Sulfur hexafluoride explained

Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF₆. It is a colorless, odorless, non-flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule.

Typical for a nonpolar gas, is poorly soluble in water but quite soluble in nonpolar organic solvents. It has a density of 6.12 g/L at sea level conditions, considerably higher than the density of air (1.225 g/L). It is generally stored and transported as a liquefied compressed gas.

has 23,500 times greater global warming potential (GWP) than as a greenhouse gas (over a 100-year time-frame) but exists in relatively minor concentrations in the atmosphere. Its concentration in Earth's troposphere reached 11.50 parts per trillion (ppt) in October 2023, rising at 0.37 ppt/year. The increase since 1980 is driven in large part by the expanding electric power sector, including fugitive emissions from banks of gas contained in its medium- and high-voltage switchgear. Uses in magnesium, aluminium, and electronics manufacturing also hastened atmospheric growth. The 1997 Kyoto Protocol, which came into force in 2005, is supposed to limit emissions of this gas. In a somewhat nebulous way it has been included as part of the carbon emission trading scheme. In some countries this has led to the defunction of entire industries.^[1]

Synthesis and reactions

See also: Fluorochemical industry. Sulfur hexafluoride on Earth exists primarily as a synthetic industrial gas, but has also been found to occur naturally.^[2]

can be prepared from the elements through exposure of to . This was the method used by the discoverers Henri Moissan and Paul Lebeau in 1901. Some other sulfur fluorides are cogenerated, but these are removed by heating the mixture to disproportionate any (which is highly toxic) and then scrubbing the product with NaOH to destroy remaining

Alternatively, using bromine, sulfur hexafluoride can be synthesized from SF₄ and CoF₃ at lower temperatures (e.g. 100 °C), as follows:^[3]

There is virtually no reaction chemistry for . A main contribution to the inertness of SF₆ is the steric hindrance of the sulfur atom, whereas its heavier group 16 counterparts, such as SeF₆ are more reactive than SF₆ as a result of less steric hindrance.^[4] It does not react with molten sodium below its boiling point,^[5] but reacts exothermically with lithium. As a result of its inertness, has an atmospheric lifetime of around 3200 years, and no significant environmental sinks other than the ocean.^[6]

Applications

By 2000, the electrical power industry is estimated to use about 80% of the sulfur hexafluoride produced, mostly as a gaseous dielectric medium.^[7] Other main uses as of 2015 included a silicon etchant for semiconductor manufacturing, and an inert gas for the casting of magnesium.^[8]

Dielectric medium

is used in the electrical industry as a gaseous dielectric medium for high-voltage sulfur hexafluoride circuit breakers, switchgear, and other electrical equipment, often replacing oil-filled circuit breakers (OCBs) that can contain harmful polychlorinated biphenyls (PCBs). gas under pressure is used as an insulator in gas insulated switchgear (GIS) because it has a much higher dielectric strength than air or dry nitrogen. The high dielectric strength is a result of the gas's high electronegativity and density. This property makes it possible to significantly reduce the size of electrical gear. This makes GIS more suitable for certain purposes such as indoor placement, as opposed to air-insulated electrical gear, which takes up considerably more room.

Gas-insulated electrical gear is also more resistant to the effects of pollution and climate, as well as being more reliable in long-term operation because of its controlled operating environment. Exposure to an arc chemically breaks down though most of the decomposition products tend to quickly re-form, a process termed "self-healing". Arcing or corona can produce disulfur decafluoride (), a highly toxic gas, with toxicity similar to phosgene. was considered a potential chemical warfare agent in World War II because it does not produce lacrimation or skin irritation, thus providing little warning of exposure.

is also commonly encountered as a high voltage dielectric in the high voltage supplies of particle accelerators, such as Van de Graaff generators and Pelletrons and high voltage transmission electron microscopes.

