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.

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. 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.This video shows the effects of the molecule.

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). 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), 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.

Physiological effects and precautions
Another effect is the gas's 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.

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. In general, dense, odourless gases in confined areas present the hazard of suffocation. 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.