Osmium

• 1. Introduction

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  Osmium, element 76 with the symbol Os, is the densest stable element known. A lump of it weighs about twice as much as a lump of lead the same size. Yet this hard, brittle, bluish-white metal is almost invisible in everyday life, and it ranks among the scarcest stable elements in Earth's crust.

  That contrast raises questions. How did two metals come to share the title of densest, and what finally settled it? What was hiding in the dark dregs that chemists kept finding at the bottom of their flasks? Why has a metal prized for toughness been pushed out of job after job by cheaper rivals? And what do scientists still ask of it today, from the inside of living cells to the age of ancient rocks?

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• 2. A Close Race

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  The numbers for osmium and iridium at 20 °C are 22.587 and 22.562, a gap so thin that each metal was once counted as the densest element. Only in the 1990s did X-ray crystallography deliver measurements accurate enough to show that osmium is the denser one.

  The same stubbornness shows up whenever anyone tries to shape it. Osmium's melting point is the fourth highest of any element, behind only carbon, tungsten and rhenium. Its vapor pressure is the lowest among the platinum-group metals, and it barely compresses. Its bulk modulus is reported between 395 and 462, a range that takes in diamond's 443. Its hardness is moderately high, at 4, and it stays lustrous even when hot. Solid osmium is therefore difficult to machine, form or work.

  Light behaves oddly on a single crystal of osmium. Reflectivity depends strongly on direction, and it changes with how the light is polarized relative to the crystal's c axis. For light polarized parallel to that axis, reflectivity falls to a sharp minimum near 1.5 eV, in the near-infrared. For the perpendicular polarization, the minimum sits at 2.0 eV, an orange wavelength. Both peak in the visible spectrum at around 3.0 eV, which is blue-violet.

  Before any of these measurements existed, osmium was just a dark, insoluble residue at the bottom of a chemist's flask.

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• 3. The Black Residue

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  Platinum arrived in Europe under the name platina, meaning small silver, after being found in the late 17th century in silver mines around the Chocó Department of Colombia. A paper published in 1748 established that it was a distinct metal and not an alloy of something else.

  Chemists who studied platinum dissolved it in aqua regia, a mixture of hydrochloric and nitric acids, to make soluble salts. Every time, a small amount of dark residue refused to dissolve. Joseph Louis Proust thought it was graphite. In 1803, Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin noticed iridium in the black residue but could not gather enough material to go further. Fourcroy and Vauquelin also spotted a metal they called ptène.

  Vauquelin treated the powder alternately with alkali and acids and obtained a volatile oxide. He believed it came from a new metal, and he named that metal ptene, from the Greek word ptènos, meaning winged.

  The credit for the discovery goes to Smithson Tennant and William Hyde Wollaston, working in London in 1803. Tennant had a much larger supply of residue than his French rivals, and it made the difference. He found not one new element but two, iridium and osmium. He reacted the residue with sodium hydroxide at red heat and got a yellow solution. After acidifying it, he distilled off the volatile oxide, osmium tetroxide.

  Tennant named the element osmium, after the Greek osme, meaning a smell. The oxide gave off an odor that was chlorine-like and slightly garlic-like.

  He documented the discovery of both elements in a letter to the Royal Society dated the 21st of June 1804.

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• 4. A Chemistry of Extremes

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  Osmium forms compounds with oxidation states running all the way from -2 to +8, and the most common are +2, +3, +4 and +8. At the low end sits sodium tetracarbonylosmate, with osmium at -2. Reactive compounds of this kind are used to build osmium cluster compounds. Triosmium dodecacarbonyl represents oxidation state 0, and osmium monoiodide is the only compound known with osmium at +1.

  The halogens show how fussy the metal can be. Osmium pentafluoride is known, but osmium trifluoride has never been synthesized. The lower oxidation states are stabilized by the larger halogens, so the trichloride, tribromide, triiodide and even the diiodide all exist.

  At the high end, osmium tetroxide, with osmium at +8, is a volatile, water-soluble solid. Reacting it with a base produces red perosmates, and reacting it with ammonia produces nitrido-osmates. Its relative at +4, osmium dioxide, is darkly colored, non-volatile and far less reactive.

  Despite all this range, bulk osmium at ordinary temperatures and pressures is stable in air. It resists most acids and bases, even aqua regia. Fluorine and chlorine attack it at high temperatures, and hot concentrated nitric acid converts it to the tetroxide. Molten alkalis fused with an oxidizer such as sodium peroxide or potassium chlorate will dissolve it into osmates.

  That tetroxide is also the metal's main hazard. It is very poisonous, and its formation from the metal is thermodynamically favorable even at room temperature. The speed depends on temperature and surface area. A solid piece is not considered hazardous, but a powder reacts fast enough that a handled sample can smell of the tetroxide.

  Used with care, that same compound turns out to be a laboratory workhorse.

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• 5. Rare by Nature

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  The average mass fraction of osmium in the continental crust is 50 parts per trillion, which makes it one of the least abundant stable elements on Earth. It turns up either uncombined or inside natural alloys with iridium. Osmiridium is the iridium-rich version, and iridosmium is the osmium-rich one. In nickel and copper deposits, small amounts of osmium and iridium replace platinum inside sulfides, tellurides, antimonides and arsenides.

  Like iridium, osmium concentrates in three kinds of geologic setting: igneous deposits pushed up from below, impact craters, and deposits reworked from either of those. The largest known primary reserves lie in the Bushveld Igneous Complex in South Africa. The copper-nickel deposits near Norilsk in Russia and the Sudbury Basin in Canada are also significant, and smaller reserves exist in the United States. The alluvial deposits in Colombia's Chocó Department, worked by pre-Columbian people, still yield platinum-group metals. The second large alluvial deposit, in the Ural Mountains of Russia, is still mined.

