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Sulfur hexafluoride is SF6. This species is a gas at standard conditions. SF6 has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is colorless, odorless, non-toxic non-flammable gas. 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 suflur 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 SF6 is used by the electricity 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. SF6 also has the desirable property of "self healing". 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 plasma is also used in the semiconductor industry as an etchant and in the magnesium industry. It has been used successfully as a tracer in oceanography to study diapycnal mixing and air-sea gas exchange. It is also emitted during the aluminium smelting process. 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. 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 concentation of SF6. However its density (5 times heavier than air) necessitates proper mixing. 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 over a 100 year period* (for countries reporting their emissions to the UNFCCC, a GWP of 23,900 for SF6 is used as it was decreted at the third Conference of the Parties *). However, because its mixing ratio in the atmosphere is lower than that of CO2 (about 0.005 ppm versus 365 ppm), its contribution to global warming is accordingly low. Physiological effects and precautions Sulfur hexafluoride can affect the sound of a person's voice if it is inhaled in small quantities. When SF6 is inhaled, the pitch of a person's voice decreases dramatically because the speed of sound in SF6 is considerably less than it is in air. Sound travels through air at about 340 m/s but through SF6 at only about 120 m/s. This is the opposite of what is heard when a person inhales helium (about 900 m/s). Although xenon gas can also do this, its high cost makes it prohibitive for this type of entertainment, and it also has (poorly understood) anaesthetic side effects (similar to nitrous oxide). 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. There is a danger associated with any heavy gas in large quantities: it may sit invisibly in a container, and if a person enters a container filled with an odorless, colorless gas, they may find themselves breathing it unknowingly. SF6 is rarely used in large enough quantities for this to be a concern, though the potential for danger exists any time a tank or container of SF6 is kept in an unventilated space. It is not advisable to inhale or release SF6 in any quantity without proper training and precautions. Under no circumstances should one attempt to inhale SF6 -- or any other gas, for that matter -- directly from the pressurised cylinders used for storage. The high flow rate can fatally overpressure the lungs and rupture the alveoli in a fraction of a second, without time to react. Especially for SF6, trace amounts of toxic sulfur tetrafluoride, might have severe health effects. See also | |||||||||||||
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