# Sulfur hexafluoride

Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF$_6$. It is a colorless, odorless, non-flammable, and non-toxic gas. 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, SF $_6$ 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 stored and transported as a liquefied compressed gas.
SF $_6$ has 23,500 times greater global warming potential (GWP) than CO$_2$ as a greenhouse gas (over a 100-year time-frame) but exists in relatively minor concentrations in the atmosphere. Its concentration in Earth's troposphere reached 11.50 parts per trillion (ppt) in October 2023, rising at 0.37 ppt/year. The increase since 1980 is driven in large part by the expanding electric power sector, including fugitive emissions from banks of SF $_6$ gas contained in its medium- and high-voltage switchgear. Uses in magnesium, aluminium, and electronics manufacturing also hastened atmospheric growth. The 1997 Kyoto Protocol, which came into force in 2005, is supposed to limit emissions of this gas. In a somewhat nebulous way it has been included as part of the carbon emission trading scheme. In some countries this has led to the defunction of entire industries.

## Synthesis and reactions

Sulfur hexafluoride on Earth exists primarily as a synthetic industrial gas, but has also been found to occur naturally.
SF $_6$ can be prepared from the elements through exposure of S $_8$ to F $_2$. This was 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, using bromine, sulfur hexafluoride can be synthesized from SF$_4$ and CoF$_3$ at lower temperatures (e.g. 100 °C), as follows:
There is virtually no reaction chemistry for SF $_6$. A main contribution to the inertness of SF$_6$ is the steric hindrance of the sulfur atom, whereas its heavier group 16 counterparts, such as SeF$_6$ are more reactive than SF$_6$ as a result of less steric hindrance. It does not react with molten sodium below its boiling point, but reacts exothermically with lithium. As a result of its inertness, SF $_6$ has an atmospheric lifetime of around 3200 years, and no significant environmental sinks other than the ocean.

## Applications

By 2000, the electrical power industry is estimated to use about 80% of the sulfur hexafluoride produced, mostly as a gaseous dielectric medium. Other main uses as of 2015 included a silicon etchant for semiconductor manufacturing, and an inert gas for the casting of magnesium.

### Dielectric medium

SF $_6$ is used in the electrical industry as a gaseous dielectric medium for high-voltage sulfur hexafluoride circuit breakers, switchgear, and other electrical equipment, often replacing oil-filled circuit breakers (OCBs) that can contain harmful polychlorinated biphenyls (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". 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. Compact GIS technology that combines vacuum switching with clean air insulation has been introduced for a subset of applications up to 420 kV.

### Medical use

SF $_6$ is used to provide a tamponade or plug of a retinal hole in retinal detachment repair operations in the form of a gas bubble. It is inert in the vitreous chamber. The bubble initially doubles its volume in 36 hours due to oxygen and nitrogen entering it, before being absorbed in the blood in 10–14 days.
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. It remains visible in the blood for 3 to 8 minutes, and is exhaled by the lungs.

### 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. 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. 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 SF$_6$ to verify the containment properties of the fume hood.
It has been used successfully as a transient tracer in oceanography to study diapycnal mixing and air-sea gas exchange. The concentration of sulfur hexafluoride in seawater (typically on the order of femtomoles per kilogram) has been classified by the international oceanography community as a "level one" measurement, denoting the highest priority data for observing ocean changes.

### Other uses

* The magnesium industry uses SF $_6$ as an inert "cover gas" to prevent oxidation during casting, and other processes including smelting. Once the largest user, consumption has declined greatly with capture and recycling.
* Insulated glazing windows have used it as a filler to improve their thermal and acoustic insulation performance.
* SF $_6$ plasma is used in the semiconductor industry as an etchant in processes such as deep reactive-ion etching. A small fraction of the SF $_6$ breaks down in the plasma into sulfur and fluorine, with the fluorine ions performing a chemical reaction with silicon.
* Tires filled with it take longer to deflate from diffusion through rubber due to the larger molecule size.
* Nike likewise used it to obtain a patent and to fill the cushion bags in all of their "Air"-branded shoes from 1992 to 2006. 277 tons was used during the peak in 1997.
* The United States Navy's Mark 50 torpedo closed Rankine-cycle propulsion system is powered by sulfur hexafluoride in an exothermic reaction with solid lithium.
* Waveguides in high-power microwave systems are pressurized with it. The gas electrically insulates the waveguide, preventing internal arcing.
* Electrostatic loudspeakers have used it because of its high dielectric strength and high molecular weight.
* Disulfur decafluoride, a chemical weapon, is produced with it as a feedstock.
* For entertainment purposes, when breathed, SF $_6$ causes the voice to become significantly deeper, due to its density being so much higher than air. This phenomenon 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.
* It is used for benchmark and calibration measurements in Associative and Dissociative Electron Attachment (DEA) experiments

## Greenhouse gas

* Sulfur hexafluoride (SF$_6$) measured by the Advanced Global Atmospheric Gases Experiment (AGAGE) in the lower atmosphere (troposphere) at stations around the world. Abundances are given as pollution free monthly mean mole fractions in parts-per-trillion.
* Abundance and growth rate of SF $_6$ in Earth's troposphere (1978-2018).
* Atmospheric concentration of SF$_6$ vs. similar man-made gases (right graph). Note the log scale.
According to the Intergovernmental Panel on Climate Change, SF $_6$ is the most potent greenhouse gas. Its global warming potential of 23,900 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.
Measurements of SF$_6$ show that its global average mixing ratio has increased from a steady base of about 54 parts per quadrillion prior to industrialization, to over 11.5 parts per trillion (ppt) as of October 2023, and is increasing by about 0.4 ppt (3.5%) per year. Average global SF$_6$ concentrations increased by about 7% 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 SF$_6$ released compared to carbon dioxide, its overall individual contribution to global warming is estimated to be less than 0.2%, however the collective contribution of it and similar man-made halogenated gases has reached about 10% as of 2020. Alternatives are being tested.
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. 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

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 SF $_6$.
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.
Sulfur hexafluoride has an anesthetic potency slightly lower than nitrous oxide; it is classified as a mild anesthetic.