Sulfur hexafluoride

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Sulfur hexafluoride
Skeletal formula of sulfur hexafluoride with assorted dimensions Sulfur-hexafluoride-2D-dimensions.png
Skeletal formula of sulfur hexafluoride with assorted dimensions
Spacefill model of sulfur hexafluoride Sulfur-hexafluoride-3D-vdW.png
Spacefill model of sulfur hexafluoride
Sulfur-hexafluoride-3D-balls.png
Names
IUPAC name
Sulfur hexafluoride
Systematic IUPAC name
Hexafluoro-λ6-sulfane [1]
Other names
Elagas

Esaflon
Sulfur(VI) fluoride

Sulfuric fluoride
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.018.050
EC Number
  • 219-854-2
2752
KEGG
MeSH Sulfur+hexafluoride
PubChem CID
RTECS number
  • WS4900000
UNII
UN number 1080
Properties
SF6
Molar mass 146.06 g/mol
AppearanceColorless gas
Odor odorless [2]
Density 6.17 g/L
Melting point −64 °C; −83 °F; 209 K
Boiling point −50.8 °C (−59.4 °F; 222.3 K)
Critical point (T, P)45.51±0.1 °C, 3.749±0.01 MPa [3]
0.003% (25 °C) [2]
Solubility slightly soluble in water, very soluble in ethanol, hexane, benzene
Vapor pressure 2.9 MPa (at 21.1 °C)
−44.0×10−6 cm3/mol
Thermal conductivity
  • 13.45 mW/(m·K) at 25 °C [4]
  • 11.42 mW/(m·K) at 0 °C
Viscosity 15.23 μPa·s [5]
Structure
Orthorhombic, oP28
Oh
Orthogonal hexagonal
Octahedral
0 D
Thermochemistry
0.097 kJ/(mol·K) (constant pressure)
292 J·mol−1·K−1 [6]
−1209 kJ·mol−1 [6]
Pharmacology
V08DA05 ( WHO )
License data
Hazards
Safety data sheet External MSDS
S-phrases (outdated) S38
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazard SA: Simple asphyxiant gas. E.g. nitrogen, heliumSulfur hexafluoride
0
1
0
SA
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 1000 ppm (6000 mg/m3) [2]
REL (Recommended)
TWA 1000 ppm (6000 mg/m3) [2]
IDLH (Immediate danger)
N.D. [2]
Related compounds
Related sulfur fluorides
Disulfur decafluoride

Sulfur tetrafluoride

Related compounds
Selenium hexafluoride

Sulfuryl fluoride
Tellurium hexafluoride

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Sulfur hexafluoride (SF6) is an inorganic, colorless, odorless, non-flammable, non-toxic but extremely potent greenhouse gas, and an excellent electrical insulator. [7] SF
6
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 quite 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, considerably higher than the density of air (1.225 g/L).

Contents

Synthesis and reactions

SF
6
can be prepared from the elements through exposure of S
8
to F
2
. 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 S
2
F
10
(which is highly toxic) and then scrubbing the product with NaOH to destroy remaining SF
4
.

Alternatively, utilizing bromine, sulfur hexafluoride can be synthesized from SF4 and CoF3 at lower temperatures (e.g. 100 °C), as follows: [8]

2 CoF3 + SF4 + [Br2] → SF6 + 2 CoF2 + [Br2]

There is virtually no reaction chemistry for SF
6
. A main contribution to the inertness of SF6 is the steric hindrance of the sulfur atom, whereas its heavier group 16 counterparts, such as SeF6 are more reactive than SF6 as a result of less steric hindrance (See hydrolysis example). [9] It does not react with molten sodium below its boiling point, [10] but reacts exothermically with lithium.

Applications

More than 10,000 tons of SF
6
are produced per year, most of which (over 8,000 tons) is used as a gaseous dielectric medium in the electrical industry. [11] Other main uses include an inert gas for the casting of magnesium, and as an inert filling for insulated glazing windows.

Dielectric medium

SF
6
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. SF
6
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. The high dielectric strength is a result of the gas's high electronegativity and density. 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. Exposure to an arc chemically breaks down SF
6
though most of the decomposition products tend to quickly re-form SF
6
, a process termed "self-healing". [12] Arcing or corona can produce disulfur decafluoride (S
2
F
10
), a highly toxic gas, with toxicity similar to phosgene. S
2
F
10
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.

SF
6
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.

Alternatives to SF
6
as a dielectric gas include several fluoroketones. [13] [14]

Medical use

SF
6
is used to provide a tamponade or plug of a retinal hole in retinal detachment repair operations [15] in the form of a gas bubble. It is inert in the vitreous chamber [16] and initially doubles its volume in 36 hours before being absorbed in the blood in 10–14 days. [17]

SF
6
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 used to examine the vascularity of tumours. [18] It remains visible in the blood for 3 to 8 minutes, and is exhaled by the lungs. [19]

Tracer compound

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. [20] Gaseous SF
6
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 SF
6
.

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. [21] 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.

