−). This
oxyanion condenses at lower pH values, forming polyoxotungstates.
The broad range of oxidation states of tungsten is reflected in its
various chlorides:
* Tungsten(II) chloride, which exists as the hexamer W6Cl12
* Tungsten(III) chloride, which exists as the hexamer W6Cl18
* Tungsten(IV) chloride, WCl4, a black solid, which adopts a polymeric
structure.
* Tungsten(V) chloride WCl5, a black solid which adopts a dimeric
structure.
* Tungsten(VI) chloride WCl6, which contrasts with the instability of
MoCl6.
Organotungsten compounds are numerous and also span a range of
oxidation states. Notable examples include the trigonal prismatic and
octahedral .
Reserves
==========
The world's reserves of tungsten are 3,200,000 tonnes; they are mostly
located in China (1,800,000 t), Canada (290,000 t), Russia (160,000
t), Vietnam (95,000 t) and Bolivia. As of 2017, China, Vietnam and
Russia are the leading suppliers with 79,000, 7,200 and 3,100 tonnes,
respectively. Canada had ceased production in late 2015 due to the
closure of its sole tungsten mine. Meanwhile, Vietnam had
significantly increased its output in the 2010s, owing to the major
optimization of its domestic refining operations, and overtook Russia
and Bolivia.
China remains the world's leader not only in production, but also in
export and consumption of tungsten products. Tungsten production is
gradually increasing outside China because of the rising demand.
Meanwhile, its supply by China is strictly regulated by the Chinese
Government, which fights illegal mining and excessive pollution
originating from mining and refining processes.
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. Following increases in tungsten prices,
this mine was reactivated in 2014, but ceased activities in 2018.
Within the EU, the Austrian Felbertal scheelite deposit is one of the
few producing tungsten mines. Portugal is one of Europe's main
tungsten producers, with 121 kt of contained tungsten in mineral
concentrates from 1910 to 2020, accounting for roughly 3.3% of the
global production.
Tungsten is considered to be a conflict mineral due to the unethical
mining practices observed in the Democratic Republic of the Congo.
South Korea's Sangdong mine, one of the world's largest tungsten mines
with 7,890,000 tonnes of high-grade tungsten reportedly buried, was
closed in 1994 due to low profitability but has since re-registered
mining rights and is scheduled to resume activities in 2024.
Extraction
============
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. 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:
:WF6 → W + 3 F2 (Δ'H'r = +)
Tungsten is not traded as a futures contract and cannot be tracked on
exchanges like the London Metal Exchange. The tungsten industry often
uses independent pricing references such as Argus Media or Metal
Bulletin as a basis for contracts. The prices are usually quoted for
tungsten concentrate or WO3.
Applications
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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. Because of the high ductile-brittle transition
temperature of tungsten, its products are conventionally manufactured
through powder metallurgy, spark plasma sintering, chemical vapor
deposition, hot isostatic pressing, and thermoplastic routes. A more
flexible manufacturing alternative is selective laser melting, which
is a form of 3D printing and allows creating complex three-dimensional
shapes.
Industrial
============
Tungsten is mainly used in the production of hard materials based on
tungsten carbide (WC), one of the hardest carbides. 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, dies, milling and turning tools used by
the metalworking, woodworking, mining, petroleum and construction
industries. 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.
The jewelry industry makes rings of sintered tungsten carbide,
tungsten carbide/metal composites, and also metallic tungsten.
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. 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.
Alloys
========
The hardness and heat resistance of tungsten can contribute to useful
alloys. A good example is high-speed steel, which can contain as much
as 18% tungsten. 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. Tungsten alloys
are used in a wide range of applications, including the aerospace and
automotive industries and radiation shielding. Superalloys containing
tungsten, such as Hastelloy and Stellite, are used in turbine blades
and wear-resistant parts and coatings.
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.
Permanent magnets
===================
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 is), when it is present in
steel in these proportions, it stabilizes the martensite phase, which
has greater ferromagnetism than the ferrite (iron) phase due to its
greater resistance to magnetic domain wall motion.
Military
==========
Tungsten, usually alloyed with nickel, 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 desired (for example, in ordinary small
arms bullets designed to penetrate body armor). Similarly, tungsten
alloys have also been used in 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
bore 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, caused in part by the Wolfram Crisis,
limited their use.
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.
Chemical applications
=======================
Tungsten(IV) sulfide is a high temperature lubricant and is a
component of catalysts for hydrodesulfurization. MoS2 is more commonly
used for such applications.
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.
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. Tungsten
containing catalysts are promising for epoxidation, oxidation, and
hydrogenolysis reactions. Tungsten heteropoly acids are key component
of multifunctional catalysts. Tungstates can be used as
photocatalyst, while the tungsten sulfide as electrocatalyst.
Niche uses
============
Applications requiring its high density include weights,
counterweights, ballast keels for yachts, tail ballast for commercial
aircraft, rotor weights for civil and military helicopters, and as
ballast in race cars for NASCAR and Formula One. Being slightly less
than twice the density, tungsten is seen as an alternative (albeit
more expensive) to lead fishing sinkers. 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 (to allow for a smaller diameter and thus
tighter groupings) or for artificial flies (tungsten beads allow the
fly to sink rapidly). Tungsten is also used as a heavy bolt to lower
the rate of fire of the SWD M11/9 sub-machine gun from 1300 RPM to 700
RPM. Some string instrument strings incorporates tungsten. Tungsten
is used as an absorber on the electron telescope on the Cosmic Ray
System of the two Voyager spacecraft.