Alternatives to as a dielectric gas include several fluoroketones.^{[9] [10]} Compact GIS technology that combines vacuum switching with clean air insulation has been introduced for a subset of applications up to 420 kV.^[11]

Medical use

is used to provide a tamponade or plug of a retinal hole in retinal detachment repair operations^[12] in the form of a gas bubble. It is inert in the vitreous chamber.^[13] The bubble initially doubles its volume in 36 hours due to oxygen and nitrogen entering it, before being absorbed in the blood in 10–14 days.^[14]

is used as a contrast agent for ultrasound imaging. Sulfur hexafluoride microbubbles are administered in solution through injection into a peripheral vein. These microbubbles enhance the visibility of blood vessels to ultrasound. This application has been used to examine the vascularity of tumours.^[15] It remains visible in the blood for 3 to 8 minutes, and is exhaled by the lungs.^[16]

Tracer compound

Sulfur hexafluoride was the tracer gas used in the first roadway air dispersion model calibration; this research program was sponsored by the U.S. Environmental Protection Agency and conducted in Sunnyvale, California on U.S. Highway 101.^[17] Gaseous is used as a tracer gas in short-term experiments of ventilation efficiency in buildings and indoor enclosures, and for determining infiltration rates. Two major factors recommend its use: its concentration can be measured with satisfactory accuracy at very low concentrations, and the Earth's atmosphere has a negligible concentration of .

Sulfur hexafluoride was used as a non-toxic test gas in an experiment at St John's Wood tube station in London, United Kingdom on 25 March 2007.^[18] The gas was released throughout the station, and monitored as it drifted around. The purpose of the experiment, which had been announced earlier in March by the Secretary of State for Transport Douglas Alexander, was to investigate how toxic gas might spread throughout London Underground stations and buildings during a terrorist attack.

Sulfur hexafluoride is also routinely used as a tracer gas in laboratory fume hood containment testing. The gas is used in the final stage of ASHRAE 110 fume hood qualification. A plume of gas is generated inside of the fume hood and a battery of tests are performed while a gas analyzer arranged outside of the hood samples for SF₆ to verify the containment properties of the fume hood.

It has been used successfully as a tracer in oceanography to study diapycnal mixing and air-sea gas exchange.^[19]

Other uses

• The magnesium industry uses as an inert "cover gas" to prevent oxidation during casting,^[20] and other processes including smelting.^[21] Once the largest user, consumption has declined greatly with capture and recycling.
• Insulated glazing windows have used it as a filler to improve their thermal and acoustic insulation performance.^{[22] [23]}
• plasma is used in the semiconductor industry as an etchant in processes such as deep reactive-ion etching. A small fraction of the breaks down in the plasma into sulfur and fluorine, with the fluorine ions performing a chemical reaction with silicon.^[24]
• Tires filled with it take longer to deflate from diffusion through rubber due to the larger molecule size.
• Nike likewise used it to obtain a patent and to fill the cushion bags in all of their "Air"-branded shoes from 1992 to 2006.^[25] 277 tons was used during the peak in 1997.
• The United States Navy's Mark 50 torpedo closed Rankine-cycle propulsion system is powered by sulfur hexafluoride in an exothermic reaction with solid lithium.^[26]
• Waveguides in high-power microwave systems are pressurized with it. The gas electrically insulates the waveguide, preventing internal arcing.
• Electrostatic loudspeakers have used it because of its high dielectric strength and high molecular weight.^[27]
• The chemical weapon disulfur decafluoride is produced with it as a feedstock.
• For entertainment purposes, when breathed, causes the voice to become significantly deeper, due to its density being so much higher than air. This phenomenon is related to the more well-known effect of breathing low-density helium, which causes someone's voice to become much higher. Both of these effects should only be attempted with caution as these gases displace oxygen that the lungs are attempting to extract from the air. Sulfur hexafluoride is also mildly anesthetic.^{[28] [29]}
• For science demonstrations / magic as "invisible water" since a light foil boat can be floated in a tank, as will an air-filled balloon.
• It is used for benchmark and calibration measurements in Associative and Dissociative Electron Attachment (DEA) experiments^{[30] [31]}

