Sulfur hexafluoride

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Sulfur hexafluoride
IUPAC name

Sulfur hexafluoride
Systematic name

Hexafluoro-λ6-sulfane[1]
Other names

Elagas

Esaflon
Sulfur(VI) fluoride

Sulfuric fluoride
Identifiers
CAS number 2551-62-4 YesY
PubChem 17358 YesY
ChemSpider 16425 YesY
UNII WS7LR3I1D6 N
EC number 219-854-2
UN number 1080
KEGG D05962 N
MeSH Sulfur+hexafluoride
ChEBI CHEBI:30496 YesY
RTECS number WS4900000
ATC code V08DA05
Gmelin Reference 2752
Jmol-3D images Image 1
Properties
Molecular formula F6S
Molar mass 146.06 g mol−1
Appearance Colorless, odorless gas
Density 6.17 g/l
Boiling point

-64 °C, 209 K, -83 °F
Solubility slightly soluble in water, very soluble in ethanol, hexane, benzene
Vapor pressure 2.9 MPa (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 enthalpy of
formation ΔfHo298		 −1209 kJ·mol−1[2]
Standard molar
entropy So298		 292 J·mol−1·K−1[2]
Hazards
MSDS External MSDS
S-phrases S38
NFPA 704
NFPA 704.svg
0
0
0
Related compounds
Related sulfur fluorides Disulfur decafluoride

Sulfur tetrafluoride
Related compounds Selenium hexafluoride

Sulfuryl fluoride
Tellurium hexafluoride
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Sulfur hexafluoride (SF6) is an inorganic, colorless, odorless, and non-flammable greenhouse gas. SF6 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 (1.225 g/L).

Contents

Synthesis and reactions [edit]

SF6 can be prepared from the elements through exposure of S8 to F2. 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 S2F10 (which is highly toxic) and then scrubbing the product with NaOH to destroy remaining SF4.

There is virtually no reaction chemistry for SF6. 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 (See hydrolysis example).[3] It does not react with molten sodium, but reacts exothermically with lithium.

For example, reactions of SF6 with water to produce sulfuric acid and hydrofluoric acid (a hydrolysis reaction, which would be thermodynamically favourable) does not occur as a result of steric hindrance: SF6 + 4H2O(l) → no reaction

Applications [edit]

Of the 8,000 tons of SF6 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 [edit]

SF6 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. SF6 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 SF6, arcing or corona can produce disulfur decafluoride (S2F10), a highly toxic gas, with toxicity similar to phosgene. S2F10 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.

SF6 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 [edit]

SF6 is used to provide a tamponade or plug of a retinal hole in retinal detachment repair operations[4] in the form of a gas bubble. It is inert in the vitreous chamber[5] and initially doubles its volume in 36 hours before being absorbed in the blood in 10–14 days.[6]

SF6 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.[7]

Tracer compound [edit]

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.[8] Gaseous SF6 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 SF6.

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.[9] 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.

Other uses [edit]

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.[10]

SF6 plasma is also used in the semiconductor industry as an etchant. SF6 breaks down in the plasma into sulfur and fluorine, the fluorine plasma performing the etching.[11]

The magnesium industry uses large amounts of SF6 as inert gas to fill casting forms.

Sulfur hexafluoride is also used to pressurize waveguides in high power microwave systems. The gas insulates the waveguide preventing internal arcing.

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

Sulfur hexafluoride was used to fill Nike Air bags in all of their shoes from 1990-1996.

Greenhouse gas [edit]

Mauna Loa sulfur hexafluoride timeseries.

According to the Intergovernmental Panel on Climate Change, SF6 is the most potent greenhouse gas that it has evaluated, with a global warming potential of 23,900[12] times that of CO2 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.[13] Sulfur hexafluoride is also extremely long-lived, is inert in the troposphere and stratosphere and has an estimated atmospheric lifetime of 800–3200 years.[14] SF6 is very stable (for countries reporting their emissions to the UNFCCC, a GWP of 23,900 for SF6 was suggested at the third Conference of the Parties: GWP used in Kyoto protocol).[15] 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.[16]

In Europe, SF6 falls under the F-Gas directive which ban or control its usage for several applications. Since 1 January 2006, SF6 is banned as a tracer gas and in all applications except high-voltage switchgear.[17]

Physiological effects and precautions [edit]

