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Sulfur hexafluoride

Names
IUPAC name Sulfur hexafluoride
Systematic IUPAC name Hexafluoro-λ-sulfane
Other names Elagas Esaflon Sulfur(VI) fluoride Sulfuric fluoride
Identifiers
CAS Number	2551-62-4
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30496
ChemSpider	16425
ECHA InfoCard	100.018.050
EC Number	219-854-2
Gmelin Reference	2752
KEGG	D05962
MeSH	Sulfur+hexafluoride
PubChem CID	17358
RTECS number	WS4900000
UNII	WS7LR3I1D6
UN number	1080
CompTox Dashboard (EPA)	DTXSID8029656
InChI InChI=1S/F6S/c1-7(2,3,4,5)6Key: SFZCNBIFKDRMGX-UHFFFAOYSA-N
SMILES FS(F)(F)(F)(F)F
Properties
Chemical formula	F6S
Molar mass	146.05 g·mol
Appearance	Colorless, odorless gas
Density	6.17 g/l
Boiling point	−64 °C; −83 °F; 209 K
Vapor pressure	2.9 kPa (at 21.1°C)
Structure
Crystal structure	Orthorhombic, oP28
Space group	Oh
Coordination geometry	Orthogonal hexagonal
Molecular shape	Octahedral
Dipole moment	0 D
Thermochemistry
Std molar entropy (S298)	292 J·mol·K
Std enthalpy of formation (ΔfH298)	−1209 kJ·mol
Hazards
NFPA 704 (fire diamond)	0 0 0
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). N verify (what is  ?) Infobox references

Sulfur hexafluoride (SF
₆) is an inorganic, colorless, odorless, and non-flammable greenhouse 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 reactions

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) 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.

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 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. 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

SF
₆ is used to provide a tamponade or plug of a retinal hole in retinal detachment repair operations. It is inert in the vitreous chamber and initially doubles its volume in 36 hours before being absorbed in the blood in 10–14 days.

SF
₆ 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 utilized to examine the vascularity of tumours.

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. Gaseous SF
₆ 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 SF
₆.

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. 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. SF
₆ breaks down in the plasma into sulfur and fluorine, the fluorine plasma performing the etching.

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. The gas insulates the waveguide preventing internal arcing. The same use of sulfur hexafluoride is applied in transmission waveguides of medical linear accelerators, which are used for delivery of external beam radiotherapy.

Sulfur hexafluoride has been used in electrostatic loudspeakers because of its high dielectric strength and high molecular weight.

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 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 over 7 ppt. Sulfur hexafluoride is also extremely long-lived, is inert in the troposphere and stratosphere and has an estimated atmospheric lifetime of 800–3200 years. 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). 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.

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.

Physiological effects and precautions

As with all gasses, the density of SF
₆ affects the frequency at which vocal chords produce sound vibrations. 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 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.

As with all non-oxygen bearing gasses, inhalation can result in asphyxiation, even leading to death. While seemingly unlikely, this consequence is important to keep in mind at all times because the effect of a gas on the vocal chords is greatest when the least amount of air possible is mixed with it; attempting to achieve the greatest transformation in vocal quality implies also achieving the least availability of oxygen - that is to say, the most "successful" experiments in changing vocal qualities may be among the more dangerous ones.

Other properties

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

See also

• Selenium hexafluoride
• Tellurium hexafluoride
• Hypervalent molecule
• Process for Measuring the Degradation of Sulfur Hexafluoride in High-voltage Systems U.S. patent 4,633,082
• Halocarbon—another group of major greenhouse gases

References

1. "Sulfur Hexafluoride - PubChem Public Chemical Database". The PubChem Project. USA: National Center for Biotechnology Information.
2. ^ Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 061894690X.
3. Daniel A. Brinton, C. P. Wilkinson, George F. Hilton, Retinal detachment: principles and practice Oxford University Press, 2009 ISBN 019533082X, page 183
4. Thompson, John T. (2001). "17: Intraocular gases and techniques for air-fluid exchage". In Peyman, Gholam A.; Meffert, Stephen A.; Conway, Mandi D.; Chou, Famin (eds.). Vitreoretinal Surgical Techniques. London: Martin Dunitz. p. 157. ISBN 1-85317-585-4. Retrieved 11 November 2011.
5. Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 9018990, please use {{cite journal}} with |pmid=9018990 instead.
6. 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)
7. 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
8. "'Poison gas' test on Underground". BBC News. 25 March 2007. Retrieved 2007-03-31.
9. Y. Tzeng and T.H. Lin. "Dry Etching of Silicon Materials in SF
   ₆ Based Plasmas" (PDF).
10. Intergovernmental Panel on Climate Change, Working Group 1, Climate Change 2007, Chapter 2.10.2.
11. "Mauna Loa and Global SF₆". Retrieved 2011-03-06.
12. "Atmospheric Lifetimes of Long-Lived Halogenated Species".
13. "Climate Change 2001: Working Group I: The Scientific Basis". Intergovernmental Panel on Climate Change. 2001. Retrieved 2007-03-31.
14. F-gas and SF₆ restrictions
15. "Physics in speech". phys.unsw.edu.au. Retrieved 2008-07-20.
16. ^ "Air Liquide Gas Encyclopedia Sulfur hexafluoride". Retrieved 2008-10-26.

Further reading

• Gaseous Dielectrics VI. Plenum Press. 1991. ISBN 0-306-43894-1. {{cite book}}: Unknown parameter |editors= ignored (|editor= suggested) (help)
• Holleman, A. F.; Wiberg, E. Inorganic Chemistry Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
• Khalifa, Mohammad (1990). High-Voltage Engineering: Theory and Practice. New York: Marcel Dekker. ISBN 0824781287. OCLC 20595838.
• Maller, V. N.; Naidu, M. S. (1981). Advantages in High Voltage Insulation and Arc Interruption in SF₆ and Vacuum. Oxford; New York: Pergamon Press. ISBN 0080247261. OCLC 7866855.
• SF₆ Reduction Partnership for Electric Power Systems

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

• Sulfur fluorides
• Dielectric gases
• Greenhouse gases
• Octahedral compounds