Osmium

History

Osmium was discovered in 1803 by Smithson Tennant and William Hyde Wollaston in London, England, intertwined with the discovery of platinum and other platinum-group metals. Platinum reached Europe as platina from silver mines in Colombia, and its nature as a new element was published in 1748. Chemists dissolving platinum in aqua regia observed a dark insoluble residue; Joseph Louis Proust thought it was graphite, while Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin observed iridium in the residue in 1803 but lacked enough material for further work.

Fourcroy and Vauquelin later identified a metal they called ptène. Smithson Tennant analyzed the residue and concluded it contained a new metal. Vauquelin obtained a volatile new oxide he named ptene, but Tennant, with more residue, identified two new elements: iridium and osmium. He obtained a yellow solution by reactions with sodium hydroxide at red heat, then distilled osmium tetroxide after acidification, naming it osmium from Greek osme meaning a smell, due to the chlorine-like and garlic-like odor.

Discovery was documented in a letter to the Royal Society on June 21, 1804. Uranium and osmium were early successful catalysts in the Haber process for ammonia production, and a BASF group led by Carl Bosch bought most of the world's osmium supply for this use. In 1908, cheaper iron-based catalysts replaced osmium. Osmium is now obtained primarily from processing platinum and nickel ores.

Characteristics

Osmium is a hard, brittle, blue-gray metal and the densest stable element, roughly twice as dense as lead. Its density is slightly greater than that of iridium, and the two are so similar that each was once considered the densest; only in the 1990s did X-ray crystallography confirm osmium as denser. The reflectivity of single crystals of osmium is complex and strongly direction-dependent, with light in red and near-infrared wavelengths absorbed more when polarized parallel to the c crystal axis than perpendicular; reflectivity reaches a sharp minimum at around 1.5 eV for c-parallel and 2.0 eV for c-perpendicular, peaking in the visible at about 3.0 eV.

Osmium is hard but brittle, remains lustrous at high temperatures, and has very low compressibility. Its bulk modulus is extremely high, rivaling that of diamond, and its hardness is moderately high. Due to its hardness, brittleness, low vapor pressure (lowest among platinum-group metals), and very high melting point (fourth highest of all elements), solid osmium is difficult to machine or work. Osmium forms compounds with oxidation states from −2 to +8, with +2, +3, +4, and +8 being most common.

The +8 compound osmium tetroxide is toxic, volatile, water-soluble, and has a pronounced nauseating smell; it forms red perosmates with alkali and nitrido-osmates with ammonia. The +4 oxide, osmium dioxide, is dark, non-volatile, and much less reactive.

Production

Osmium is obtained commercially as a by-product from nickel and copper mining and processing. During electrorefining of copper and nickel, noble metals including silver, gold, and platinum-group metals settle as anode mud, which is the starting material for extraction. Separating the metals requires bringing them into solution, achieved by methods such as fusion with sodium peroxide followed by dissolution in aqua regia, or 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 is separated by treatment with molten sodium bisulfate, and the remaining residue containing ruthenium, osmium, and iridium is treated with sodium oxide, in which iridium is insoluble, producing water-soluble ruthenium and osmium salts. After oxidation to volatile oxides, osmium tetroxide is separated from ruthenium tetroxide by precipitation with ammonium chloride.

Osmium is then separated by distillation or extraction with organic solvents of the volatile osmium tetroxide. Modern methods reduce ammonium hexachloroosmate(IV) using hydrogen to yield osmium powder or sponge. Annual worldwide production is estimated at several hundred to a few thousand kilograms; US imports from 2014 to 2021 averaged 155 kg annually.

Applications

Osmium is rarely used in its pure state due to its fragility when sintered and difficulty in forging when fully dense, but is often alloyed with other metals for high-wear applications. Osmium alloys such as osmiridium are very hard and used in fountain pen nib tips, instrument pivots, and electrical contacts. They were also used for phonograph styli tips from about 1945 to 1955, being more durable than steel and chromium points but wearing out faster than sapphire and diamond tips.

Only two osmium compounds have major applications: osmium tetroxide for staining tissue in electron microscopy and for oxidizing alkenes in organic synthesis, and non-volatile osmates for organic oxidation reactions. Osmium tetroxide is used in fingerprint detection and staining fatty tissue for microscopy; it cross-links lipids by reacting with unsaturated carbon–carbon bonds, fixing biological membranes and enhancing contrast in transmission electron microscopy.

Another compound, osmium ferricyanide, exhibits similar fixing and staining action. The tetroxide and its derivative potassium osmate are important oxidants in organic synthesis; for the Sharpless asymmetric dihydroxylation using osmate, Karl Barry Sharpless received the Nobel Prize in Chemistry in 2001. In 1898, Auer von Welsbach developed the Oslamp with an osmium filament, introduced commercially in 1902, but osmium was soon replaced by tungsten, which is more abundant and stable.

Did You Know?

• 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 metal is generally harmless, as it is unaffected by oxygen below 400°C, but finely divided powder can slowly react with air to form toxic osmium tetroxide.