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Osmium
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Osmium is a chemical element that has the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-black transition metal in the platinum family, and is the densest natural element. The density of osmium is 22610 kg/m³ (22.61 g/cm³), slightly greater than the density of iridium, the second densest element. Osmium is used in alloys with platinum, iridium and other platinum group metals.
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Encyclopedia
Osmium is a chemical element that has the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-black transition metal in the platinum family, and is the densest natural element. The density of osmium is 22610 kg/m³ (22.61 g/cm³), slightly greater than the density of iridium, the second densest element. Osmium is used in alloys with platinum, iridium and other platinum group metals. Osmium is found in nature as an alloy in platinum ore. Alloys of osmium are employed in fountain pen tips, electrical contacts and in other applications where extreme durability and hardness are needed.
History
Osmium (Greek osme meaning "smell") 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. The French chemists Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for further experiments.
In 1803, British scientist Smithson Tennant analysed 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 (ptènos) for winged. However, Tennant, who had the advantage of a much greater 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 osmium after Greek osme meaning "a smell", because of the 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. However, in 1908 cheaper catalysts based on iron and iron oxides were introduced for the first pilot plants.
Characteristics
Physical
Osmium is an extremely dense, blue-grey, hard but brittle metal that remains lustrous even at high temperatures. It proves to be extremely difficult to make. Powdered osmium is easier to make, but when exposed to air leads to the formation of osmium tetroxide (OsO4), which is very toxic. Osmium powder has a characteristic smell of osmium tetroxide. The tetroxide is a very volatile, water-soluble, pale yellow, crystalline solid with a strong smell that boils at 130 °C, and is a powerful oxidizing agent. By contrast osmium dioxide (OsO2) is black, non-volatile and much less reactive or toxic.
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. The extraordinary density of osmium is a consequence of the lanthanide contraction.
Osmium has a very low compressibility. Correspondingly, its bulk modulus is extremely high, commonly quoted as 462 GPa, which is higher than that of diamond but lower than that of aggregated diamond nanorods (there is some debate in the academic community about whether it is in fact this high). A paper by Cynn et al. reported that osmium had this bulk modulus, based on an experimental result, but other authors have cast doubt upon this.
Osmium metal has the highest melting point and the lowest vapor pressure of the platinum family.
Chemical
Common oxidation states of osmium are +4 and +3, but oxidation states from +1 to +8 are observed.
Osmium forms compounds in the oxidation states range from 0 to +8; the most common oxidation states are +2, +3 and +4 as well as +8. The most common compound is the tetrahedral osmium tetroxide with osmium in oxidation state +8. Red osmates [OsO4(OH)2]2- are formed if osmium tetroxide reacts with a base. With ammonia the nitrido osmates [OsO3N]- are formed.
Only two compounds have major applications, osmium tetroxide for staining tissue for electron microscopy and the non volatile osmates for organic oxidation reactions.
The osmium heptafluoride OsF7 and osmium pentafluoride OsF5 are known while the osmium trifluoride OsF3 has not yet been synthesised. The lower oxidation states are stabilized by the larger halogens. Therefore the trichloride, tribromide and triiodide and even an osmium diiodide is known. The oxidation state +1 is only known for the osmium iodide (OsI), while several carbonyl complexes of osmium are known representing the oxidation state 0.
| Chemical name | Chemical formula |
|---|
| Osmium(III) chloride | OsCl3 | | Osmium(IV) oxide | OsO2 | | Osmium(VIII) oxide | OsO4 | | Osmium carbonyl | Os3(CO)12 |
Isotopes
Osmium has seven naturally occurring isotopes, 6 of which are stable: 184Os, 187Os, 188Os, 189Os, 190Os, and (most abundant) 192Os. 186Os undergoes alpha decay with enormously long half-life of (2.0±1.1)×1015 yr and for many practical purposes can be considered to be stable as well. Alpha decay is predicted for all 7 naturally occurring isotopes, but due to very long half-lives, it was observed only for 186Os. It is predicted also that 184Os and 192Os can undergo double beta decay but this radioactivity is not yet observed.
187Os is the daughter of 187Re (half-life 4.56×1010 years) 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 contain an abnormally high isotopic abundance of 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 K-T boundary that marks the extinction of the dinosaurs 65 million years ago.
Occurrence
Osmium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridiosmium (iridium rich). In the nickel and copper deposits the platinum group metals occur as sulfides (i.e. (Pt,Pd)S)), tellurides (i.e. PtBiTe), antimonides (PdSb), and arsenides (i.e. PtAs2), in all of 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 the 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
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 including selenium and tellurium settle to the bottom of the cell as anode mud, which forms the starting point 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 and 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 bisulphate. 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, 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 Tennant and Wollastone used for their separation. 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 (62 kg), would suggest that the production is still less than 1 t per year.
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 that are used in high-wear applications. Osmium alloys such as osmiridium are very hard and, along with other platinum group metals, are almost exclusively used in alloys employed in the tips of fountain pens, instrument pivots, and electrical contacts, as they can resist wear from frequent use. The stylus (needle) in early phonograph designs was also made of osmium, especially for 78-rpm records, until sapphire and industrial diamond replaced the metal in later designs for 45-rpm and 33-rpm long-playing records.
Osmium tetroxide has been used in fingerprint detection and in staining fatty tissue for microscope slides. 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, since osmium atoms are extremely electron dense, making OsO4 an important stain for transmission electron microscopy (TEM) studies of many biological materials. An alloy of 90% platinum and 10% osmium (90/10) is used in surgical implants such as pacemakers and replacement of pulmonary valves.
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.
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 (also 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.
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.
Like palladium, powdered osmium will densely absorb hydrogen atoms, perhaps making it a potential candidate as a metal hydride battery electrode substance, but it will react with potassium hydroxide, the most common battery electrolyte.
Precautions
Finely divided metallic osmium is pyrophoric.
Osmium 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 for the contact to 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 vapour 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.
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