Sulfur hexafluoride is also routinely used as a tracer gas in laboratory fume hood containment testing. The gas is used in the final stage of ASHRAE 110 fume hood qualification. A plume of gas is generated inside of the fume hood and a battery of tests are performed while a gas analyzer arranged outside of the hood samples for SF6 to verify the containment properties of the fume hood.

It has been used successfully as a tracer in oceanography to study diapycnal mixing and air-sea gas exchange.

Other uses

Greenhouse gas

Mauna Loa sulfur hexafluoride timeseries. Mauna Loa Sulfur Hexafluoride.png
Mauna Loa sulfur hexafluoride timeseries.

According to the Intergovernmental Panel on Climate Change, SF
6
is the most potent greenhouse gas that it has evaluated, with a global warming potential of 23,900 [28] times that of CO
2
when compared over a 100-year period. Sulfur hexafluoride is inert in the troposphere and stratosphere and is extremely long-lived, with an estimated atmospheric lifetime of 800–3,200 years. [29]

Measurements of SF6 show that its global average mixing ratio has increased by about 0.2 parts per trillion per year to over 9 ppt as of February 2018. [30] [31] 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 small amounts of SF6 released compared to carbon dioxide, its overall contribution to global warming is estimated to be less than 0.2 percent. [32] Alternatives are being tested. [33]

In Europe, SF
6
falls under the F-Gas directive which ban or control its use for several applications. Since 1 January 2006, SF
6
is banned as a tracer gas and in all applications except high-voltage switchgear. [34] It was reported in 2013 that a three-year effort by the United States Department of Energy to identify and fix leaks at its laboratories in the United States such as the Princeton Plasma Physics Laboratory, where the gas is used as a high voltage insulator, had been productive, cutting annual leaks by 16,000 kilograms (35,000 pounds). This was done by comparing purchases with inventory, assuming the difference was leaked, then locating and fixing the leaks. [7]

Physiological effects and precautions

Like xenon, sulfur hexafluoride is a nontoxic gas, but by displacing oxygen in the lungs, it also carries the risk of asphyxia if too much is inhaled. [35] Since it is more dense than air, a substantial quantity of gas, when released, will settle in low-lying areas and present a significant risk of asphyxiation if the area is entered. That is particularly relevant to its use as an insulator in electrical equipment since workers may be in trenches or pits below equipment containing SF
6
. [36]

As with all gases, the density of SF
6
affects the resonance frequencies of the vocal tract, thus changing drastically the vocal sound qualities, or timbre, of those who inhale it. It does not affect the vibrations of the vocal folds. The density of sulfur hexafluoride is relatively high at room temperature and pressure due to the gas's large molar mass. Unlike helium, which has a molar mass of about 4 g/mol and pitches the voice up, SF
6
has a molar mass of about 146 g/mol, and the speed of sound through the gas is about 134 m/s at room temperature, pitching the voice down. For comparison, the molar mass of air, which is about 80% nitrogen and 20% oxygen, is approximately 30 g/mol which leads to a speed of sound of 343 m/s. [37]

Sulfur hexafluoride has an anesthetic potency slightly lower than nitrous oxide; [38] Sulfur hexafluoride is classified as a mild anesthetic. [39]

See also

Related Research Articles

Fluorocarbon class of chemical compounds

Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are, strictly speaking, organofluorine compounds with the formula CxFy, i.e. they contain only carbon and fluorine, though the terminology is not strictly followed. Compounds with the prefix perfluoro- are hydrocarbons, including those with heteroatoms, wherein all C-H bonds have been replaced by C-F bonds. Fluorocarbons can be perfluoroalkanes, fluoroalkenes and fluoroalkynes and perfluoroaromatic compounds. Fluorocarbons and their derivatives are used as fluoropolymers, refrigerants, solvents, and anesthetics.

Circuit breaker electrical switch designed to open when exposed to excess current

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset to resume normal operation.

Electrical breakdown when current flows through an electrical insulator when the voltage applied across it exceeds the breakdown voltage

Electrical breakdown or dielectric breakdown is when current flows through an electrical insulator when the voltage applied across it exceeds the breakdown voltage. This results in the insulator becoming electrically conductive. Electrical breakdown may be a momentary event, or may lead to a continuous arc if protective devices fail to interrupt the current in a power circuit.

Switchgear electrical switches that protect or control other parts

In an electric power system, switchgear is composed of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is directly linked to the reliability of the electricity supply.

Trigatron

A trigatron is a type of triggerable spark gap switch designed for high current and high voltage,. It has very simple construction and in many cases is the lowest cost high energy switching option. It may operate in open air, it may be sealed, or it may be filled with a dielectric gas other than air or a liquid dielectric. The dielectric gas may be pressurized, or a liquid dielectric may be substituted to further extend the operating voltage. Trigatrons may be rated for repeated use, or they may be single-shot, destroyed in a single use.

