Osmium

0.02 %

{ syn. }

1.58%

1.6%

13.3%

16.1%

26.4 %

{ syn. }

41.0 %

Powder

Risk

Powder

Osmium is a chemical element with the element symbol Os and atomic number 76; in the periodic table of the elements, it is in the 8th group of the iron group. It is a hard, brittle, steel- blue transition metal and is one of the platinum metals. Osmium has 462 GPa with the highest compressive modulus of all elements, exceeded only aggregated diamond nanorods, and 22.6 g/cm3, the highest density.

Biological functions of osmium are known neither in human nor in other organisms. Technically, osmium is used only because of its high price, when durability and hardness are crucial.

• 9.1 osmium tetroxide
• 9.2 Further osmium
• 9.3 Complexes

History

Osmium, the heaviest homologues of group 8 in the periodic table, was discovered in 1804 by Smithson Tennant along with iridium in the residue of platinum dissolved in aqua regia. The name " osmium " comes from the radish -like odor ( Greek: ὀσμή osme ) of its low concentration of volatile Tetroxids.

The first important application of the metal was at the beginning of the 20th century its use as a material for filaments in incandescent lamps by Carl Auer von Welsbach. The name of the company derives from the Osram to metals used osmium and tungsten. In the application, however, the use of osmium had some disadvantages. Besides the high price especially difficult processing has been a problem. Osmium is brittle and can not be drawn into threads. Therefore, the filaments were prepared by spraying an osmium-containing paste and subsequent annealing of the organic components. The filaments thus obtained were too thick for high voltages and also sensitive to vibration and voltage fluctuations. After a short time they were first replaced by tantalum and finally by tungsten.

Occurrence

Osmium is very rare, with a share of 1 × 10-6 % of the earth's crust. It is almost always associated with the other platinum group metals ruthenium, rhodium, iridium, palladium and platinum. Osmium is often the dignified, but also bound as sulfide, selenide or telluride.

The Osmiumvorkommen to distinguish between primary and secondary deposits. Primary deposits are copper, nickel, chromium or iron ores, in which small amounts of platinum metals are contained in bound form. There are no independent Osmiumerze. In addition to these ores exist Secondary deposits or placer deposits, which occur in osmium and other platinum metals tasteful. The metals have been leached by weathering from water and - due to their high density - enriched at suitable locations. Osmium comes here mainly in the natural alloys osmiridium and Iridosmium that are primarily contain iridium and osmium in addition distinguished by their predominant component.

The major deposits are the platinum metal-rich nickel ores in Canada ( Sudbury, Ontario ), Russia ( Ural ) and South Africa ( Witwatersrand ). Secondary deposits are located at the foot of the Urals, in Colombia, Ethiopia, and on Borneo.

Production and representation

The preparation of osmium -consuming and takes place during the recovery of other noble metals such as gold or platinum. The method used for this purpose take advantage of the different properties of the individual precious metals and their compounds, are being gradually the elements separated.

The starting material used precious metal ores or anode sludge from the nickel or gold mining. The ore is first dissolved in aqua regia. Here, gold, palladium and platinum go into solution, the other platinum group metals and silver remain. The silver reacts initially to form insoluble silver chloride which can be removed with lead carbonate and nitric acid ( the formation of silver nitrate). By fusion with sodium hydrogen sulfate and then leaching out of rhodium as rhodium can be solved and separated. Thereafter, the remaining residue is fused with sodium peroxide, thereby osmium and ruthenium are dissolved and the insoluble iridium remains. When this solution is treated with chlorine caused the volatiles osmium tetroxide and ruthenium tetroxide. With the addition of alcoholic sodium hydroxide only osmium tetroxide dissolves and can be separated in this way from the ruthenium. The osmium is precipitated as a complex with ammonium chloride and then reduced with hydrogen to form metallic osmium:

Osmium is obtained only in very small amounts, the amount of production worldwide is about 100 kg per year.

Properties

Physical Properties

Metallic osmium is a lustrous even at high temperatures heavy metal from steel blue color. It crystallizes in a hexagonal closest packing of spheres in space group 6/mmm with the lattice parameters a = 373.5 pm, c = 413.9 pm and two formula units per unit cell.

Osmium iridium before the item with the highest density. Crystallographic calculations yield of 22.59 g/cm3 osmium and iridium 22.56 g/cm3 for the natural isotope ratio. Thus, osmium is the densest naturally occurring element on earth.

Osmium has all platinum group metals the highest melting point and the lowest vapor pressure. Its bulk modulus of 462 GPa is the highest of all known elements and compounds; so that it is even less compressible than diamond with 443 GPa. Below the transition temperature of 0.66 K. osmium is used for the superconductor.

Chemical Properties

Osmium is one of the precious metals and is therefore relatively inert. It reacts directly with the non-metals fluorine, chlorine and oxygen. A reaction of oxygen and compact osmium takes place only at a red heat. Depending on the reaction conditions produced osmium tetroxide (less high temperatures, high oxygen pressure) or Osmiumtrioxid. Finely divided osmium is highly toxic osmium tetroxide at room temperature in traces.

