Sulfur hexafluoride

Chembox new
Name = Sulfur hexafluoride
ImageFile = Sulfur-hexafluoride-2D-dimensions.png ImageSize = 100px
ImageName = Structure and dimensions of the sulfur hexafluoride molecule
ImageFile1 = Sulfur-hexafluoride-3D-balls.png ImageSize1 = 100px
ImageName1 = Ball-and-stick model of sulfur hexafluoride
ImageFile2 = Sulfur-hexafluoride-3D-vdW.png ImageSize2 = 100px
ImageName2 = Space-filling model of sulfur hexafluoride
IUPACName = sulfur(VI) fluoride
OtherNames = sulphur hexafluoride
Section1 = Chembox Identifiers
SMILES = FS(F)(F)(F)(F)F
CASNo = 2551-62-4
RTECS = WS4900000
Section2 = Chembox Properties
Formula = SF₆
MolarMass = 146.06 g/mol
Appearance = colorless, odorless gas
Density = 6.164 g/L, gas phase at 1 bar
(~5.1 times denser than air)
1.329 kg/L, liquid phase at 25 °C
2,510 kg/m³ or 2.510 kg/L, solid phase at −50.8 °C
Solubility = low
MeltingPt = −64 °C (209 K) (Sublimes),
Decomposes at 500 °C (773 K)
BoilingPt = Sublimes at Standard Pressure
Section3 = Chembox Structure
Coordination = octahedral
Dipole = 0 D
Coordination = O_h
Section7 = Chembox Hazards
ExternalMSDS = [http://ptcl.chem.ox.ac.uk/MSDS/SU/sulfur_hexafluoride.html External MSDS]
MainHazards = Inert gas, simple asphyxiant in high concentrations
Section8 = Chembox Related
OtherCpds = chem|SF|4, chem|CF|4

Sulfur hexafluoride is an inorganic compound with the formula chem|SF|6. It is a colorless, odorless, non-toxic and non-flammable gas (under standard conditions). chem|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 liquified compressed gas. It has a density of 6.13 g/L at sea level conditions.

ynthesis and chemistry

chem|SF|6 can be prepared from the elements through exposure of chem|S|8 to chem|F|2. This is 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 chem|S|2|F|10 and then scrubbing the product with NaOH to destroy remaining chem|SF|4.

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

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

Applications

Of the 8,000 tons produced per year, most of the chem|SF|6 goes into three applications: firstly as a gaseous dielectric medium or other use in the electrical industry, which accounts for 6,000 tons; secondly as an inert gas for the casting of magnesium; and thirdly as an inert filling for windows.

Dielectric medium

chem|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. chem|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 chem|SF|6 breakers in industry as they are safer and require less maintenance. Although most of the decomposition products tend to quickly re-form chem|SF|6 , arcing or corona can produce disulfur decafluoride (chem|S|2|F|10), a highly toxic gas, with toxicity similar to phosgene. chem|S|2|F|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.

chem|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 chem|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, chem|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.Fact|date=September 2008

Tracer compound

Gaseous chem|SF|6 is still a 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 chem|SF|6.

Sulfur hexafluoride was used as a harmless test gas in an experiment at St John's Wood tube station in London, England on 25 March 2007. [cite web |title='Poison gas' test on Underground |url=http://news.bbc.co.uk/1/hi/england/london/6492501.stm |accessdate=2007-03-31 |date=25 March 2007 |publisher=BBC News ] 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.

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

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

Greenhouse gas

According to the Intergovernmental Panel on Climate Change, chem|SF|6 is the most potent greenhouse gas that it has evaluated, with a global warming potential of 22,200 times that of chem|CO|2 when compared over a 100 year period--chem|SF|6 is very stable (for countries reporting their emissions to the UNFCCC, a GWP of 23,900 for chem|SF|6 was suggested at the third Conference of the Parties: GWP used in Kyoto protocol). [cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/248.htm |title=Climate Change 2001: Working Group I: The Scientific Basis |accessdate=2007-03-31 |date=2001 |publisher=Intergovernmental Panel on Climate Change ] Its mixing ratio in the atmosphere is lower than that of chem|CO|2 about 6.5 parts per trillion (ppt) in 2008 versus 380 ppm of carbon dioxide, but has steadily increased (from a figure of 4.0 parts per trillion in the late 1990s) [cite web |url=http://www.esrl.noaa.gov/gmd/ccgg/iadv/ |title=NOAA ESRL GMD Carbon Cycle - Interactive Atmospheric Data Visualisation |accessdate=2008-04-21 |date=21 April 2008 |publisher=US National Oceanic and Atmospheric Administration ] . Its atmospheric lifetime is 3200 years.

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, too, is due to the gas density. Unlike helium, which is much less dense than air, chem|SF|6 is approximately 5 times more dense than air, and the velocity of sound through the gas is 0.44 times the speed of sound in air. Unlike a gas such as helium, the speed of sound in which is greater than the speed of sound in air, the result of inhaling chem|SF|6 is the opposite of inhaling helium, a lowering of the frequency of the formants of the vocal tract. [http://www.physics.umd.edu/lecdem/services/demos/demosh6/h6-05.htm] [cite web |url=http://www.stevespanglerscience.com/experiment/from-donald-duck-to-barry-white-how-gases-change-your-voice |title=Anti-Helium - Sulfur Hexafluoride |accessdate=2007-06-29 |date= |author=Steve Spangler |publisher= Steve Spangler Science]

This was demonstrated (Sept. 3, 2008) on the Mythbusters television program (along with an inhalation of helium, to show higher pitched sound). [cite web
url=http://www.youtube.com/watch?v=d-XbjFn3aqE
title=Mythbusters - Fun With Gas
publisher=YouTube
accessdate=2008-09-09
date=2008-06-02]

Although inhaling chem|SF|6 can be a novel amusement, the practice can be dangerous because, like other inert gases, it displaces not only the oxygen needed for life, but also the chem|CO|2 that is the primary trigger of the breathing reflex. In general, dense, odourless gases in confined areas present the hazard of suffocation. A myth exists that chem|SF|6 is too heavy for the lungs to expel unassisted, and that after inhaling chem|SF|6, it is necessary to bend over completely at the waist to allow the excess gas to "spill" out of the body. In fact, the lungs mix gases very effectively and rapidly, such that chem|SF|6 would be purged from the lungs within a breath or two. [cite web |url=http://yarchive.net/med/helium_breathe.html |title=helium inhalation |accessdate=2007-03-31 |date=6 June 1995 |author=Steven B. Harris |publisher=Usenet Archives ]

References

Further reading

*
*Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
*Khalifa, from Maller and Naidu (1981)
* [http://www.epa.gov/electricpower-sf6/ SF₆ Reduction Partnership for Electric Power Systems]

ee also

* Selenium hexafluoride
* Tellurium hexafluoride
* Hypervalent molecule
* Paschen's Law
* Electric power
* Process for Measuring the Degradation of Sulfur Hexafluoride in High-voltage Systems US patent|4633082

External links

* [http://www.npi.gov.au/database/substance-info/profiles/44.html National Pollutant Inventory - Fluoride and compounds fact sheet]
* [http://www.epa.gov/highgwp/scientific.html High GWP Gases and Climate Change] from the U.S. EPA website.
* [http://www.epa.gov/highgwp/magnesium-sf6/workshops.html International Conference on SF₆ and the Environment]

United States Environmental Protection Agency website