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Sulfur hexafluoride

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Sulfur hexafluoride
Structure and dimensions of the sulfur hexafluoride molecule
Ball-and-stick model of sulfur hexafluoride
Ball-and-stick model of sulfur hexafluoride
Space-filling model of sulfur hexafluoride
Space-filling model of sulfur hexafluoride
Names
Preferred IUPAC name
Sulfur hexafluoride
Systematic IUPAC name
Hexafluoro-λ6-sulfane
Other names
Elagas

Esaflon
Sulfur(VI) fluoride

Sulfur fluoride
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.018.050 Edit this at Wikidata
EC Number
  • 219-854-2
2752
KEGG
MeSH Sulfur+hexafluoride
RTECS number
  • WS4900000
UNII
UN number 1080
  • Key: SFZCNBIFKDRMGX-UHFFFAOYSA-N checkY
  • InChI=1S/F6S/c1-7(2,3,4,5)6
  • FS(F)(F)(F)(F)F
Properties
SF6
Molar mass 146.06 g/mol
Appearance colorless, odorless gas
Density 6.164 g/L (gas, 1 bar: ~5.1 times denser than air)
1.329 g/ml (liquid, 25 °C)
2.510 g/cm3 (solid, −50.8 °C)
Melting point −50.7 °C (triple point)
Boiling point −64 °C (209 K) (subl.)
decomp. at ca. 500 °C (773 K)
slightly soluble
Solubility in ethanol soluble
Structure
Orthorhombic, oP28
Pnma, No. 62
octahedral (Oh)
Octahedral
0 D
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Asphyxiant in high concentrations, no odour warning
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
0
0
Related compounds
Other cations
 Selenium hexafluoride
Tellurium hexafluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Sulfur hexafluoride (SF
6) is an inorganic, colorless, odorless, non-toxic and non-flammable gas. SF
6 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, it is poorly soluble in water but soluble in nonpolar organic solvents. It is generally transported as a liquefied compressed gas. It has a density of 6.12 g/L at sea level conditions, which is considerably higher than the density of air.

Synthesis and chemistry

SF
6 can be prepared from the elements through exposure of S
8 to F
2. This was also 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 S
2F
10 (which is highly toxic, unlike SF
6) and then scrubbing the product with NaOH to destroy remaining SF
4.

There is virtually no reaction chemistry for SF
6. It does not react with molten sodium, but reacts exothermically with lithium.

Starting from SF
4, one can prepare SF
5Cl, which is structurally related to SF
6. The monochloride is, however, a strong oxidant and readily hydrolyzed to sulfate.

Applications

Of the 8,000 tons of SF
6 produced per year, most (6,000 tons) is used as a gaseous dielectric medium in the electrical industry, an inert gas for the casting of magnesium, and as an inert filling for insulated glazing windows.

Dielectric medium

SF
6 is used in the electrical industry as a gaseous dielectric medium for high-voltage (35 kV and above) circuit breakers, switchgear, and other electrical equipment, often replacing oil filled circuit breakers (OCBs) that can contain harmful PCBs. SF
6 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. 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. Vacuum circuit breakers (VCBs) are displacing sulfur hexafluoride circuit breakers in industry as they are safer and require less maintenance[citation needed]. Although most of the decomposition products tend to quickly re-form SF
6, arcing or corona can produce disulfur decafluoride (S
2F
10), a highly toxic gas, with toxicity similar to phosgene. S
2F
10 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.

SF
6 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.

Medical use

Because SF
6 is relatively slowly absorbed by the bloodstream, it is used to provide a long-term tamponade or plug of a retinal hole in retinal detachment repair operations.

In a further medical application, SF
6 is employed 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 utilized to examine the vascularity of tumours amongst other things.[1]

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.[2] Gaseous SF
6 is an ongoing commonly used tracer gas for use 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 SF
6.

Sulfur hexafluoride was used as a harmless test gas in an experiment at St John's Wood tube station in London, United Kingdom on 25 March 2007.[3] 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.

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

Other uses

Sulfur hexafluoride is also used as a reagent for creating thrust in a closed Rankine cycle propulsion system, reacting with solid lithium as used in the United States Navy's Mark 50 torpedo.

SF
6 plasma is also used in the semiconductor industry as an etchant.

The magnesium industry uses large amounts of SF
6 as inert gas to fill casting forms.

Sulfur hexafluoride is also used to pressurize waveguides in radar systems.