In non-oxidizing mineral acids osmium is insoluble, even aqua regia osmium can not dissolve at low temperatures. However, access to strong oxidizing agents, for example, concentrated nitric acid, hot sulfuric acid, and alkaline oxidation of melts, such as sodium and Kaliumchloratschmelzen osmium.

Isotopes

From a total of 34 osmium isotopes and 6 Kernisomere are known, of which the seven isotopes come with the masses 184, 186, 187, 188, 189, 190 and 192 before, of course. 192Os, with a share of 40.78 % of the natural osmium the most abundant isotope, 184Os with 0.02% the rarest. As the only natural isotope 186Os is radioactive, but only weakly with over 2 quadrillion years half-life. In addition to these, there are another 27 short-lived isotopes of 162Os to 196Os, which, like the short-lived Kernisomere only be artificially manufactured.

The two isotopes 187Os and 189Os can be used for magnetic resonance imaging studies. Of the artificial nuclides 185Os (half life 96.6 days ) and are 191Os (15 days ) is used as a tracer. The ratio of 187Os to 188Os can be used in rhenium - osmium chronometers for age determination in iron meteorites, as 187Re to 187Os decays slowly (half-life: 4.12 · 1010 years ).

Use

For the element there is because of its rarity, its complicated production process and the consequent high price of 380 U.S. dollars per troy ounce ( as of Dec. 2013), relatively few applications. Due to the high toxicity of the oxides of osmium is rarely used in its pure state. In abrasive and wearable applications such as writing balls in pens, phonograph stylus, shafts and pins for instrument and electrical contacts hard osmium alloys of platinum metals are used. An alloy of 90% platinum and 10% osmium is processed to medical implants, and artificial heart valves and used in cardiac pacemakers. Sometimes osmium is used as a catalyst for hydrogenation.

Proof

Possible Evidence of osmium can be made through the osmium tetroxide. A simple, but not due to the toxicity of best evidence would be the characteristic smell of osmium tetroxide. But there are also chemical evidence possible. Here, a osmium sample is matched on filter paper with benzidine or potassium hexacyanoferrate. With benzidine the paper in the presence of osmium tetroxide purple, with potassium ferrocyanide turned light green.

In the modern laboratory, these documents are not obsolete; today can osmium by instrumental methods such as neutron activation analysis, voltammetry, atomic spectrometry or mass spectrometry not only proven but are determined with high accuracy quantitatively. NMR spectroscopy and X-ray diffraction enable the structural analysis of organic and inorganic osmium.

Safety

Osmium tetroxide is highly toxic. Dusts can cause lung irritation, pulmonary edema with hyperemia and to cause skin or eye damage. As always small amounts of osmium tetroxide occur at the air of powdered metallic osmium, caution should be exercised in this form of the element.

Metallic osmium is highly flammable as a finely divided powder or dust, in a compact form but not flammable. To delete Osmiumbränden metal fire extinguisher ( Class D) or dry powder must be used, under no circumstances should water be used, because of the explosion hazard due to hydrogen formed.

Compounds

Are compounds and complexes in the oxidation states of II to VIII known to be the most stable oxidation state IV Osmium along with ruthenium and one xenon one of the few elements that reach the highest known oxidation state VIII. Osmium, together with the most non-metals, compounds such as oxides, halides, sulphides, tellurides, and phosphides.

Osmium tetroxide

Osmium tetroxide OsO4 is the best known compound of osmium and one of the few stable compounds in the osmium oxidation state has VIII. The compound is formed by the action of oxidizing agents such as nitric acid to metallic osmium. It is a volatile solid, which has a very strong oxidizing. In contrast to many alternative oxidants, the reaction can proceed under stereochemical control. Due to these properties, the compound has been found despite the high toxicity and the high price of some applications. Therefore osmium tetroxide is frequently used in only catalytic amounts.

It is used for fixation and contrast enhancement of lipids ( fats) and cell membranes in electron microscopy and the securing of evidence (fingerprints ) are used. In organic chemistry, it is used as an oxidizing agent for cis -hydroxylation of alkenes to give vicinal diols and at the Jacobsen - Katsuki reaction or in the Sharpless epoxidation for the stereoselective epoxidation.

More osmium

Oxygen- osmium forms more compounds, the oxides Osmiumtrioxid OSO 3 and Osmiumdioxid OsO2. Osmiumtrioxid is only stable in the gas phase, Osmiumdioxid other hand, a stable, high- melting solid in the rutile structure.

The halogens fluorine, chlorine, bromine and iodine, a variety of compounds are known. The possible oxidation states of osmium range from VII in osmium (VII ) fluoride to I in osmium (I ) iodide.

Complex compounds

Besides these compounds, a number of complex compounds are known. From the Osmate osmium tetroxide, oxygen anionic complexes of osmium derived. With other ligands, such as ammonia, carbon monoxide, cyanide and nitric oxide are well known, many complexes in various oxidation states. With organic ligands, such as cyclopentadiene, the osmium osmocene which belongs to the metallocenes, are formed. In addition to classic complexes in which each metal -ligand bond can be clearly determined, there are also non-classical complexes. In these the metal cluster in front of several osmium atoms. Your concrete shape can be determined with the help of Wade's rules.