  Nobody mines for osmium itself. It comes as a by-product of nickel and copper processing. During electrorefining of those metals, silver, gold, the platinum-group metals, selenium and tellurium settle to the bottom of the cell as anode mud. That sludge is the starting material.

  Osmium, ruthenium, rhodium and iridium resist aqua regia and remain as a solid residue, which separates them from platinum, gold and base metals. Treating the residue with sodium oxide dissolves ruthenium and osmium as water-soluble salts, while iridium stays behind. Osmium is then separated by distilling its volatile tetroxide or by extracting it with organic solvents. The distillation route resembles the procedure Tennant and Wollaston used. Modern methods reduce ammonium hexachloroosmate(IV) with hydrogen, yielding a powder or sponge that powder metallurgy can handle.

  Annual world output is estimated at several hundred to a few thousand kilograms. Demand is limited and the by-product supply can meet it, so figures are poorly reported and are often lumped with iridium and ruthenium. United States imports from 2014 to 2021 averaged 155 kilograms a year.

  Because osmium is barely traded, its price is seldom reported. Between 1990 and 2010 the nominal price stayed almost constant. Inflation meanwhile cut the real value from about 950 US dollars per troy ounce to about 600.

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• 6. Beaten by Cheaper Rivals

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  A fountain pen tipped with an osmium alloy is the textbook use of the metal. Pure osmium is virtually unforgeable when fully dense and very fragile when sintered, so it is usually alloyed. Osmiridium and its relatives tip pens, form instrument pivots and make electrical contacts that must withstand frequent operation.

  From about 1945 to 1955, osmium alloys also tipped phonograph styli, during the late 78 rpm and early LP and 45 record years. They outlasted steel and chromium needle points. They wore out far faster than the costlier sapphire and diamond tips, and they were discontinued.

  The Austrian chemist Auer von Welsbach developed the Oslamp in 1898, with a filament made of osmium. He introduced it commercially in 1902. Within a few years tungsten replaced osmium, because tungsten is more abundant, cheaper and more stable. Tungsten has the highest melting point of any metal, and in light bulbs it raises luminous efficacy and extends the life of incandescent lamps.

  The lamp left a name behind. Osram was founded in 1906 when three German companies combined their lamp production: Auer-Gesellschaft, AEG and Siemens & Halske. Its name joins osmium with Wolfram, the German word for tungsten.

  Osmium also had a brief moment in the Haber process, the reaction of nitrogen and hydrogen to make ammonia. Uranium and osmium were early successful catalysts, with yields high enough to make the process economical. A BASF group led by Carl Bosch bought most of the world's osmium supply for this purpose. In 1908 the same group introduced cheaper iron-based catalysts for the first pilot plants, and the need for the rare metal vanished.

  Space offered another chance. At 600 Å, osmium reflects twice as much ultraviolet light as gold does, which suits space-based spectrometers where mirror size is limited. Osmium-coated mirrors flew on several Space Shuttle missions. It soon became clear that oxygen radicals in low Earth orbit were abundant enough to badly deteriorate the osmium layer.

  A battery electrode was proposed as well. Like palladium, powdered osmium readily adsorbs hydrogen atoms, which suggests a metal-hydride battery use. The metal is expensive, though, and it would react with potassium hydroxide, the most common battery electrolyte.

  Only two families of osmium compounds, it turns out, hold major applications.

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• 7. Tools for Science

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  In electron microscopy labs, osmium tetroxide is used to stain fatty tissue. As a strong oxidant, it cross-links lipids, mainly by reacting with unsaturated carbon-carbon bonds. That fixes biological membranes in place and stains them at the same time. Osmium atoms are extremely electron-dense, so the stain greatly improves image contrast in transmission electron microscopy, where carbon materials otherwise show very weakly. Osmium ferricyanide has a similar fixing and staining action. The tetroxide has also been used to detect fingerprints.

  In organic synthesis, the tetroxide and its derivative potassium osmate are important oxidants. The Sharpless asymmetric dihydroxylation uses osmate to convert a double bond into a vicinal diol. Karl Barry Sharpless was awarded the Nobel Prize in Chemistry in 2001 for it. The tetroxide is so expensive for this job that potassium permanganate is often used instead, even though it gives lower yields. The non-volatile osmates are also used in other organic oxidation reactions.

  Osmium has seven naturally occurring isotopes, five of them stable: osmium-187, 188, 189, 190 and, the most abundant, 192. Osmium-187 is the decay product of rhenium-187, and geologists use it extensively to date both terrestrial and meteoric rocks. It has also been used to gauge the intensity of continental weathering over geologic time. It helps fix minimum ages for the stabilization of the mantle roots of continental cratons. The decay explains why rhenium-rich minerals are abnormally rich in osmium-187.

  The same isotope frustrates a different technique. Osmium-187 has a nuclear spin of 1/2, a low magnetic moment and a natural abundance of only 1.64 percent. That makes it one of the hardest naturally abundant isotopes to study by NMR spectroscopy.

  Beyond the natural isotopes, at least 37 artificial radioisotopes and 20 nuclear isomers exist, with mass numbers from 160 to 203. The longest-lived of them, osmium-194, has a half-life of 6.0 years.

  Two primordial isotopes go the other way. Osmium-184 and osmium-186 undergo alpha decay, with current best half-lives of 1.12e13 and 2.0e15 years, about 140000 times the age of the universe. For practical purposes they count as stable.

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