iridium

This article deals with the chemical element iridium, for the communication system of the same name see Iridium (communication system) .

properties
[ Xe ] 4 f 14 5 d 7 6 s 2 77 Ir Periodic table
General
Name , symbol , atomic number	Iridium, Ir, 77
Element category	Transition metals
Group , period , block	9 , 6 , d
Appearance	silvery white
CAS number	7439-88-5
EC number	231-095-9
ECHA InfoCard	100.028.269
Mass fraction of the earth's envelope	0.001 ppm
Atomic
Atomic mass	192.217 (3) and
Atomic radius (calculated)	135 (180) pm
Covalent radius	141 pm
Electron configuration	[ Xe ] 4 f 14 5 d 7 6 s 2
1. Ionization energy	8th.96702 (22) eV ≈ 865.19 kJ / mol
2. Ionization energy	17th.0 (3) eV ≈ 1 640 kJ / mol
3. Ionization energy	28.0 (1.6 eV) ≈ 2 700 kJ / mol
4. Ionization energy	40.0 (1.7 eV) ≈ 3 860 kJ / mol
5. Ionization energy	57.0 (1.9) eV ≈ 5 500 kJ / mol
Physically
Physical state	firmly
Crystal structure	Cubic area-centered
density	22.56 g / cm 3
Mohs hardness	6.5
magnetism	paramagnetic ( Χ m = 3.8 10 −5 )
Melting point	2739 K (2466 ° C)
boiling point	4403 K (4130 ° C)
Molar volume	8.52 · 10 −6 m 3 · mol −1
Heat of evaporation	564 kJ / mol
Heat of fusion	26 kJ mol −1
Speed ​​of sound	4825 m s −1
Electric conductivity	19.7 · 10 6 A · V −1 · m −1
Thermal conductivity	150 W m −1 K −1
Chemically
Oxidation states	−3, −1, 0, 1, 2, 3 , 4 , 5, 6, 8, 9
Normal potential	1.156 V (Ir 3+ + 3e - → Ir)
Electronegativity	2.20 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP 189 Ir {syn.} 13.2 d ε 0.532 189 Os 190 Ir {syn.} 11.78 d ε 2,000 190 Os 191 Ir 37.3% Stable 192 Ir {syn.} 73.830 d β - 1,460 192 pt ε 1.046 192 Os 192 m Ir {syn.} 241 a IT 0.155 192 Ir 193 Ir 62.7  % Stable 194 Ir {syn.} 19.15 h β - 2.247 194 pt 195 Ir {syn.} 2.5 h β - 1.120 195 pt
For other isotopes see list of isotopes
NMR properties
Spin quantum number I γ in rad · T −1 · s −1 E r  ( 1 H) f L at B = 4.7 T in MHz 191 Ir 3/2 4.812 · 10 6 1.09 · 10 −5 1.72 193 Ir 3/2 5.227 · 10 6 2.34 · 10 −5 1.87
safety instructions
GHS labeling of hazardous substances powder danger H and P phrases H: 228 P: 210
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Iridium is a chemical element with the symbol Ir and the atomic number 77. It is one of the transition metals , in the periodic table it is in group 9 (in the older counting part of the 8th subgroup ) or cobalt group . The very heavy, hard, brittle, silver-white shiny precious metal from the group of platinum metals is considered to be the most corrosion-resistant element. Below 0.11 Kelvin it changes to a superconductive state.

history

Iridium ( ancient Greek ἰριοειδής irio-eides "rainbow -like " after the many colors of its compounds) was discovered in London in 1804 by Smithson Tennant together with osmium . When raw platinum was dissolved in aqua regia , both platinum metals were found in the insoluble black residue. The variety of colors of the iridium salts inspired Tennant to the name Iridium. The “ original kilogram ” and the “ original meter ” also consist of an iridium alloy; Both have been kept in Paris in the Bureau International des Poids et Mesures since 1898 .

Occurrence

Iridium is rarer than gold or platinum . After rhenium, together with rhodium and ruthenium, it is the rarest non-radioactive metal. Its proportion in the continental crust is only 1  ppb . In nature it occurs elementally in the form of small grains or accompanied by platinum. With osmium it forms two naturally occurring minerals:

• Osmiridium , which consists of 50% iridium and the rest of osmium, platinum, ruthenium and rhodium , and
• Iridosmium , which is composed of 55–80% osmium and 20–45% iridium.

Important deposits are in South Africa , the Urals , North and South America , Tasmania , Borneo and Japan .

Free iridium as well as other elements of the platinum group are found in river sands. In addition, iridium is produced in the smelting of nickel ores .

properties

Iridium 8.3 g arc melted

Iridium foil

Physical Properties

Because of its hardness and brittleness , iridium is difficult to work with. In the naturally occurring isotopic composition, iridium is the second most dense element after osmium .

