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History

Osmium (Greek osme meaning "a smell") was discovered in 1803 by Smithson Tennat and William Hyde Wollaston in London, England.

Wollaston and Tennant were looking for a way to purify platinum by dissolution of native platinum ore in aqua regia.  Large amounts of insoluble black powder remained as a byproduct of this operation.

Wollaston concentrated on the soluble portion and discovered Palladium (in 1802) and Rhodium (in 1804), while Tennant examined the insoluble residue.  In the summer of 1803, Tennant identified two new elements; osmium and iridium.  Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.

Characteristics

Osmium in a metallic form is extremely dense, blue white, brittle and lustrous even at high temperatures, but proves to be extremely difficult to make.  Powdered osmium is easier to make, but powdered osmium exposed to air leads to the formation of osmium tetroxide (OsO₄), which is toxic.  The oxide is also a powerful oxidizing agent, emits a strong smell and boils at 130°C.

Due to its very high density osmium is generally considered to be the densest known element, narrowly defeating iridium.  However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22650 kg/m³ for iridium versus 22610 kg/m³ for osmium.  Definitive selection between the two is therefore not possible at this time.   If one distinguishes different isotopes, then the highest density ordinary substance would be ¹⁹²Os.

Osmium also has a very low compressibility.  Correspondingly, its bulk modulus is extremely high -- commonly quoted as 462GPa, which is higher than that of diamond but lower than that of aggregated diamond nanorods - although there is some debate in the academic community about whether it is in fact this high.

This metal has the highest melting point and the lowest vapor pressure of the platinum family.  Common oxidation sates of osmium are +4 and +3, but oxidation states from +1 to +8 are observed.

Occurrence

Turkey with 127,000 tons has the world's largest known reserve of osmium.   Bulgaria also has substantial reserves of about 2500 tons.  This transition metal is also found in iridiosmium, a naturally occurring alloy of iridium and osmium, and in platinum-bearing river sands in the Ural Mountains, and North and South America.   It also occurs in nickel-bearing ores found in the Sudbury, Ontario, Canada region with other platinum group metals.  Even though the quantity of platinum metals found in these ores is small, the large volume of nickel ores processed makes commercial recovery possible.

Value

Osmium is quite valuable, costing about US $400 per troy ounce.  One of the stable isotopes, ¹⁸⁷Os, is worth about $25,000 per gram.

Applications

Because of the extreme toxicity of its oxide, osmium is rarely used in its pure state, and is instead often alloyed with other metals that are used in high wear applications.   Osmium alloys such as osmiridium are very hard and, along with other platinum group metals, is almost entirely used in alloys employed in the tips of fountain pens, phonograph needles, instrument pivots, and electrical contacts, as they can resist wear from frequent use.

Osmium tetroxide has been used in fingerprint detection and in staining fatty tissue for microscope slides.  As a strong oxidant, it cross-links lipids by fixing biological membranes in place.  Furthermore, osmium atoms are extremely electron dense, making OsO₄ an important stain for transmission electron microscopy (TEM) studies of a wide range of biological materials.  An alloy of 90% platinum and 10% osmium (90/10) is used in surgical implants such as pacemakers and replacement pulmonary valves.

The tetroxide (and a related compound, potassium osmate) are important oxidants for chemical synthesis, despite being very poisonous.

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 (originally known as Wolfram).   Tungsten has the highest melting point of any metal, and using it in light bulbs increases the luminous efficacy and life of incandescent lamps.

Isotopes

Osmium has seven naturally-occurring isotopes, 6 of which are stable: ¹⁸⁴Os, ¹⁸⁷Os, ¹⁸⁸Os, ¹⁸⁹Os, ¹⁹⁰Os, and (most abundant) ¹⁹²Os.  The other, ¹⁸⁶Os, has an enormously long half-life of 2.0 x 10¹⁵ years and for practical purposes can be considered to be stable as well.  ¹⁸⁷Os is the daughter of ¹⁸⁷Re (half-life 4.56 x 10¹⁰ years) and is most often measured in an ¹⁸⁷Os/¹⁸⁸Os ratio.  This ratio, as well as the ¹⁸⁷Re/¹⁸⁷Os ratio, have been 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.  However, the most notable application of Os in dating has been in conjunction with iridium, to analyze the layer of shocked quartz along the K-T boundary that marks the extinction of the dinosaurs 65 million years ago.

Precautions

Osmium tetroxide is highly toxic.  Airborne low concentrations of osmium can cause lung congestion, skin or eye damage.