Greenhouse gas

According to the Intergovernmental Panel on Climate Change, SF
6 is the most potent greenhouse gas that it has evaluated, with a global warming potential of 22,800[4] times that of CO
2 when compared over a 100 year period. Measurements of SF6 show that its global average mixing ratio has increased by about 0.2 ppt per year to nearly 7 ppt in 2010.[5] Sulfur hexafluoride is also extremely long-lived, it is inert in the troposphere and stratosphere and has an estimated atmospheric lifetime of 800–3200 years.[6] SF
6 is very stable (for countries reporting their emissions to the UNFCCC, a GWP of 23,900 for SF
6 was suggested at the third Conference of the Parties: GWP used in Kyoto protocol).[7] Average global SF6 concentrations increased by about seven percent per year during the 1980s and 1990s, mostly as the result of its use in the magnesium production industry, and by electrical utilities and electronics manufacturers. Given the low amounts of SF6 released compared to carbon dioxide, its overall contribution to global warming is estimated to be less than 0.2 percent.[citation needed]

In Europe, SF
6 falls under the F-Gas directive which ban or control its usage for several applications. Since 1 January 2006, SF
6 is banned as a tracer gas and in all applications except high voltage switchgear[8].

Physiological effects and precautions

Another effect is the gas's ability to alter vocal sound waves. The gas can be inhaled in a small, safe amount and cause the breather's voice to sound very deep. This is due to the gas's large molar mass. Unlike helium, which has a molar mass of about 4 grams/mol, SF
6 has a molar mass of about 146 g/mol, and the velocity of sound through the gas is 0.44 times the speed of sound in air due to the large inertia of a SF
6 molecule. For comparison, the molar mass of air, which is about 80% nitrogen and 20% oxygen, is approximately 30 g/mol. Inhalation of SF
6 causes a lowering of the timbre, or frequency of the formants, of the vocal tract, by contrast with inhalation of helium, which raises it.[9] Other gases such as Freon or xenon have a similar effect, although xenon is generally not used for such demonstrations due to its high cost and anaesthetic properties.

It is possible to safely breathe heavy gases such as xenon or sulfur hexafluoride as long as they include a 20% mixture of oxygen. The lungs mix the gases very effectively and rapidly so that the heavy gases are purged along with the oxygen and do not accumulate at the bottom of the lungs.[10] There is, however, a danger associated with any heavy gas in large quantities: it may sit invisibly in a container, and if a person enters a container filled with an odorless, colorless gas, they may find themselves breathing it unknowingly.[11]

Other properties

Thermal Conductivity at STP (101.3 kPa and 0 °C) = 12.058 mW/(m.K)[12]
Heat capacity at constant pressure (Cp) (101.3 kPa and 21 °C) = 0.097 kJ/(mol.K)[12]

See also

References

Notes
  1. ^ Lassau N, Chami L, Benatsou B, Peronneau P, Roche A (2007). "Dynamic contrast-enhanced ultrasonography (DCE-US) with quantification of tumor perfusion: a new diagnostic tool to evaluate the early effects of antiangiogenic treatment". Eur Radiol. 17 (Suppl 6): F89–98. doi:10.1007/s10406-007-0233-6. PMID 18376462. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  2. ^ C.Michael Hogan, Leda C. Patmore, Richard Venti, Gary Latshaw et al. (1973) Calibration of a line source model for air pollutant dispersal from roadways, ESL Inc., U.S. Environmental Protection Agency Technology Series, Government Printing Office, Washington, DC
  3. ^ "'Poison gas' test on Underground". BBC News. 25 March 2007. Retrieved 2007-03-31.
  4. ^ Intergovernmental Panel on Climate Change, Working Group 1, Climate Change 2007, Chapter 2.10.2.
  5. ^ "Mauna Loa and Global SF6". Retrieved 2010-03-05.
  6. ^ "Atmospheric Lifetimes of Long-Lived Halogenated Species".
  7. ^ "Climate Change 2001: Working Group I: The Scientific Basis". Intergovernmental Panel on Climate Change. 2001. Retrieved 2007-03-31.
  8. ^ F-gas and SF6 restrictions
  9. ^ "Physics in speech". phys.unsw.edu.au. Retrieved 2008-07-20.
  10. ^ Yamaguchi, K. (2001). "Inhaling Gas With Different CT Densities Allows Detection of Abnormalities in the Lung Periphery of Patients With Smoking-Induced COPD". Chest Journal. 51 (6): 1907–1916. doi:10.1378/chest.120.6.1907. PMID 11742921. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ Staff (August 1, 2007). "Cryogenic and Oxygen Deficiency Hazard Safety". Stanford Linear Accelerator Center. Archived from the original on 2007-06-09. Retrieved 2007-10-10.
  12. ^ a b "Air Liquide Gas Encyclopedia Sulfur hexafluoride". Retrieved 2008-10-26.
Further reading


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