As with all gases, the density of SF6 affects the resonance frequencies of the vocal tract, thus changing drastically the vocal sound qualities, or timbre, of those who inhale it. On the other hand, 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 grams/mol and gives the voice a childish and "Donald Duck" quality, SF6 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 SF6 molecule. For comparison, the molar mass of air, which is about 80% nitrogen and 20% oxygen, is approximately 30 g/mol. Inhalation of SF6 causes a lowering of the timbre, or frequency of the formants, of the vocal tract, by contrast with inhalation of helium, which raises it.[18]

Like helium, sulfur hexafluoride is a non-toxic gas, yet by displacing oxygen in the lungs, it also carries the risk of asphyxia if too much is inhaled.[19]

Other properties [edit]

  • Thermal conductivity at STP (101.3 kPa and 0 °C) = 12.058 mW/(m.K)[20]
  • Heat capacity at constant pressure (Cp) (101.3 kPa and 21 °C) = 0.097 kJ/(mol.K)
  • Critical temperature: 45.5 °C
  • Critical pressure: 37.59 bar (3.759 MPa)[20]

See also [edit]

References [edit]

  1. ^ "Sulfur Hexafluoride - PubChem Public Chemical Database". The PubChem Project. National Center for Biotechnology Information. Retrieved 22 February 2013. 
  2. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X. 
  3. ^ Duward Shriver, Peter Atkins (2010). Inorganic Chemistry. W. H. Freeman. p. 409. ISBN 1429252553. 
  4. ^ Daniel A. Brinton, C. P. Wilkinson (2009). Retinal detachment: principles and practice. Oxford University Press. p. 183. ISBN 0199716218. 
  5. ^ Gholam A. Peyman, M.D., Stephen A. Meffert, M.D., Mandi D. Conway (2007). Vitreoretinal Surgical Techniques. Informa Healthcare. p. 157. ISBN 1841846260. 
  6. ^ Hilton, G. F.; Das, T.; Majji, A. B.; Jalali, S. (1996). "Pneumatic retinopexy: Principles and practice". Indian Journal of Ophthalmology 44 (3): 131–143. PMID 9018990.  edit
  7. ^ Lassau N, Chami L, Benatsou B, Peronneau P, Roche A (December 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. 
  8. ^ C Michael Hogan (September 10, 2011). "Air pollution line source". Encyclopedia of Earth. Retrieved 22 February 2013. 
  9. ^ "'Poison gas' test on Underground". BBC News. 25 March 2007. Retrieved 22 February 2013. 
  10. ^ Hughes, T.G.; Smith, R.B. and Kiely, D.H. (1983). "Stored Chemical Energy Propulsion System for Underwater Applications". Journal of Energy 7 (2): 128–133. doi:10.2514/3.62644. 
  11. ^ Y. Tzeng and T.H. Lin (September 1987). "Dry Etching of Silicon Materials in SF6 Based Plasmas". Journal of the Electrochemical Society. Retrieved 22 February 2013. 
  12. ^ "2.10.2 Direct Global Warming Potentials". Intergovernmental Panel on Climate Change. 2007. Retrieved 22 February 2013. 
  13. ^ "Chromatograph for Atmospheric Trace Species SF6 Mixing Ratio". US National Oceanic and Atmospheric Administration. Retrieved 22 February 2013. 
  14. ^ A. R. Ravishankara, S. Solomon, A. A. Turnipseed, R. F. Warren; Solomon; Turnipseed; Warren (8 January 1993). "Atmospheric Lifetimes of Long-Lived Halogenated Species". Science 259 (5092): 194–199. Bibcode:1993Sci...259..194R. doi:10.1126/science.259.5092.194. PMID 17790983. Retrieved 22 February 2013. 
  15. ^ "6.12.2 Direct GWPs". Intergovernmental Panel on Climate Change. 2001. Retrieved 22 February 2013. 
  16. ^ "SF6 Sulfur Hexafluoride". PowerPlantCCS Blog. 19 March 2011. Retrieved 22 February 2013. 
  17. ^ "Climate: MEPs give F-gas bill a 'green boost'". EurActiv.com. 13 October 2005. Retrieved 22 February 2013. 
  18. ^ "Physics in Speech". University of New South Wales. Retrieved 22 February 2013. 
  19. ^ "Sulfur Hexaflouride". Hazardous Substances Data Bank. U.S. National Library of Medicine. Retrieved 26 March 2013. 
  20. ^ a b "Sulfur hexafluoride". Air Liquide Gas Encyclopedia. Retrieved 22 February 2013. 

Further reading [edit]

External links [edit]

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