Chemical properties

In red heat, iridium oxidizes incompletely to black IrO ₂ , which breaks down again above 1140  ° C. Like osmium, iridium is also volatile when exposed to heat and especially when the oxygen content is higher than oxide IrO ₃ . In contrast to osmium, however, it is deposited again as metal or IrO ₂ in cold spots . In powder form, it is a flammable solid that can be easily ignited by brief exposure to an ignition source. The more finely the substance is distributed, the greater the risk of ignition. In compact form, it is not flammable. It is stable in mineral acids , including aqua regia . In chloride melts, however, it is converted to double chlorides in the presence of chlorine , e.g. B. Na ₂ [IrCl ₆ ].

Isotopes

There are two natural isotopes of iridium, 34 radioisotopes and 21 core isomers , of which the core isomer ^192m2 Ir is the most stable with a half-life of 241 years. It decays through internal conversion to ¹⁹² Ir, which is the isotope with the longest half-life with a half-life of 73.831 days. ¹⁹² Ir decays as a beta ^emitter to the platinum ^{isotope 192m} Pt, most of the others to osmium . The remaining isotopes and core isomers have half-lives between 300 µs at ¹⁶⁵ Ir and 11.78 days at ¹⁹⁰ Ir.

Because of its gamma radiation with an energy of about 550 keV (kiloelectron volt), ¹⁹² Ir is suitable for radiographic testing of components. For workpieces with a wall thickness of more than 20 mm, this isotope is usually used (regulated by standards, see e.g. DIN EN ISO 5579).

For non-destructive testing (NDT), the iridium emitter is usually welded in the form of a 2–3 mm large tablet in an emitter holder, and this is housed in a lockable type B work container, which is lined with depleted uranium to shield the gamma radiation .

Work containers for iridium emitters have the following dimensions: 20 cm long, 10 cm wide and 15 cm high. Due to the uranium jacket, the weight is around 13 to 20 kg, depending on the activity.

→ List of iridium isotopes

use

Iridium is often a component of alloys , which it gives hardness and / or brittleness. Platinum-iridium alloys are used for precision measurements, in medicine and mechanical engineering .

It is also used:

• as part of the alloy of the original kilogram and the third original meter ,
• for encasing plutonium dioxide in isotope batteries ,
• in the form of containers and crucibles for high temperature applications,
• as electrical contact,
• in jewelry as a platinum- iridium alloy (PtIr 800 and PtIr 900) for heavily used parts ( wedding rings , tie pins, clasps , mechanisms and springs),
• for spark plug electrodes,
• in nibs, an Os / Ir alloy is usually used at the front of the tip for fountain pens ,
• as part of ballpoint pen refills (writing ball ),
• in alloy with platinum as atomizer tip in flame atomic absorption spectrometry ,
• when measuring surface tension using the Du Noüy ring method, where it is used in an alloy with platinum because of its optimal wettability,
• in sputter targets for the production of electron-dissipating coatings of electrical non-conductors in high-resolution scanning electron microscopy ,
• as a UV protective layer on high-quality sunglasses ,
• in dental alloys,
• increasingly as a catalyst for chemical reactions (industrially important use in the synthesis of acetic acid ),
• because of its high density and its high melting point as a target in nuclear physics, for example for antiproton generation at CERN .
• In the dyes of organic light-emitting diodes , to generate singlet-triplet transitions

Cretaceous-Paleogene Impact

Iridium occurs in a relatively high concentration in that 66 million year old sediment layer that separates the chronostratigraphic systems of the Cretaceous and the Paleogene , and is considered an indication of a large meteorite impact, which, according to predominant scientific opinion, about 75 percent of the species of that time in addition to the dinosaurs fell victim; see also Iridium anomaly , Chicxulub crater and Cretaceous-Paleogene boundary .

safety instructions

Metallic iridium is non-toxic because of its durability. As a powder or dust it is highly flammable, in compact form it is non-combustible. Iridium compounds must be classified as toxic.

links

Many iridium salts are colored: with chlorine it forms olive-green iridium (III) chloride or dark blue-black, not quite defined iridium (IV) chloride . It reacts with fluorine to form yellow, volatile iridium (VI) fluoride or yellow-green iridium (V) fluoride . The oxidation state + IX predicted for Iridium years ago was also confirmed experimentally in 2014 by the synthesized [IrO ₄ ] ⁺ . It is the only element in which this oxidation state is known in a compound. In addition to the oxides and halides of iridium, numerous octahedral, diamagnetic iridium (III) complexes, such as amine complexes and chloro complexes, are known. The reduction of iridium (III) chloride in alcohol in the presence of π-acceptor ligands leads to square planar iridium (I) complexes, of which the Vaskas complex is best studied. By reducing iridium (III) chloride with carbon monoxide , one obtains the iridium carbonyls like the canary yellow [Ir ₄ (CO) ₁₂ ] and the red [Ir ₆ (CO) ₁₆ ]. Iridium complexes can be used in homogeneous catalysis, especially in hydrogenation reactions.

