Osmium meaning "smell") is a chemical element with the 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 is the densest naturally occurring
element, with a density of 22.59 g/cm3. Its alloys with platinum, iridium, and other platinum
group metals are employed in fountain pen tips, electrical contacts, and other applications
where extreme durability and hardness are needed.
Characteristics Physical properties
Osmium has a blue-gray tint and is the densest stable element, slightly denser than iridium.
Calculations of density from the X-ray diffraction data may produce the most reliable data for
these elements, giving a value of 22.562±0.009 g/cm3 for iridium versus 22.587±0.009 g/cm3 for
osmium. 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 395 and 462 GPa, which rivals that
of diamond. The hardness of osmium is moderately high at 4 GPa. Because of its hardness, brittleness,
low vapor pressure, and very high melting point, solid osmium is difficult to machine,
form, or work. Chemical properties
Osmium forms compounds with oxidation states ranging from −2 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 and, aside from osmium, is encountered only in xenon, ruthenium, hassium,
iridium, plutonium, and curium. The oxidation states −1 and −2 represented by the two
reactive compounds Na 2[Os
4(CO) 13] and Na
2[Os(CO) 4] are used in the synthesis of osmium cluster
compounds. The most common compound exhibiting the +8
oxidation state is osmium tetroxide. This toxic compound is formed when powdered osmium
is exposed to air, and is a very volatile, water-soluble, pale yellow, crystalline solid
with a strong smell. Osmium powder has the characteristic smell of osmium tetroxide.
Osmium tetroxide forms red osmates OsO 4(OH)2−
2 upon reaction with a base. With ammonia, it forms the nitrido-osmates OsO
3N−. Osmium tetroxide boils at 130 °C and is a powerful oxidizing agent. By contrast,
osmium dioxide is black, non-volatile, and much less reactive and toxic.
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 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 iodide, whereas several carbonyl
complexes of osmium, such as triosmium dodecacarbonyl (Os
3(CO) 12), represent oxidation state 0.
In general, the lower oxidation states of osmium are stabilized by ligands that are
good σ-donors and π-acceptors. The higher oxidation states are stabilized by strong
σ- and π-donors, such as O2− and N3−. Despite its broad range of compounds in numerous
oxidation states, osmium in bulk form at ordinary temperatures and pressures resists attack
by all acids and alkalis, and even aqua regia. Isotopes
Osmium has seven naturally occurring isotopes, six of which are stable: 184Os, 187Os, 188Os,
189Os, 190Os, and 192Os. 186Os undergoes alpha decay with such a long half-life×1015 years)
that for practical purposes it can be considered stable. Alpha decay is predicted for all seven
naturally occurring isotopes, but due to very long half-lives, it has been observed only
for 186Os. It is predicted that 184Os and 192Os can undergo double beta decay but this
radioactivity has not been observed yet. 187Os is the daughter of 187Re and is used
extensively in dating terrestrial as well as meteoric rocks. 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 187Os. However, the most notable application of Os in dating
has been in conjunction with iridium, to analyze the layer of shocked quartz along the Cretaceous–Paleogene
boundary that marks the extinction of the dinosaurs 66 million years ago.
History 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, 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 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 the
black residue in 1803, but did not obtain enough material for further experiments.
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 to be of this new metal—which he named ptene,
from the Greek word πτηνος 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 solution2O4]2−)
by reactions with sodium hydroxide at red heat. After acidification he was able to distill
the formed OsO4. He named osmium after Greek osme meaning "a smell", because of the ashen
and smoky 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.
Nowadays osmium is obtained primarily from the processing of platinum and nickel ores.
Occurrence
Osmium is the least abundant stable element 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, and iridosmium. In nickel and copper deposits, the platinum
group metals occur as sulfidesS)), tellurides, antimonides, and arsenides; 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, 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
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.
In order to separate the metals, they must first be brought into solution. Several methods
are available 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 Ru, Os and Ir, is treated
with sodium oxide, in which Ir is insoluble, producing water-soluble Ru and Os salts. After
oxidation to the volatile oxides, RuO 4 is separated from OsO
4 by precipitation of3RuCl6 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. In either case, the product is reduced using hydrogen, yielding
the metal as a powder or sponge that can be treated using powder metallurgy techniques.
Neither the producers nor the United States Geological Survey published any production
amounts for osmium. Estimations of the United States consumption date published from 1971,
which gives a consumption in the United States of 2000 troy ounces, would suggest that the
production is still less than 1 ton per year. In 2012, the estimated US production of osmium
was 75 kg. Applications
Because of the volatility and extreme toxicity of its oxide, osmium is rarely used in its
pure state, and is instead often alloyed with other metals. Those alloys are utilized in
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. Although very durable compared to steel and chromium needle points,
osmium alloy tips wore out far more rapidly than competing but costlier sapphire and diamond
tips and were discontinued. 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 studies of biological materials. Those carbon
materials have otherwise very weak TEM contrast. Another osmium compound, osmium ferricyanide,
exhibits similar fixing and staining action. The tetroxide and a related compound, potassium
osmate, are important oxidants for chemical synthesis, despite being very poisonous. For
the Sharpless asymmetric dihydroxylation, which uses osmate for the conversion of a
double bond into a vicinal diol, Karl Barry Sharpless won the Nobel Prize in Chemistry
in 2001. Apparently, 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 an 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 the more stable metal tungsten. Tungsten has the highest melting point of
any metal, and using it in light bulbs increases the luminous efficacy and life of incandescent
lamps. The light bulb manufacturer Osram derived
its name from the elements of osmium and Wolfram. 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 the low earth orbit are abundant enough to significantly deteriorate
the osmium layer. The only known clinical use of osmium appears
to be for synovectomy in arthritic patients in Scandinavia. It involves the local administration
of osmium tetroxide which is a highly toxic compound. The lack of reports of long-term
side effects suggest that osmium itself can be biocompatible, although this depends on
the osmium compound administered. In 2011, osmium(VI) and osmium(II) compounds were reported
to show anticancer activity in vivo, it indicated a promising future for using osmium compounds
as anticancer drugs.
Precautions Finely divided metallic osmium is pyrophoric
and reacts with oxygen at room temperature forming volatile osmium tetroxide. Some osmium
compounds are also converted to the tetroxide if oxygen is present. This makes osmium tetroxide
the main source of contact with the environment. Osmium tetroxide is highly volatile and penetrates
skin readily, and is very toxic by inhalation, ingestion, and skin contact. Airborne low
concentrations of osmium tetroxide vapor can cause lung congestion and skin or eye damage,
and should therefore be used in a fume hood. Osmium tetroxide is rapidly reduced to relatively
inert compounds by polyunsaturated vegetable oils, such as corn oil.
Price Osmium is usually sold as a minimum 99.9%
pure powder. Like other precious metals, it is measured by troy weight and by grams. Its
price in 2012 was about $400 per troy ounce, depending on the quantity and its supplier.
References
External links Chemistry in its element podcast from the
Royal Society of Chemistry's Chemistry World: Osmium
Osmium at The Periodic Table of Videos FLEGENHEIMER, J.. The mystery of the disappearing
isotope. Revista Virtual de Química. V. XX. Available at http:www.uff.brindex.phparticle660/450