Greenhouse gas

According to the Intergovernmental Panel on Climate Change, is the most potent greenhouse gas. Its global warming potential of 23,900 times that of when compared over a 100-year period.^[32] Sulfur hexafluoride is inert in the troposphere and stratosphere and is extremely long-lived, with an estimated atmospheric lifetime of 800–3,200 years.^[33]

Measurements of SF₆ show that its global average mixing ratio has increased from a steady base of about 54 parts per quadrillion prior to industrialization, to over 11.5 parts per trillion (ppt) as of October 2023, and is increasing by about 0.4 ppt (3.5%) per year.^{[34] [35]} Average global SF₆ concentrations increased by about 7% per year during the 1980s and 1990s, mostly as the result of its use in magnesium production, and by electrical utilities and electronics manufacturers. Given the small amounts of SF₆ released compared to carbon dioxide, its overall individual contribution to global warming is estimated to be less than 0.2%,^[36] however the collective contribution of it and similar man-made halogenated gases has reached about 10% as of 2020.^[37] Alternatives are being tested.^{[38] [39]}

In Europe, falls under the F-Gas directive which ban or control its use for several applications.^[40] Since 1 January 2006, is banned as a tracer gas and in all applications except high-voltage switchgear.^[41] It was reported in 2013 that a three-year effort by the United States Department of Energy to identify and fix leaks at its laboratories in the United States such as the Princeton Plasma Physics Laboratory, where the gas is used as a high voltage insulator, had been productive, cutting annual leaks by 2280order=flipNaNorder=flip. This was done by comparing purchases with inventory, assuming the difference was leaked, then locating and fixing the leaks.^[42]

Physiological effects and precautions

Sulfur hexafluoride is a nontoxic gas, but by displacing oxygen in the lungs, it also carries the risk of asphyxia if too much is inhaled.^[43] Since it is more dense than air, a substantial quantity of gas, when released, will settle in low-lying areas and present a significant risk of asphyxiation if the area is entered. That is particularly relevant to its use as an insulator in electrical equipment since workers may be in trenches or pits below equipment containing .^[44]

As with all gases, the density of affects the resonance frequencies of the vocal tract, thus changing drastically the vocal sound qualities, or timbre, of those who inhale it. It does not affect the vibrations of the vocal folds. The density of sulfur hexafluoride is relatively high at room temperature and pressure due to the gas's large molar mass. Unlike helium, which has a molar mass of about 4 g/mol and pitches the voice up, has a molar mass of about 146 g/mol, and the speed of sound through the gas is about 134 m/s at room temperature, pitching the voice down. For comparison, the molar mass of air, which is about 80% nitrogen and 20% oxygen, is approximately 30 g/mol which leads to a speed of sound of 343 m/s.^[45]

Sulfur hexafluoride has an anesthetic potency slightly lower than nitrous oxide;^[46] it is classified as a mild anesthetic.^[47]

See also

• Selenium hexafluoride
• Tellurium hexafluoride
• Uranium hexafluoride
• Hypervalent molecule
• Halocarbon—another group of major greenhouse gases
• Trifluoromethylsulfur pentafluoride, a similar gas

Further reading

• Web site: Sulfur hexafluoride . Air Liquide Gas Encyclopedia . 22 February 2013 . live . https://web.archive.org/web/20120331215233/http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=34 . 31 March 2012 .
• Book: Christophorou, Loucas G. . Isidor Sauers . Isidor Sauers . 1991 . Gaseous Dielectrics VI . Plenum Press . 978-0-306-43894-3 .
• Book: Holleman . A. F. . Wiberg . E. . Inorganic Chemistry . Academic Press . San Diego . 2001 . 0-12-352651-5.
• Book: High-Voltage Engineering: Theory and Practice . Khalifa . Mohammad . 1990 . Marcel Dekker . New York . 978-0-8247-8128-6 . 20595838.
• Book: Advantages in High Voltage Insulation and Arc Interruption in SF₆ and Vacuum . Maller . V. N. . Naidu . M. S. . 1981 . Pergamon Press . Oxford; New York . 978-0-08-024726-7 . 7866855.
• SF₆ Reduction Partnership for Electric Power Systems
• Web site: Climate change: Electrical industry's 'dirty secret' boosts warming . . September 13, 2019 . September 14, 2019 . Matt McGrath.

