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Sulfur hexafluoride

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Template:Chembox new Sulfur hexafluoride is an inorganic compound with the formula SF
6. It is a colorless, odorless, non-toxic and non-flammable gas (under standard conditions). 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.

Synthesis and chemistry

SF
6 can be prepared from the elements through exposure of S
8 to 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 S
2F
10 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 produced per year, most of the 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

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 SF
6 breakers in industry as they are safer and require less maintenance. 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.[citation needed]

Tracer compound

Sulfur hexafluride 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.[1] 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, England on 25 March 2007.[2] 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 SF6 as inert gas to fill casting forms.

Sulfur Hexafluoride is also used to pressurize the waveguide in radar systems.

Sometimes Sulfur Hexafluoride is inhaled to make one's voice lower as a party gag, similar to the more common practice of inhaling helium.

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 22,200 times that of CO
2 when compared over a 100 year period. However, due to its high density relative to air, SF
6 flows to the bottom of the atmosphere which limits its ability to heat the atmosphere. 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).[3] Its mixing ratio in the atmosphere is lower than that of 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)[4]. 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, SF
6 is approximately 5 times denser 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 SF
6 is the opposite of inhaling helium, a lowering of the frequency of the formants of the vocal tract.[5][6]

This was demonstrated (Sept. 3, 2008) on the Mythbusters television program (along with an inhalation of helium, to show higher pitched sound).[7]

Although inhaling 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 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 SF
6 is too heavy for the lungs to expel unassisted, and that after inhaling 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 SF
6 would be purged from the lungs within a breath or two.[8]

Other Properties

Thermal Conductivity = (1.013 bar and 0°C)) = 12.058 mW/(m.K)[9]
Heat capacity at constant pressure (Cp) (1.013 bar and 21 °C) = 0.097 kJ/(mol.K)[9]

References

  1. ^ 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
  2. ^ "'Poison gas' test on Underground". BBC News. 25 March 2007. Retrieved 2007-03-31.
  3. ^ "Climate Change 2001: Working Group I: The Scientific Basis". Intergovernmental Panel on Climate Change. 2001. Retrieved 2007-03-31.
  4. ^ "NOAA ESRL GMD Carbon Cycle - Interactive Atmospheric Data Visualisation". US National Oceanic and Atmospheric Administration. 21 April 2008. Retrieved 2008-04-21.
  5. ^ http://www.physics.umd.edu/lecdem/services/demos/demosh6/h6-05.htm
  6. ^ Steve Spangler. "Anti-Helium - Sulfur Hexafluoride". Steve Spangler Science. Retrieved 2007-06-29.
  7. ^ "Mythbusters - Fun With Gas". YouTube. 2008-06-02. Retrieved 2008-09-09.
  8. ^ Steven B. Harris (6 June 1995). "helium inhalation". Usenet Archives. Retrieved 2007-03-31.
  9. ^ a b "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, from Maller and Naidu (1981)
  • SF6 Reduction Partnership for Electric Power Systems

See also

United States Environmental Protection Agency website

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