Sulfur hexafluoride

Sulfur hexafluoride is an inorganic compound with the formula SF₆. It is a colorless, odorless, non-toxic and non-flammable gas (at standard conditions). 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 liquified compressed gas. It has a density of 6.13 g/L at sea level conditions.

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Synthesis and chemistry

SF₆ can be prepared from the elements, that is exposure of S₈ to F₂. Some other sulfur fluorides are cogenerated, but these are removed by heating the mixture to disproportionate any S₂F₁₀ and then scrubbing the product with NaOH to destroy remaining SF₄.

There is virtually no reaction chemistry for SF₆. It does not react with molten sodium.

Starting from SF₄, one can prepare SF₅Cl, which is structurally related to SF₆. The monochloride is, however, a strong oxidant and readily hydrolyzed to sulfate.

Applications

Of the produced 8000 t per year most of the SF₆ goes into three applications. First as gaseous dielectric medium or other use in the electrical industry with 6000 t , second as inert gas for the casting of magnesium and third it is used as inert filling of windows.

Dielectric medium

SF₆ is used by the electrical industry as a gaseous dielectric medium for high-voltage (1 kV and above) circuit breakers, switchgear, and other electrical equipment, often replacing 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 Graff generators and Pelletrons.

Medical use

Because SF₆ is relatively slowly absorbed by the bloodstream, it is used to provide a long-term tamponade (plug) of a retinal hole in retinal detachment repair operations.

In a further medical application, SF₆ 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 their visibility of blood vessels, to ultrasound. This application has been utilised to examine the vascularity of tumours amongst other things.

Tracer compound

Gaseous SF₆ 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. Several 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 harmless test gas in an experiment at St John's Wood tube station in London, England on 25 March 2007.^[1] 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 use

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.

SF₆ 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.

Due to its high density sulfur hexafluoride is often used in public "magic" tricks, where it is renowned for being "invisible water". One famous video shows a small 'boat' made from aluminium foil being carefully placed in a container of sulfur hexafluoride, where it floats almost exactly as if the container was filled with water. A beaker is then used to scoop up the gas from the container and into the foil boat, making it heavier and heavier, until finally, it sinks to the bottom of the tank. Another fun use for this substance is through inhalation, in which it has the inverse effect to that of helium. Meaning, after inhalation, one's voice becomes significantly lower-pitched.Youtube video of sulfur hexafluoride inhalation+sulfur hexaflouride in a tank

It is also emitted during the aluminium smelting process.

Greenhouse gas

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 22,200 times that of CO₂ when compared over a 100 year period--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).^[2] Its mixing ratio in the atmosphere is lower than that of CO₂ (about 6 parts per trillion (ppt) in 2007 versus 380 ppm of carbon dioxide),^[3] and its contribution to global warming is accordingly low.

Notable characteristics

Because the gas has a high density (over five times denser than air), it can be poured into open containers, like beakers and fishtanks. Moreover, light objects, e.g. ship-like vessels made out of light wood or aluminium foil containing air inside, can float on the gas.^[4]

Physiological effects and precautions

Another effect is the gas' ability to alter vocal sound waves. The gas can be breathed 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, SF₆ 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 SF₆ is the opposite of inhaling helium, a reduction in the pitch of the voice.^[5]

Although inhaling SF₆ can be a novel amusement, the practice can be dangerous because, like all gases other than oxygen, the SF₆ displaces the oxygen needed for breathing (a phenomenon known as asphyxiation). A myth exists that SF₆ is too heavy for the lungs to expel unassisted, and that after inhaling SF₆, 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 SF₆ would be purged from the lungs within a breath or two.^[6] In general, dense, odourless gases in confined areas present the hazard of suffocation.

References

1. ^ 'Poison gas' test on Underground. BBC News (25 March 2007). Retrieved on 2007-03-31.
2. ^ Climate Change 2001: Working Group I: The Scientific Basis. Intergovernmental Panel on Climate Change (2001). Retrieved on 2007-03-31.
3. ^ High GWP Gases and Climate Change. US Environmental Protection Agency (19 October 2006). Retrieved on 2007-03-31.
4. ^ TheTvelvethMonkey (6 January 2007). Ship floating on nothing! :: Physikshow Uni Bonn. YouTube. Retrieved on 2007-03-31.
5. ^ Steve Stangler. Anti-Helium - Sulfur Hexafluoride. Steve Stangler Science. Retrieved on 2007-06-29.
6. ^ Steven B. Harris (6 June 1995). helium inhalation. Usenet Archives. Retrieved on 2007-03-31.

Further reading

• (1991) Gaseous Dielectrics VI. Plenum Press. ISBN 0-306-43894-1. 
• 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

• Selenium hexafluoride
• Tellurium hexafluoride
• hypervalent molecule
• Paschen's Law
• Electric power
• Process for Measuring the Degradation of Sulfur Hexafluoride in High-voltage Systems U.S. Patent 4,633,082