External links

• Fluoride and compounds fact sheet— National Pollutant Inventory
• High GWP Gases and Climate Change from the U.S. EPA website
• International Conference on SF₆ and the Environment (related archive)
• CDC - NIOSH Pocket Guide to Chemical Hazards

Notes and References

1. Book: Kapusta . Joël . Mackey . Phillip . Stubina . Nathan . The Canadian Metallurgical & Materials Landscape 1960 - 2011 . Canadian Institute of Metallurgy . 2011 . Magnesium Metal Production in Canada . D. . Creber . B. . Davis . S. . Kashani-Nejad.
2. Busenberg, E. and Plummer, N. . Dating young groundwater with sulfur hexafluoride: Natural and anthropogenic sources of sulfur hexafluoride . Water Resources Research . American Geophysical Union . 36 . 3011–3030 . 2000 . 10 . 10.1029/2000WR900151 . 2000WRR....36.3011B . free.
3. Winter . R. W. . Pugh . J. R. . Cook . P. W. . January 9–14, 2011 . SF₅Cl, SF₄ and SF₆: Their Bromine−facilitated Production & a New Preparation Method for SF₅Br . 20th Winter Fluorine Conference.
4. Book: Duward Shriver . Peter Atkins . Inorganic Chemistry . 2010 . W. H. Freeman . 409 . 978-1429252553 .
5. Book: Advanced Inorganic Chemistry: Volume II . 12th . Gurdeep . Raj . GOEL Publishing House . 2010 . 160 . Extract of page 160
6. Stöven . T. . Tanhua . T. . Hoppema . M. . Bullister . J. L. . 2015-09-18 . Perspectives of transient tracer applications and limiting cases . Ocean Science . en . 11 . 5 . 699–718 . 10.5194/os-11-699-2015 . free . 2015OcSci..11..699S . 1812-0792.
7. Constantine T. Dervos . Panayota Vassilou . Sulfur Hexafluoride: Global Environmental Effects and Toxic Byproduct Formation . Journal of the Air & Waste Management Association . 2000 . 50 . 1 . 137–141 . Taylor and Francis. 10.1080/10473289.2000.10463996. 10680375 . 8533705 . free . 2000JAWMA..50..137D .
8. Deborah Ottinger . Mollie Averyt . Deborah Harris . US consumption and supplies of sulphur hexafluoride reported under the greenhouse gas reporting program . Journal of Integrative Environmental Sciences . 2015 . 12 . sup1 . 5–16 . Taylor and Francis. 10.1080/1943815X.2015.1092452. free .
9. Web site: Archived copy . 2017-10-12 . live . https://web.archive.org/web/20171012150303/http://cired.net/publications/cired2015/papers/CIRED2015_0587_final.pdf . 2017-10-12 .
10. Book: 1 June 2015. 379–383. IEEE Xplore. 10.1109/ICACACT.2014.7223577. SF₆ alternative development for high voltage switchgears. Kieffel. Yannick. Biquez. Francois. 2015 IEEE Electrical Insulation Conference (EIC) . 978-1-4799-7352-1. 15911515.
11. Web site: Sustainable switchgear technology for a CO₂ neutral future . Siemens Energy . 2020-08-31 . 2021-04-27.
12. Book: Daniel A. Brinton . C. P. Wilkinson . Retinal detachment: principles and practice . 183 . 2009 . Oxford University Press . 978-0199716210 .
13. Book: Gholam A. Peyman, M.D., Stephen A. Meffert, M.D., Mandi D. Conway . Vitreoretinal Surgical Techniques . 157 . 2007 . Informa Healthcare . 978-1841846262 .
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16. Web site: SonoVue, INN-sulphur hexafluoride - Annex I - Summary of Product Characteristics. European Medicines Agency. 2019-02-24.
17. Web site: Air pollution line source . September 10, 2011 . C Michael Hogan . Encyclopedia of Earth . 22 February 2013 . dead. https://web.archive.org/web/20130529064051/http://www.eoearth.org/article/Air_pollution_line_source?topic=49506 . 29 May 2013 .