literature

• AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 .
• Hans Breuer: dtv-Atlas Chemie. Volume 1, 9th edition. Munich 2000, ISBN 3-423-03217-0 .
• M. Binnewies: General and Inorganic Chemistry. 1st edition. Heidelberg 2004, ISBN 3-8274-0208-5 .
• NN Greenwood, A. Earnshaw: Chemistry of the Elements. 1st edition. Weinheim 1988, ISBN 3-527-26169-9 .
• Harry H. Binder: Lexicon of the chemical elements - the periodic table in facts, figures and data. S. Hirzel Verlag, Stuttgart 1999, ISBN 3-7776-0736-3 .

Web links

Wiktionary: Iridium  - explanations of meanings, word origins, synonyms, translations

Commons : Iridium  - album with pictures, videos and audio files

• Image in the collection of Heinrich Pniok
• Mineral Atlas: Iridium (Wiki)

Individual evidence

1. ^ Harry H. Binder: Lexicon of the chemical elements. S. Hirzel Verlag, Stuttgart 1999, ISBN 3-7776-0736-3 .
2. ↑ The values ​​for the properties (info box) are taken from www.webelements.com (Iridium) , unless otherwise stated .
3. ↑ CIAAW, Standard Atomic Weights Revised 2013 .
4. ↑ ^{a b c d e} Entry on iridium in Kramida, A., Ralchenko, Yu., Reader, J. and NIST ASD Team (2019): NIST Atomic Spectra Database (ver. 5.7.1) . Ed .: NIST , Gaithersburg, MD. doi : 10.18434 / T4W30F ( https://physics.nist.gov/asd ). Retrieved June 13, 2020.
5. ↑ ^{a b c d e} entry on iridium at WebElements, https://www.webelements.com , accessed on June 13, 2020.
6. ↑ JW Arblaster: Densities of osmium and iridium. In: Platinum Metals Review. 33, 1, 1989, pp. 14-16 ( full text ; PDF; 209 kB).
7. ↑ Robert C. Weast (Ed.): CRC Handbook of Chemistry and Physics . CRC (Chemical Rubber Publishing Company), Boca Raton 1990, ISBN 0-8493-0470-9 , pp. E-129 to E-145. Values ​​there are based on g / mol and given in cgs units. The value specified here is the SI value calculated from it, without a unit of measure.
8. ↑ ^{a b} Yiming Zhang, Julian RG Evans, Shoufeng Yang: Corrected Values ​​for Boiling Points and Enthalpies of Vaporization of Elements in Handbooks. In: Journal of Chemical & Engineering Data . 56, 2011, pp. 328-337, doi: 10.1021 / je1011086 .
9. ↑ ^{a b} Guanjun Wang, Mingfei Zhou, James T. Goettel, Gary G. Schrobilgen, Jing Su, Jun Li, Tobias Schlöder, Sebastian Riedel: Identification of an iridium-containing compound with a formal oxidation state of IX . In: Nature . tape 514 , August 21, 2014, p. 475-477 , doi : 10.1038 / nature13795 .
10. ↑ ^{a b c} Entry on Iridium, powder in the GESTIS substance database of the IFA , accessed on April 12, 2020(JavaScript required) .
11. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 85th edition. CRC Press, Boca Raton, Florida, 2005. Section 14, Geophysics, Astronomy, and Acoustics; Abundance of Elements in the Earth's Crust and in the Sea.
12. ↑ CERN Neutrino Factory Working Group Technical Note princeton.edu, accessed February 1, 2011.
13. ↑ Michael J. Henehan, Andy Ridgwell, Ellen Thomas, Shuang Zhang, Laia Alegret, Daniela N. Schmidt, James WB Rae, James D. Witts, Neil H. Landman, Sarah E. Greene, Brian T. Huber, James R. Super, Noah J. Planavsky, Pincelli M. Hull: Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact . In: PNAS . 116, No. 43, October 2019. doi : 10.1073 / pnas.1905989116 .
14. ↑ Steve Ritter: Iridium Dressed To The Nines - Periodic Table: IrO ₄ ⁺ is the first molecule with an element in the +9 oxidation state . In: Chemical & Engineering News . 2014.
15. ↑ Lexicon of Chemistry: Iridium Compounds - Lexicon of Chemistry , accessed on February 20, 2018