Gold substitution
===================
Its density, similar to that of gold, allows tungsten to be used in
jewelry as an alternative to gold or platinum. 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), and its price of the order of one-thousandth,
tungsten can also be used in counterfeiting of gold bars, such as by
plating a tungsten bar with gold, which has been observed since the
1980s, or taking an existing gold bar, drilling holes, and replacing
the removed gold with tungsten rods. The densities are not exactly the
same, and other properties of gold and tungsten differ, but
gold-plated tungsten will pass superficial tests.
Gold-plated tungsten is available commercially from China (the main
source of tungsten), both in jewelry and as bars.
Electronics
=============
Because it retains its strength at high temperatures and has a high
melting point, elemental tungsten is used in many high-temperature
applications, such as incandescent light bulb, cathode-ray tube, and
vacuum tube filaments, heating elements, and rocket engine nozzles.
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).
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.
The electronic structure of tungsten makes it one of the main sources
for X-ray targets, 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. In addition to its high melting point, when
tungsten is doped with potassium, it leads to an increased shape
stability (compared with non-doped tungsten). This ensures that the
filament does not sag, and no undesired changes occur.
Tungsten is used in producing vibration motors, also known as mobile
vibrators. These motors are integral components that provide tactile
feedback to users, alerting them to incoming calls, messages, and
notifications. Tungsten's high density, hardness, and wear resistance
property helps to endure the high-speed rotational vibrations these
motors generate.
Nanowires
===========
Through top-down nanofabrication processes, tungsten nanowires have
been fabricated and studied since 2002. 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. Such tungsten
nanowires have potential applications in nanoelectronics and
importantly as pH probes and gas sensors. 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. Using the Deal-Grove
model it is possible to predict the oxidation kinetics of nanowires
fabricated through such thermal oxidation processing.
Fusion power
==============
Due to its high melting point and good erosion resistance, tungsten is
a lead candidate for the most exposed sections of the plasma-facing
inner wall of nuclear fusion reactors. Tungsten, as a plasma-facing
component material, features exceptionally low tritium retention
through co-deposition and implantation, which enhances safety by
minimizing radioactive inventory, improves fuel efficiency by making
more fuel available for fusion reactions, and supports operational
continuity by reducing the need for frequent fuel removal from
surfaces. It will be used as the plasma-facing material of the
divertor in the ITER reactor, and is currently in use in the JET test
reactor.
Biological role
======================================================================
Tungsten, at atomic number 'Z' = 74, is the heaviest element known to
be biologically functional. It is used by some bacteria and archaea,
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. 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 molybdopterin-requiring enzymes. 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. 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.
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.
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. The soil's chemistry determines how the tungsten
polymerizes; alkaline soils cause monomeric tungstates; acidic soils
cause polymeric tungstates.
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.
Tungsten has been studied as a biological copper metabolic antagonist,
in a role similar to the action of molybdenum. It has been found that
salts may be used as biological copper chelation chemicals, similar to
the tetrathiomolybdates.
Early epidemiologic association with cancer
=============================================
On 20 August 2002, officials representing the U.S.-based Centers for
Disease Control and Prevention announced that urine tests on leukemia
patient families and control group families in the Fallon, Nevada area
had shown elevated levels of tungsten in the bodies of both groups.
Sixteen recent cases of cancer in children were discovered in the
Fallon area, which has now been identified as a cancer cluster;
although the majority of the cancer victims are not longtime residents
of Fallon. However, there is not enough data to support a link between
tungsten and leukemia at this time. -->
In archaea
============
Tungsten is essential for some archaea. The following
tungsten-utilizing enzymes are known:
* Aldehyde ferredoxin oxidoreductase (AOR) in 'Thermococcus' strain
ES-1
* Formaldehyde ferredoxin oxidoreductase (FOR) in 'Thermococcus
litoralis'
* Glyceraldehyde-3-phosphate ferredoxin oxidoreductase (GAPOR) in
'Pyrococcus furiosus'
A 'wtp' system is known to selectively transport tungsten in archaea:
* WtpA is tungsten-binding protein of ABC family of transporters
* WtpB is a permease
* WtpC is ATPase
Health factors
======================================================================
Because tungsten is a rare metal and its compounds are generally
inert, the effects of tungsten on the environment are limited. The
abundance of tungsten in the Earth's crust is thought to be about 1.5
parts per million. It is the 58th most abundant element found on
Earth.
It was at first believed to be relatively inert and an only slightly
toxic metal, but beginning in the year 2000, the risk presented by
tungsten alloys, its dusts and particulates to induce cancer and
several other adverse effects in animals as well as humans has been
highlighted from in vitro and in vivo experiments.
The median lethal dose LD50 depends strongly on the animal and the
method of administration and varies between 59 mg/kg (intravenous,
rabbits) and 5000 mg/kg (tungsten metal powder, intraperitoneal,
rats).
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.
See also
======================================================================
* Field emission gun
* List of chemical elements name etymologies
* List of chemical elements naming controversies
* Tungsten oxide
External links
======================================================================
* [
http://www.tungsten.com/mtstung.html Properties, Photos, History,
MSDS]
* [
https://www.cdc.gov/niosh/npg/npgd0645.html CDC - NIOSH Pocket
Guide to Chemical Hazards]
* [
http://www.periodicvideos.com/videos/074.htm Tungsten] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://www.pniok.de/w.htm Picture in the collection from Heinrich
Pniok] ()
* [
http://elements.vanderkrogt.net/element.php?sym=W Elementymology
& Elements Multidict by Peter van der Krogt - Tungsten]
* of the International Tungsten Industry Association
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Tungsten