18. News: 'Poison gas' test on Underground . 25 March 2007 . BBC News . 22 February 2013 . live . https://web.archive.org/web/20080215005754/http://news.bbc.co.uk/1/hi/england/london/6492501.stm . 15 February 2008 .
19. Fine. Rana A.. Rana Fine. 2010-12-15. Observations of CFCs and SF6 as Ocean Tracers. Annual Review of Marine Science. 3. 1. 173–195. 10.1146/annurev.marine.010908.163933. 21329203. 1941-1405.
20. Web site: Update on EPA's manesium industry partnership for climate protection . Scott C. Bartos . . February 2002 . December 14, 2013 . dead. https://web.archive.org/web/20121010163051/http://www.epa.gov/magnesium-sf6/documents/tms_paper.pdf . October 10, 2012 .
21. News: John . Ayres . Environment Canada . Canadian Perspective on SF6 Management from Magnesium Industry . 2000.
22. Web site: Final report on the costs and the impact on emissions of potential regulatory framework for reducing emissions of hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride . J. Harnisch and W. Schwarz . 2003-02-04 . Ecofys GmbH.
23. Book: Sound insulation - Google Books . 9780750665261 . Hopkins . Carl . 2007 . 504–506. Elsevier / Butterworth-Heinemann .
24. Web site: Dry Etching of Silicon Materials in Based Plasmas . September 1987 . Y. Tzeng . T.H. Lin . Journal of the Electrochemical Society . 22 February 2013 . dead. https://web.archive.org/web/20120406111056/http://www.eng.auburn.edu/~tzengy/Publications/Plasma%20Lab%20Publications/Dry%20etching%20of%20silicon%20materials%20in%20SF6%20based%20plasma.pdf . 6 April 2012 .
25. Web site: Nike Goes For The Green . Stanley Holmes . Bloomberg Business Week Magazine . September 24, 2006 . December 14, 2013 . dead . https://web.archive.org/web/20130603225645/http://www.businessweek.com/stories/2006-09-24/nike-goes-for-the-green . June 3, 2013 .
26. Stored Chemical Energy Propulsion System for Underwater Applications. Hughes, T.G. . Smith, R.B. . Kiely, D.H. . Journal of Energy. 1983. 7. 2 . 128–133. 10.2514/3.62644. 1983JEner...7..128H .
27. Web site: Advances in loudspeaker technology - A 50 year prospective . Dick Olsher . The Absolute Sound . October 26, 2009 . December 14, 2013 . live . https://web.archive.org/web/20131214091113/http://www.theabsolutesound.com/articles/advances-in-loudspeaker-technology-a-50-year-perspective-tas-196/ . December 14, 2013 .
28. Anesthetic Potencies of Sulfur Hexafluoride, Carbon Tetrafluoride, Chloroform and Ethrane in Dogs: Correlation with the Hydrate and Lipid Theories of Anesthetic Action. Anesthesiology: The Journal of the American Society of Anesthesiologists. 30. 2. 127–134. Anesthesiology - The Journal of the American Society of Anesthesiologists, Inc. September 10, 1968. Edmond I Eger MD . etal .
29. WTOL. Imagination Station. 2015-01-27. Sound Like Darth Vader with Sulfur Hexafluoride. YouTube.
30. Braun. M. Marienfeld. S. Ruf. M-W. Hotop. H. 2009-05-26. High-resolution electron attachment to the molecules CCl4and SF6over extended energy ranges with the (EX)LPA method. Journal of Physics B: Atomic, Molecular and Optical Physics. en. 42. 12. 125202. 10.1088/0953-4075/42/12/125202. 2009JPhB...42l5202B. 122242919. 0953-4075.
31. Fenzlaff. Marita. Gerhard. Rolf. Illenberger. Eugen. 1988-01-01. Associative and dissociative electron attachment by SF6 and SF5Cl. The Journal of Chemical Physics. 88. 1. 149–155. 10.1063/1.454646. 1988JChPh..88..149F. 0021-9606.
