Sulfur hexafluoride
- Sulfur ( VI ) fluoride
- R -846 (refrigerant)
Colorless and odorless gas
Gaseous
6.63 kg · m 3 (0 ° C, 1013 hPa)
-63.8 ° C
2.11 MPa ( 20 ° C)
Very poor in water (40 mg · l -1)
0
Attention
22,800 (based on 100 years)
-1220 KJ · mol -1
Template: Infobox chemical / molecular formula search available
Sulfur hexafluoride is an inorganic chemical compound consisting of the elements sulfur and fluorine, with molecular formula SF6. It is stable under normal conditions a colorless, odorless, poisonous gas, that is non-flammable and extremely inert, like nitrogen, behaves. At normal pressure and a temperature of -63.8 ° C it goes through sublimation directly from solid to gaseous state.
Production and representation
Sulfur hexafluoride can be synthesized directly from the elements by reaction of elemental sulfur (S8 ) in the fluorine gas ( F2). The reaction is highly exothermic.
In addition to SF6 formed in this pathway, other sulfur fluorides, such as disulphur decafluoride ( S2F10 ). Therefore, in the industrial production, the gas is heated to 400 ° C to produce a disproportionation of disulphur decafluoride in sulfur hexafluoride and sulfur tetrafluoride ( SF4 ) is carried out.
By washing the gas mixture in liquor, the sulfur tetrafluoride is destroyed while SF6 is not attacked by the lye.
Subsequent pressure distillation, the pure SF6 is separated.
Properties
Physical Properties
Sulfur hexafluoride is a gas under normal conditions. It has about five times denser than air. Its sublimation point is -63.8 ° C, which means that it goes directly from solid to gaseous state.
The triple point where the three phases, liquid and gas are determined in equilibrium, is at a temperature of -50.8 ° C and at a pressure of 2.26 bar. Only above this pressure, a liquid phase is possible.
The critical point can no longer be distinguished from the liquid and gas is at a temperature of 45.6 ° C, a critical pressure of 3.76 MPa and a critical density of 0.735 g · cm -3.
Crystal and Molecular Structure
At low temperatures it crystallizes in the monoclinic crystal system.
In the gaseous state, the molecule of SF6 is octahedral (Oh ); the S -F bond length is 156.4 pm.
Bond lengths and angles in the gaseous sulfur hexafluoride
Chemical Properties
Due to its structure, it is chemically inert and therefore is similar to molecular nitrogen or the noble gases. It is almost insoluble in water and non-flammable.
Because of its inertness reactions are usually carried out only under more drastic than the standard condition. Thus, sodium interspersed with SF6 in liquid ammonia to sodium sulfide and sodium fluoride to:
In the presence of hydrogen sulfide to elemental sulfur and the comproportionation of hydrogen fluoride ( HF) is known:
SF6 is isoelectronic to the anions hexafluorophosphate ( PF6 - ), hexafluorosilicate ( SiF62 ) and hexafluoroaluminate ( AlF63 ).
Use
Sulfur hexafluoride ( SF6) is used as an insulating gas in the medium and high- voltage technology, for example, in gas-insulated switchgear (GIS ) with high voltage circuit breakers and gas-insulated pipe lines ( GIL) in fully contained systems with operating voltages from 6 kV to 800 kV. It also serves as a quenching gas to interrupt the switching arc.
The dielectric strength is almost three times higher than in air or nitrogen at atmospheric pressure. These characteristics as well as low dielectric loss make it suitable as an insulating coaxial cables and in gas-insulated high -frequency power capacitors. As an insulation gas in electrical switchgear it is to 10 held under elevated pressure of 5 bar bar to ensure the necessary high insulation ability. The increased gas pressure is necessary because so that the mean free path of the free electrons in the gas is reduced (see Paschen's law ); thus electrons can not be accelerated so much as at atmospheric pressure and thus more likely to encounter together with the SF6 molecules.
