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Typical for a nonpolar gas, is poorly soluble in water but quite soluble in nonpolar organic solvents. It has a density of 6.12 g/L at sea level conditions, considerably higher than the density of air (1.225 g/L). It is generally transported as a liquefied compressed gas.
The concentration of in Earth's troposphere reached 10 parts per trillion (ppt) in year 2019, rising at 0.35 ppt/year. The increase over the prior 40 years was driven in large part by the expanding electric power sector, including fugitive emissions from banks of gas contained in its medium- and high-voltage switchgear. Uses in magnesium, aluminum, and electronics manufacturing also hastened atmospheric growth.
Synthesis and reactions
Sulfur hexafluoride on Earth exists primarily as a man-made industrial gas and has also been found to occur naturally.
can be prepared from the elements through exposure of to . 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 (which is highly toxic) and then scrubbing the product with NaOH to destroy remaining .
Alternatively, utilizing bromine, sulfur hexafluoride can be synthesized from SF4 and CoF3 at lower temperatures (e.g. 100 °C), as follows:
2 CoF3 + SF4 + [Br2] -> SF6 + 2 CoF2 + [Br2]
There is virtually no reaction chemistry for . 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). It does not react with molten sodium below its boiling point, but reacts exothermically with lithium.
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. It remains visible in the blood for 3 to 8 minutes, and is exhaled by the lungs.
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.
For entertainment purposes, when breathed, causes the voice to become significantly deeper, due to its density being so much higher than air, as seen in this video. This is related to the more well-known effect of breathing low-density helium, which causes someone's voice to become much higher. Both of these effects should only be attempted with caution as these gases displace oxygen that the lungs are attempting to extract from the air. Sulfur hexafluoride is also mildly anesthetic.
For science demonstrations / magic as "invisible water" since a light foil boat can be floated in a tank, as will an air-filled balloon.
Abundance and growth rate of in Earth's troposphere (1978-2018).
Atmospheric concentration of SF6 vs. similar man-made gases (right graph). Note the log scale.
Measurements of SF6 show that its global average mixing ratio has increased from a steady base of about 54 parts per quadrillion prior to industrialization, to over 10 parts per trillion (ppt) as of April 2020, and is increasing by about 0.35 ppt (3.5 percent) per year. Average global SF6 concentrations increased by about seven percent per year during the 1980s and 1990s, mostly as the result of its use in magnesium production, and by electrical utilities and electronics manufacturers. Given the small amounts of SF6 released compared to carbon dioxide, its overall individual contribution to global warming is estimated to be less than 0.2 percent, however the collective contribution of it and similar man-made halogenated gases has reached about 10 percent as of year 2020. Alternatives are being tested.
In Europe, falls under the F-Gas directive which ban or control its use for several applications. Since 1 January 2006, is banned as a tracer gas and in all applications except high-voltage switchgear. 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 1,030 kilograms (2,280 pounds). This was done by comparing purchases with inventory, assuming the difference was leaked, then locating and fixing the leaks.
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. 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 .
As with all gases, the density of 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, 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.
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