32. Web site: 2.10.2 Direct Global Warming Potentials . 2007 . . 22 February 2013 . live . https://web.archive.org/web/20130302061433/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-2.html . 2 March 2013 .
33. A. R. Ravishankara, S. Solomon, A. A. Turnipseed, R. F. Warren . 8 January 1993 . Atmospheric Lifetimes of Long-Lived Halogenated Species . Science . 259 . 5092 . 194–199 . 22 February 2013 . 1993Sci...259..194R . Solomon . Turnipseed . Warren . 10.1126/science.259.5092.194 . 17790983 . 574937 . live . https://web.archive.org/web/20150924144127/http://www.sciencemag.org/content/259/5092/194.abstract . 24 September 2015 .
34. Web site: Trends in Atmospheric Sulpher Hexaflouride . . 28 December 2023.
35. Web site: [ftp://aftp.cmdl.noaa.gov/data/hats/sf6/insituGCs/CATS/monthly/smo_SF6_MM.dat Sulfur hexafluoride (SF₆) data from hourly in situ samples analyzed on a gas chromatograph located at Cape Matatulu (SMO)]. July 7, 2020. August 8, 2020.
36. Web site: SF₆ Sulfur Hexafluoride . 19 March 2011 . PowerPlantCCS Blog . 22 February 2013 . live . https://web.archive.org/web/20121230202321/http://powerplantccs.com/blog/2011/03/sf6-sulfur-hexafluoride.html . 30 December 2012 .
37. Web site: The NOAA Annual Greenhouse Gas Index (AGGI) . NOAA Global Monitoring Laboratory/Earth System Research Laboratories . Butler J. and Montzka S. . 2020 .
38. Web site: g3, the SF6-free solution in practice Think Grid . think-grid.org . 18 February 2019 . 6 February 2020 . 30 October 2020 . https://web.archive.org/web/20201030203625/https://think-grid.org/g3-sf6-free-solution-practice . dead .
39. Mohamed Rabie . Christian M. Franck . Assessment of Eco-friendly Gases for Electrical Insulation to Replace the Most Potent Industrial Greenhouse Gas SF6 . Environmental Science & Technology . 2018 . 52 . 2 . 369–380 . American Chemical Society . 10.1021/acs.est.7b03465. 29236468 . 2018EnST...52..369R . 20.500.11850/238519 . free .
40. Web site: Sulfur hexafluoride: The truths and myths of this greenhouse gas . David Nikel . phys.org . 2020-01-15 . 2020-10-18.
41. News: Climate: MEPs give F-gas bill a 'green boost' . www.euractiv.com . 13 October 2005 . . 22 February 2013 . live . https://web.archive.org/web/20130603001718/http://www.euractiv.com/sustainability/climate-meps-give-gas-bill-green-boost/article-145749 . 3 June 2013 .
42. News: Department of Energy's Crusade Against Leaks of a Potent Greenhouse Gas Yields Results. June 14, 2013. The New York Times. June 13, 2013. Michael Wines. live. https://web.archive.org/web/20130614114649/http://www.nytimes.com/2013/06/14/us/department-of-energys-crusade-against-leaks-of-a-potent-greenhouse-gas-yields-results.html . June 14, 2013. Michael Wines.
43. Web site: Sulfur Hexafluoride. Hazardous Substances Data Bank. U.S. National Library of Medicine. 26 March 2013. live. https://web.archive.org/web/20180509021853/https://toxnet.nlm.nih.gov/cgi-bin/sis/search/a?dbs+hsdb:@term+@DOCNO+825. 9 May 2018.
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46. Book: 9780398000110 . The Chemistry and Physics of Anesthesia . 2nd . Adriani . John . 1962 . Thomas Books . Illinois . 319 .
47. Weaver . Raymond H. . Virtue . Robert W. . The mild anesthetic properties of sulfur hexafluoride . Anesthesiology . 605–607 . en . 1 November 1952. 13 . 6 . 10.1097/00000542-195211000-00006 . 12986223 . 32403288 . free .