Sulfur hexafluoride

Sulfur hexafluoride
General
Systematic name	Sulfur(VI) fluoride
Other names	Sulfur hexafluoride
Molecular formula	SF₆
SMILES	?
Molar mass	146.06 g/mol
Appearance	colorless, odorless gas
CAS number	^*
Properties
Density and phase	5.11 (vs air)
Solubility in water	low
Melting point	-64 °C (209 °K) (Sublimes), Decomposes at 500 °C (773 °K)
Boiling point	Sublimes at Standard Pressure
Structure
Coordination geometry	octahedral
Dipole moment	0 D
Hazards
MSDS	External MSDS
Main hazards	highly inert, quite safe
NFPA 704
RTECS number	WS4900000
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related compounds	SF₄, CF₄
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Chemical infobox

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. 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^*. 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). Furthermore, because of its high density, it can be difficult to completely expel once it has filled the lungs. 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.

References

Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
Khalifa, from Maller and Naidu (1981)

External links

National Pollutant Inventory - Fluoride and compounds fact sheet
High GWP Gases and Climate Change from the U.S. EPA website.

Fluorides | Sulfur compounds | Greenhouse gases | Dielectrics

Schwefelhexafluorid | Hexafluorure de soufre | 六フッ化硫黄 | Rikkiheksafluoridi