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= Osmium =
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Introduction
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Osmium () is a chemical element; it has symbol Os and atomic number
76. It is a hard, brittle, bluish-white transition metal in the
platinum group that is found as a trace element in alloys, mostly in
platinum ores. Osmium has the highest density of any stable element
(). It is also one of the rarest elements in the Earth's crust, with
an estimated abundance of 50 parts per trillion (ppt). Manufacturers
use alloys of osmium with platinum, iridium, and other platinum-group
metals for fountain pen nib tipping, electrical contacts, and other
applications that require extreme durability and hardness.
Physical properties
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Osmium is a hard, brittle, blue-gray metal, and the densest stable
element--about twice as dense as lead. The density of osmium is
slightly greater than that of iridium; the two are so similar (22.587
versus at 20 °C) that each was at one time considered to be the
densest element. Only in the 1990s were measurements made accurately
enough (by means of X-ray crystallography) to be certain that osmium
is the denser of the two.
Osmium has a blue-gray tint. The reflectivity of single crystals of
osmium is complex and strongly direction-dependent, with light in the
red and near-infrared wavelengths being more strongly absorbed when
polarized parallel to the 'c' crystal axis than when polarized
perpendicular to the 'c' axis; the 'c'-parallel polarization is also
slightly more reflected in the mid-ultraviolet range. Reflectivity
reaches a sharp minimum at around 1.5 eV (near-infrared) for the
'c'-parallel polarization and at 2.0 eV (orange) for the
'c'-perpendicular polarization, and peaks for both in the visible
spectrum at around 3.0 eV (blue-violet).
Osmium is a hard but brittle metal that remains lustrous even at high
temperatures. It has a very low compressibility. Correspondingly, its
bulk modulus is extremely high, reported between and , which rivals
that of diamond (). The hardness of osmium is moderately high at .
Because of its hardness, brittleness, low vapor pressure (the lowest
of the platinum-group metals), and very high melting point (the fourth
highest of all elements, after carbon, tungsten, and rhenium), solid
osmium is difficult to machine, form, or work.
Chemical properties
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colspan=2|Oxidation states of osmium
−4 [OsIn6−'x'Sn'x']
−2
−1
0 12}
|-
| +1 ||
|-
| +2 ||
|-
| +3 ||
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| +4 || Osmium dioxide, Osmium(IV) chloride
|-
| +5 ||
|-
| +6 ||Osmium hexafluoride
|-
| +7 ||
|-
| +8 ||Osmium tetroxide,
|}
Osmium forms compounds with oxidation states ranging from −4 to +8.
The most common oxidation states are +2, +3, +4, and +8. The +8
oxidation state is notable for being the highest attained by any
chemical element aside from iridium's +9 and is encountered only in
xenon, ruthenium, hassium, iridium, and plutonium. Examples of the −1
and −2 oxidation states are and , respectively; these reactive
compounds are used to synthesize osmium cluster compounds.
The most common compound exhibiting the +8 oxidation state, osmium
tetroxide (), is a volatile, water-soluble solid with a "pronounced
and nauseating" smell. Osmium tetroxide forms red osmates upon
reaction with a base. With ammonia, it forms the nitrido-osmates . The
+4 oxide, osmium dioxide (), is a darkly-colored, non-volatile, and
much less reactive compound.
Osmium pentafluoride () is known, but osmium trifluoride () has not
yet been synthesized. The lower oxidation states are stabilized by the
larger halogens, so that the trichloride, tribromide, triiodide, and
even diiodide are known. The oxidation state +1 is known only for
osmium monoiodide (OsI), whereas several carbonyl complexes of osmium,
such as triosmium dodecacarbonyl (), represent oxidation state 0.
In general, the lower oxidation states of osmium are stabilized by
ligands that are good σ-donors (such as amines) and π-acceptors
(heterocycles containing nitrogen). The higher oxidation states are
stabilized by strong σ- and π-donors, such as and .
