Sulfur hexafluoride (SF 6) is an inorganic, colorless, odorless, non-flammable, extremely potent greenhouse gas which is an excellent electrical insulator. SF 6 has anoctahedral 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 (1.225 g/L).
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 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. Sulfur hexafluoride is also extremely long-lived, is inert in the troposphere and stratosphere and has an estimated atmospheric lifetime of 800–3200 years. SF 6 is very stable (for countries reporting their emissions to the UNFCCC, a GWP of 23,900 for SF 6 was suggested at the third Conference of the Parties: GWP used in Kyoto protocol). 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, SF 6 falls under the F-Gas directive which ban or control its usage 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 35,000 pounds. This was done by comparing purchases with inventory, assuming the difference was leaked, then locating and fixing the leaks.
PHYSOLOGICAL EFFECTS AND PRECAUTIONS
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. On the other hand, 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 grams/mol and gives the voice a childish and “Donald Duck” quality, SF 6 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 largeinertia of a SF 6 molecule. For comparison, the molar mass of air, which is about 80% nitrogen and 20% oxygen, is approximately 30 g/mol. Inhalation of SF 6 causes a lowering of the timbre, or frequency of the formants, of the vocal tract, by contrast with inhalation of helium, which raises it.
Like helium, sulfur hexafluoride is a non-toxic gas, yet by displacing oxygen in the lungs, it also carries the risk of asphyxia if too much is inhaled.