In switchboards with the principle of non-toxic SF6 gas arise in the course of the operation by the arcs in combination with impurities such as a small amount of water in addition to the non-toxic Tetrafluoromethane toxic fluoride compounds such as hydrogen fluoride and thionyl fluoride, and the highly toxic disulphur decafluoride ( S2F10 ). For these reasons, appropriate safety guidelines for ventilation must be observed in gas-tight SF6 switchgear before servicing.
Furthermore, SF6 is used for leak testing of leakage of 1x10 -9 mbar · l / s. It is used as insulating gas for routine testing ( testing) of microelectronic circuits within the framework of quality assurance.
In the manufacture of semiconductor devices it is used as an etching gas: SF6 is the reactive gas in the reactive ion etching ( RIE) and DRIE ( deep reactive ion etching of eng. ). In addition, it is used in a similar way to the cleaning records, among other things in the display manufacturing.
Further using SF6 as a cover gas for the production of magnesium. The SF6, heavier than air, here prevents contact of the hot molten metal with the ambient air. Due to the process are given very large amounts of SF6 in the atmosphere in this application.
SF6 was previously used as an insulation between insulated glass panes, and as a filler gas in soles of sports shoes. In addition, sulfur hexafluoride was used until about the year 2000 for filling car tires, although it due to the high price of sulfur Hexafluorides, per set of tires up to 100 DM (about 50 euros ) cost (see also tire gas). All three aforementioned applications are now prohibited for reasons of environmental protection.
Due to its physical properties, the low background concentration in the atmosphere and the very good detectability in SF6 gas analyzers currently used still as a tracer gas for ventilation efficiency measurements in very small amounts. However, fewer greenhouse gases are used for many applications now.
In ophthalmology, a mixture of sulfur hexafluoride and air is used to treat retinal detachment in order to achieve a re-attachment of the retina. These ( vitrectomy ), the gas mixture in the vitreous cavity (Camera vitrea bulbi) is introduced for the purpose during the surgical vitrectomy to push the retina to its support ( tamponade ).
SF6 is used as ultrasound contrast agents in medicine since 2001. Here it is particularly used the detection of liver metastases of malignant tumors. The advantages include very high temporal and spatial resolution contrast. Thyroid disease and renal insufficiency are not a contraindication for performing this investigation dar. With the help of SF6 can liver foci are detected correctly about 90%.
Climate relevance
SF6 gas is according to a study by the Intergovernmental Panel on Climate Change (Intergovernmental Panel on Climate Change ), the most powerful known greenhouse gas. 1 kg of this gas is considered a period of 100 years, as effective as 22.8 thousand kilograms of carbon dioxide ( CO2). Because of the very low concentration of SF6 in the atmosphere ( approximately 0.005 ppb by volume, which corresponds to 0.12 ppmv CO2 equivalent; . CO2 about 385 ppm ), however, considered its impact on global warming as relatively low. It does not contribute significantly to the destruction of the ozone layer. The reduction of the SF6 gas in the atmosphere by high-energy UV radiation from the sun takes about 3200 years. The emission due to electrical installations in Germany in 1997 was 10% of 238 t of the total emission.
The increase in the SF6 concentration in the last few years, however, is enormous. So he went to the station Bukit Kototabang in Indonesia from 5.3 ppt to 6.3 ppt in early 2004 the end of 2008, representing an increase of approximately 19% in just five years.
Curiosities
Because of its higher density about five times as compared to normal air, sulfur hexafluoride may be an invisible liquid is poured into the container. On the SF6 mirror then very light objects, such as shells may "float" from aluminum foil. With great care, such an attempt succeeds with easier accessible CO2.
Enjoy an interesting effect occurs when SF6 is inhaled. In contrast to helium (a factor of 0.39 129 m / s, compared to air ) is obtained due to the over air much higher density of SF6 and the consequent lower speed of sound in the gas a deep voice. Before such experiments is expressly warned as sulfur hexafluoride hinders the exhalation of carbon dioxide. The danger of carbon dioxide anesthesia or respiratory standstill is higher than in the use of other oxygen- free gas such as nitrogen or helium. The background is the lower rate of diffusion of gases in sulfur hexafluoride, that reduces the mean free path due to its large -cross- section and high molar mass.