Despite its broad range of compounds in numerous oxidation states,
osmium in bulk form at ordinary temperatures and pressures is stable
in air. It resists attack by most acids and bases including aqua
regia, but is attacked by and at high temperatures, and by hot
concentrated nitric acid to produce . It can be dissolved by molten
alkalis fused with an oxidizer such as sodium peroxide () or potassium
chlorate () to give osmates such as potassium osmate.
Isotopes
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Osmium has seven naturally occurring isotopes, five of which are
stable: , , , , and (most abundant) . At least 37 artificial
radioisotopes and 20 nuclear isomers exist, with mass numbers ranging
from 160 to 203; the most stable of these is with a half-life of 6
years.
undergoes alpha decay with such a long half-life years,
approximately times the age of the universe, that for practical
purposes it can be considered stable. is also known to undergo alpha
decay with a half-life of years. Alpha decay is predicted for all the
other naturally occurring isotopes, but this has never been observed,
presumably due to very long half-lives. It is predicted that and can
undergo double beta decay, but this radioactivity has not been
observed yet.
189Os has a spin of 5/2 but 187Os has a nuclear spin 1/2. Its low
natural abundance (1.64%) and low nuclear magnetic moment means that
it is one of the most difficult natural abundance isotopes for NMR
spectroscopy.
is the descendant of (half-life ) and is used extensively in dating
terrestrial as well as meteoric rocks (see 'Rhenium-osmium dating').
It has also been used to measure the intensity of continental
weathering over geologic time and to fix minimum ages for
stabilization of the mantle roots of continental cratons. This decay
is a reason why rhenium-rich minerals are abnormally rich in .
However, the most notable application of osmium isotopes in geology
has been in conjunction with the abundance of iridium, to characterise
the layer of shocked quartz along the Cretaceous-Paleogene boundary
that marks the extinction of the non-avian dinosaurs 65 million years
ago.
History
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Osmium was discovered in 1803 by Smithson Tennant and William Hyde
Wollaston in London, England. The discovery of osmium is intertwined
with that of platinum and the other metals of the platinum group.
Platinum reached Europe as 'platina' ("small silver"), first
encountered in the late 17th century in silver mines around the Chocó
Department, in Colombia. The discovery that this metal was not an
alloy, but a distinct new element, was published in 1748.
Chemists who studied platinum dissolved it in aqua regia (a mixture of
hydrochloric and nitric acids) to create soluble salts. They always
observed a small amount of a dark, insoluble residue. Joseph Louis
Proust thought that the residue was graphite. Victor Collet-Descotils,
Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also
observed iridium in the black platinum residue in 1803, but did not
obtain enough material for further experiments. Later the two French
chemists Fourcroy and Vauquelin identified a metal in a platinum
residue they called 'ptène'.
In 1803, Smithson Tennant analyzed the insoluble residue and concluded
that it must contain a new metal. Vauquelin treated the powder
alternately with alkali and acids and obtained a volatile new oxide,
which he believed was of this new metal--which he named 'ptene', from
the Greek word (ptènos) for winged. However, Tennant, who had the
advantage of a much larger amount of residue, continued his research
and identified two previously undiscovered elements in the black
residue, iridium and osmium. He obtained a yellow solution (probably
of 'cis'-[Os(OH)2O4]2−) by reactions with sodium hydroxide at red
heat. After acidification he was able to distill the formed OsO4. He
named it osmium after Greek 'osme' meaning "a smell", because of the
chlorine-like and slightly garlic-like smell of the volatile osmium
tetroxide. Discovery of the new elements was documented in a letter to
the Royal Society on June 21, 1804.
Uranium and osmium were early successful catalysts in the Haber
process, the nitrogen fixation reaction of nitrogen and hydrogen to
produce ammonia, giving enough yield to make the process economically
successful. At the time, a group at BASF led by Carl Bosch bought most
of the world's supply of osmium to use as a catalyst. Shortly
thereafter, in 1908, cheaper catalysts based on iron and iron oxides
were introduced by the same group for the first pilot plants, removing
the need for the expensive and rare osmium.
Osmium is now obtained primarily from the processing of platinum and
nickel ores.