Octafluorocyclobutane chemical compound

Octafluorocyclobutane, or perfluorocyclobutane, C4F8, is an organofluorine compound which enjoys several niche applications. It is related to cyclobutane by replacement of all C–H bonds with C–F bonds. Octafluorocyclobutane is produced by the dimerization of tetrafluoroethylene and the reductive coupling of 1,2-dichloro-1,1,2,2-tetrafluoroethane.

Tellurium hexafluoride is a chemical compound of tellurium and fluorine with the chemical formula TeF6. It is a colorless, highly toxic gas with an extremely unpleasant smell.

Isidor Sauers (born 1948) is an Austrian-born American who is a physicist at the Oak Ridge National Laboratory in Tennessee. He is a specialist on the properties of Sulfur hexafluoride (SF6), with an important patent and over 60 peer-reviewed academic papers.

Disulfur decafluoride chemical compound

Disulfur decafluoride (S2F10) is a chemical compound discovered in 1934 by Denbigh and Whytlaw-Gray. Each sulfur atom of the S2F10 molecule is octahedral, and surrounded by five fluorine atoms. S2F10 is highly toxic, with toxicity four times that of phosgene. It was considered a potential chemical warfare pulmonary agent in World War II because it does not produce lacrimation or skin irritation, thus providing little warning of exposure. It is produced by the electrical decomposition of sulfur hexafluoride (SF6)—an essentially inert insulator used in high voltage systems such as transmission lines, substations and switchgear. S2F10 is also made during the production of SF6, but is distilled out. It is a colorless liquid with a burnt match smell similar to sulfur dioxide.

Selenium hexafluoride is the inorganic compound with the formula SeF6. It is a colourless gas described as having a "repulsive" odor. It is not widely encountered and has no commercial applications.

A hexafluoride is a chemical compound with the general formula QXnF6, QXnF6m−, or QXnF6m+. Many molecules fit this formula. An important hexafluoride is hexafluorosilicic acid (H2SiF6), which is a byproduct of the mining of phosphate rock. In the nuclear industry, uranium hexafluoride (UF6) is an important intermediate in the purification of this element.

Sulfur hexafluoride circuit breaker Switching device used in high voltage power grids

Sulfur hexafluoride circuit breakers protect electrical power stations and distribution systems by interrupting electric currents, when tripped by a protective relay. Instead of oil, air, or a vacuum, a sulfur hexafluoride circuit breaker uses sulfur hexafluoride (SF6) gas to cool and quench the arc on opening a circuit. Advantages over other media include lower operating noise and no emission of hot gases, and relatively low maintenance. Developed in the 1950s and onward, SF6 circuit breakers are widely used in electrical grids at transmission voltages up to 800 kV, as generator circuit breakers, and in distribution systems at voltages up to 35 kV.

A dielectric gas, or insulating gas, is a dielectric material in gaseous state. Its main purpose is to prevent or rapidly quench electric discharges. Dielectric gases are used as electrical insulators in high voltage applications, e.g. transformers, circuit breakers, switchgear, radar waveguides, etc.

A liquid dielectric is a dielectric material in liquid state. Its main purpose is to prevent or rapidly quench electric discharges. Dielectric liquids are used as electrical insulators in high voltage applications, e.g. transformers, capacitors, high voltage cables, and switchgear. Its function is to provide electrical insulation, suppress corona and arcing, and to serve as a coolant.

Sulfur chloride pentafluoride chemical compound

Sulfur chloride pentafluoride is an inorganic compound with the formula SF
5
Cl
. It exists as a colorless gas at room temperature and is highly toxic, like most inorganic compounds containing the pentafluorosulfide (SF5) functional group. The compound adopts an octahedral geometry with C
4v
symmetry. Sulfur chloride pentafluoride is the only commercially available reagent for adding the SF
5
group to organic compounds.

Fluorinated gases (F-gases) are man-made gases that can stay in the atmosphere for centuries and contribute to a global greenhouse effect. There are four types: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3).

A hybrid switchgear is one that combines the components of traditional air-insulated switchgear (AIS) and SF6 gas-insulated switchgear (GIS) technologies. It is characterized by a compact and modular design, which encompasses several different functions in one module.

Perfluoromethylcyclohexane chemical compound

Perfluoromethylcyclohexane is a fluorocarbon liquid—a perfluorinated derivative of the hydrocarbon methylcyclohexane. It is chemically and biologically inert.

Perfluoromethyldecalin is a fluorocarbon liquid—a perfluorinated derivative of the hydrocarbon methyldecalin. It is chemically and biologically inert. It is mainly of interest as a blood substitute, exploiting the high solubility of air in this solvent.

Vacuum interrupter

In electrical engineering, a vacuum interrupter is a switch which uses electrical contacts in a vacuum. It is the core component of medium-voltage circuit-breakers, generator circuit-breakers & high-voltage circuit-breakers. Separation of the electrical contacts results in a metal vapour arc, which is quickly extinguished. Vacuum interrupters are widely used in utility power transmission systems, power generation unit, and power-distribution systems for railway, arc furnace application & industrial plants.

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Further reading