| Sulfur hexafluoride | |
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Sulfur hexafluoride |
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Hexafluoro-λ6-sulfane[1] |
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Other names
Elagas Esaflon |
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| Identifiers | |
| CAS number | 2551-62-4 |
| PubChem | 17358 |
| ChemSpider | 16425 |
| UNII | WS7LR3I1D6 |
| EC number | 219-854-2 |
| UN number | 1080 |
| KEGG | D05962 |
| MeSH | Sulfur+hexafluoride |
| ChEBI | CHEBI:30496 |
| RTECS number | WS4900000 |
| ATC code | V08 |
| Gmelin Reference | 2752 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | F6S |
| Molar mass | 146.06 g mol−1 |
| Exact mass | 145.962489920 g mol-1 |
| Appearance | Colorless, odorless gas |
| Density | 6.17 g/l |
| Boiling point |
-64 °C, 209 K, -83 °F |
| Vapor pressure | 2.9 kPa (at 21.1°C) |
| Structure | |
| Crystal structure | Orthorhombic, oP28 |
| Space group | Oh |
| Coordination geometry |
Orthogonal hexagonal |
| Molecular shape | Octahedral |
| Dipole moment | 0 D |
| Thermochemistry | |
| Std enthalpy of formation ΔfH |
-1220.47 kJ mol-1 |
| Standard molar entropy S |
291.52 J K-1 mol-1 |
| Hazards | |
| MSDS | External MSDS |
| S-phrases | S38 |
| NFPA 704 |
0
0
0
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| Related compounds | |
| Related sulfur fluorides | Disulfur decafluoride |
| Related compounds | Selenium hexafluoride |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Sulfur hexafluoride (SF6) is an inorganic, colorless, odorless, and non-flammable greenhouse gas. 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 liquefied compressed gas. It has a density of 6.12 g/L at sea level conditions, which is considerably higher than the density of air.
Contents |
SF6 can be prepared from the elements through exposure of S8 to F2. This was 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 S2F10 (which is highly toxic) 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, but reacts exothermically with lithium.
Of the 8,000 tons of SF6 produced per year, most (6,000 tons) is used as a gaseous dielectric medium in the electrical industry, an inert gas for the casting of magnesium, and as an inert filling for insulated glazing windows.
SF6 is used in the electrical industry as a gaseous dielectric medium for high-voltage circuit breakers, switchgear, and other electrical equipment, often replacing oil filled circuit breakers (OCBs) that can contain 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 Graaff generators and Pelletrons and high voltage transmission electron microscopes.
SF6 is used to provide a tamponade or plug of a retinal hole in retinal detachment repair operations.[2] It is inert in the vitreous chamber[3] and initially doubles its volume in 36 hours before being absorbed in the blood in 10–14 days.[4]
SF6 is used 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.[5]
Sulfur hexafluoride 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.[6] Gaseous SF6 is used as a tracer gas 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 SF6.
Sulfur hexafluoride was used as a non-toxic test gas in an experiment at St John's Wood tube station in London, United Kingdom on 25 March 2007.[7] 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.
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. SF6 breaks down in the plasma into sulfur and fluorine, the fluorine plasma performing the etching.[8]
The magnesium industry uses large amounts of SF6 as inert gas to fill casting forms.
Sulfur hexafluoride is also used to pressurize waveguides in radar systems. The gas insulates the waveguide preventing internal arcing. The same use of sulfur hexafluoride is applied in transmission waveguides of medical linear accelerators, which are used for delivery of external beam radiotherapy.
Sulfur hexafluoride has been used in electrostatic loudspeakers because of its high dielectric strength and high molecular weight.
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 22,800[9] times that of CO2 when compared over a 100-year period. Measurements of SF6 show that its global average mixing ratio has increased by about 0.2 ppt per year to over 7 ppt.[10] Sulfur hexafluoride is also extremely long-lived, is inert in the troposphere and stratosphere and has an estimated atmospheric lifetime of 800–3200 years.[11] 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).[12] Average global SF6 concentrations increased by about seven percent per year during the 1980s and 1990s, mostly as the result of its use in the magnesium production industry, and by electrical utilities and electronics manufacturers. Given the low amounts of SF6 released compared to carbon dioxide, its overall contribution to global warming is estimated to be less than 0.2 percent.
In Europe, SF6 falls under the F-Gas directive which ban or control its usage for several applications. Since 1 January 2006, SF6 is banned as a tracer gas and in all applications except high-voltage switchgear.[13]
Another effect is the gas's ability to alter vocal sound waves. This is due to the gas's large molar mass. Unlike helium, which has a molar mass of about 4 grams/mol, SF6 has a molar mass of about 146 g/mol, and the velocity of sound through the gas is 0.44 times the speed of sound in air due to the large inertia of a SF6 molecule. For comparison, the molar mass of air, which is about 80% nitrogen and 20% oxygen, is approximately 30 g/mol. Inhalation of SF6 causes a lowering of the timbre, or frequency of the formants, of the vocal tract, by contrast with inhalation of helium, which raises it.[14]
Thermal conductivity at STP (101.3 kPa and 0 °C) = 12.058 mW/(m.K)[15]
Heat capacity at constant pressure (Cp) (101.3 kPa and 21 °C) = 0.097 kJ/(mol.K)[15]
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