Occurrence
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Osmium is one of the least abundant stable elements in Earth's crust,
with an average mass fraction of 50 parts per trillion in the
continental crust.
Osmium is found in nature as an uncombined element or in natural
alloys; especially the iridium-osmium alloys, osmiridium (iridium
rich), and iridosmium (osmium rich). In nickel and copper deposits,
the platinum-group metals occur as sulfides (i.e., ), tellurides
(e.g., ), antimonides (e.g., ), and arsenides (e.g., ); in all these
compounds platinum is exchanged by a small amount of iridium and
osmium. As with all of the platinum-group metals, osmium can be found
naturally in alloys with nickel or copper.
Within Earth's crust, osmium, like iridium, is found at highest
concentrations in three types of geologic structure: igneous deposits
(crustal intrusions from below), impact craters, and deposits reworked
from one of the former structures. The largest known primary reserves
are in the Bushveld Igneous Complex in South Africa, though the large
copper-nickel deposits near Norilsk in Russia, and the Sudbury Basin
in Canada are also significant sources of osmium. Smaller reserves can
be found in the United States. The alluvial deposits used by
pre-Columbian people in the Chocó Department, Colombia, are still a
source for platinum-group metals. The second large alluvial deposit
was found in the Ural Mountains, Russia, which is still mined.
Production
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Osmium is obtained commercially as a by-product from nickel and copper
mining and processing. During electrorefining of copper and nickel,
noble metals such as silver, gold and the platinum-group metals,
together with non-metallic elements such as selenium and tellurium,
settle to the bottom of the cell as 'anode mud', which forms the
starting material for their extraction. Separating the metals requires
that they first be brought into solution. Several methods can achieve
this, depending on the separation process and the composition of the
mixture. Two representative methods are fusion with sodium peroxide
followed by dissolution in aqua regia, and dissolution in a mixture of
chlorine with hydrochloric acid. Osmium, ruthenium, rhodium, and
iridium can be separated from platinum, gold, and base metals by their
insolubility in aqua regia, leaving a solid residue. Rhodium can be
separated from the residue by treatment with molten sodium bisulfate.
The insoluble residue, containing ruthenium, osmium, and iridium, is
treated with sodium oxide, in which Ir is insoluble, producing
water-soluble ruthenium and osmium salts. After oxidation to the
volatile oxides, is separated from by precipitation of (NH4)3RuCl6
with ammonium chloride.
After it is dissolved, osmium is separated from the other
platinum-group metals by distillation or extraction with organic
solvents of the volatile osmium tetroxide. The first method is similar
to the procedure used by Tennant and Wollaston. Both methods are
suitable for industrial-scale production. Modern methods involve
reducing ammonium hexachloroosmate(IV) using hydrogen, yielding the
metal as a powder or sponge that can be treated using powder
metallurgy techniques.
Estimates of annual worldwide osmium production are on the order of
several hundred to a few thousand kilograms. Production and
consumption figures for osmium are not well reported because demand
for the metal is limited and can be fulfilled with the byproducts of
other refining processes. To reflect this, statistics often report
osmium with other minor platinum group metals such as iridium and
ruthenium. US imports of osmium from 2014 to 2021 averaged 155 kg
annually.
Applications
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Because osmium is virtually unforgeable when fully dense and very
fragile when sintered, it is rarely used in its pure state, but is
instead often alloyed with other metals for high-wear applications.
Osmium alloys such as osmiridium are very hard and, along with other
platinum-group metals, are used in the tips of fountain pens,
instrument pivots, and electrical contacts, as they can resist wear
from frequent operation. They were also used for the tips of
phonograph styli during the late 78 rpm and early "LP" and "45" record
era, circa 1945 to 1955. Osmium-alloy tips were significantly more
durable than steel and chromium needle points, but wore out far more
rapidly than competing, and costlier, sapphire and diamond tips, so
they were discontinued.
Only two osmium compounds have major applications: osmium tetroxide
for staining tissue in electron microscopy and for the oxidation of
alkenes in organic synthesis, and the non-volatile osmates for organic
oxidation reactions.
Osmium tetroxide has been used in fingerprint detection and in
staining fatty tissue for optical and electron microscopy. As a strong
oxidant, it cross-links lipids mainly by reacting with unsaturated
carbon-carbon bonds and thereby both fixes biological membranes in
place in tissue samples and simultaneously stains them. Because osmium
atoms are extremely electron-dense, osmium staining greatly enhances
image contrast in transmission electron microscopy (TEM) studies of
biological materials. Those carbon materials otherwise have very weak
TEM contrast. Another osmium compound, osmium ferricyanide (OsFeCN),
exhibits similar fixing and staining action.
The tetroxide and its derivative potassium osmate are important
oxidants in organic synthesis. For the Sharpless asymmetric
dihydroxylation, which uses osmate for the conversion of a double bond
into a vicinal diol, Karl Barry Sharpless was awarded the Nobel Prize
in Chemistry in 2001. OsO4 is very expensive for this use, so KMnO4 is
often used instead, even though the yields are less for this cheaper
chemical reagent.
In 1898, the Austrian chemist Auer von Welsbach developed the Oslamp
with a filament made of osmium, which he introduced commercially in
1902. After only a few years, osmium was replaced by tungsten, which
is more abundant (and thus cheaper) and more stable. Tungsten has the
highest melting point among all metals, and its use in light bulbs
increases the luminous efficacy and life of incandescent lamps.
The light bulb manufacturer Osram (founded in 1906, when three German
companies, Auer-Gesellschaft, AEG and Siemens & Halske, combined
their lamp production facilities) derived its name from the elements
of osmium and 'Wolfram' (the latter is German for tungsten).
Like palladium, powdered osmium effectively absorbs hydrogen atoms.
This could make osmium a potential candidate for a metal-hydride
battery electrode. However, osmium is expensive and would react with
potassium hydroxide, the most common battery electrolyte.
Osmium has high reflectivity in the ultraviolet range of the
electromagnetic spectrum; for example, at 600 Å osmium has a
reflectivity twice that of gold. This high reflectivity is desirable
in space-based UV spectrometers, which have reduced mirror sizes due
to space limitations. Osmium-coated mirrors were flown in several
space missions aboard the Space Shuttle, but it soon became clear that
the oxygen radicals in low Earth orbit are abundant enough to
significantly deteriorate the osmium layer.
File:Sharpless Dihydroxylation Scheme.png|The Sharpless
dihydroxylation: RL = largest substituent; RM = medium-sized
substituent; RS = smallest substituent
File:NASAmirroroxidation.jpg|Post-flight appearance of Os, Ag, and Au
mirrors from the front (left images) and rear panels of the Space
Shuttle. Blackening reveals oxidation due to irradiation by oxygen
atoms.
File:Osmium-2.jpg|A bead of osmium, about 0.5 cm in diameter,
displaying the metal's reflectivity
Precautions
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The primary hazard presented by metallic osmium is the potential
formation of osmium tetroxide (OsO4), which is volatile and very
poisonous. This reaction is thermodynamically favorable at room
temperature, but the rate depends on the temperature and surface area
of the metal. As a result, bulk material is not considered hazardous
while powders react quickly enough that samples can sometimes smell
like OsO4 if they are handled in air.
Price
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Between 1990 and 2010, the nominal price of osmium metal was almost
constant, while inflation reduced the real value from ~US950 $/ozt to
~US600 $/ozt Because osmium has few commercial applications, it is not
heavily traded and prices are seldom reported.
External links
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* [
http://www.periodicvideos.com/videos/076.htm Osmium] at 'The
Periodic Table of Videos' (University of Nottingham)
* Flegenheimer, J. (2014).
[
https://web.archive.org/web/20150619170958/http://www.uff.br/RVQ/index.php/rvq/article/viewFile/660/450
"The Mystery of the Disappearing Isotope"] (via the Wayback Machine).
'Revista Virtual de Química'. V. XX.
*
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Osmium
