Sulfur hexafluoride

Sulfur hexafluoride (sublimation temperature -63.9°C) is manufactured from sulfur and elemental fluorine  

The reaction is strongly exothermic. Lower sulfur fluorides are formed as byproducts together with the extremely poisonous disulfur decafluoride. Most of the lower sulfur fluorides are easily hydrolyzable, but sulfur decafluoride can only be decomposed by pyrolysis  

After pyrolysis it is scrubbed with aqueous alkali. Oxygen, nitrogen and carbon fluorides (from the fluorine) are removed by distillation under pressure. 

Several thousand tons of sulfur hexafluoride are produced worldwide per year. 

Chemical Symbol SFe CAS Registry Number 2551-62-4 DOT Classification Nonflammable gas DOT Label Nonflammable gas Transport Canada Classification 2.2 UN Number UN 1080 

Density of the liquid under its own approximate vapor pressure  

Sulfur hexafluoride is a colorless, odorless, nontoxic, nonflammable gas that has a high dielectric strength and serves widely as an insulating gas in electrical equipment. At atmospheric pressures it sublimes directly from the solid to the gas phase and does not have a stable liquid phase unless under a pressure of more than 32 psia (221 kPa abs). It is shipped as a liquefied compressed gas at its vapor pressure of 298 psig at 70°F (2055 kPa at 21.rC). 

One of the most chemically inert gases known, it is completely stable in the presence of most materials to temperatures of about 400°F (204°C) and has shown no breakdown or reaction in quartz at 900°F (482°C). Sulfur hexafluoride is slightly soluble in water and oil. No change in pH occurs when distilled water is saturated with sulfur hexafluoride. 

Sulfur hexafluoride is available for commercial and industrial use in various grades (minimum 99.8 mole percent) having much the same component proportions from one producer to another. 

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The reactivity of fluorine compounds varies from extremely stable, eg, compounds such as sulfur hexafluoride [2551-62 ] nitrogen trifluoride [7783-54-2] and the perfluorocarbons (see Fluorine compounds, organic) to extremely reactive, eg, the halogen fluorides. Another unique property of nonionic metal fluorides is great volatiUty. Volatile compounds such as tungsten hexafluoride [7783-82-6] molybdenum hexafluoride [7783-77-9] ... 

Nonmeta.ls, Sulfur reacts with fluorine to yield the remarkably stable sulfur hexafluoride, SF. Operating conditions must be controlled because a mixture of the lower fluorides such as disulfur difluoride [13709-35-8] 2 2 disulfur decafluoride [5714-22-7] 2 10 sulfur tetrafluoride [7783-60-0] SF4, may also be formed. When this reaction is carried out between 310 and 340°C, SF is primarily obtained and essentially no SF and only trace amounts of lower fluorides. Below 300°C, and preferably at ca 275°C, SF is the primary product. At 450—500°C, a mixture comprising ca 50% SF and the lower sulfur fluorides is formed (see Fluorine compounds, inorganic-sulfur). 

Sulfur hexafluoride [2551-62-4] 6 molecular weight 146.07, is a colorless, odorless, tasteless gas. It is not flammable and not particularly reactive. Its high chemical stabiUty and excellent electrical characteristics have led to widespread use in various kinds of electrical and electronic equipment such as circuit breakers, capacitors, transformers, microwave components, etc (see Electronic materials). Other properties of the gas have led to limited usage in a variety of unique appHcations ranging from medical appHcations to space research. 

Sulfur hexafluoride was first prepared in 1902 (1). The discovery in 1937 that its dielectric strength is much higher than that of air (2) led to its use as an insulating material for cables, capacitors (3), and transformers (4) (see Insulation,electric). Sulfur hexafluoride has been commercially available as AccuDri, SF (AUiedSignal Inc.) since 1948. It is also produced by Air Products and Chemicals in the United States and by others in Germany, Italy, Japan, and Russia. 

Properties. Sulfur hexafluoride is a good dielectric because a high gas density can be maintained at low temperatures. Properties are given in... 

Chemica.1 Properties. With few exceptions, SF is chemically inert at ambient temperature and atmospheric pressure. Thermodynamically SF is unstable and should react with many materials, including water, but these reactions are kineticaHy impeded by the fluorine shielding the sulfur. Sulfur hexafluoride does not react with alkah hydroxides, ammonia, or strong acids. 

At elevated temperatures SF forms the respective fluorides and sulfides with many metals (25). In quart2, it starts to decompose at 500°C (1) in copper or stainless steel, it is less stable (26). The stabiUty of SF at 200 and 250°C in the presence of aluminum, copper, siUcon steel, and mild steel is shown in Table 2 (14). Careful exclusion of moisture from the system improves the stabiUty of sulfur hexafluoride in the presence of most materials. 

Table 2. Stability of Sulfur Hexafluoride in Various Materials of Construction ...

Sulfur hexafluoride is more stable in arcs (27) than fluorocarbons such as C2F, or refrigerants such as CCI2F2, but less stable than CF, BCl, or SiF. Exposed to 1000°C temperatures, SF decomposes to SOF2 and SF to the extent of 10 mol %. In spite of its decomposition, the dielectric strength... 

Electrica.1 Properties. The electrical properties of SF stem primarily from its effectiveness as an electron scavenger. To accomplish electrical breakdown in a dielectric gas, primary electrons must gain sufficient energy to generate appreciable numbers of secondary electrons on molecular impact. Sulfur hexafluoride interferes with this process by capturing the primary electrons, resulting in the formation of SF or SF ions and F atoms (29) ... 

Table 3. Sulfur Hexafluoride Compared with other Dielectrics ...

Sulfur hexafluoride is packaged as a Hquefted gas in DOT 3AA 2015 steel cylinders containing 52 kg. Larger quantities are available in tube trailers containing ca 11,000 kg. 

Specifications and Analytical Methods. Sulfur hexafluoride is made to rigid specifications. Per ASTM D2472-81 (reapproved 1985) (50), the only permissible impurities are traces of air, carbon tetrafluoride (0.05 wt % max), and water (9 ppm by wt max dew point —45° Cmax). 

Sulfur hexafluoride may be analyzed chromatographicaHy using a molecular sieve or a Porapak QS column. Using an electron-capture detector, a sensitivity of 10 to lO " ppb is possible (51—53). 

Health and Safety Factors. Sulfur hexafluoride is a nonflammable, relatively unreactive gas that has been described as physiologically inert (54). The current OSHA standard maximum allowable concentration for human exposure in air is 6000 mg/m (1000 ppm) TWA (55). The Underwriters Laboratories classification is Toxicity Group VI. It should be noted, however, that breakdown products of SF, produced by electrical decomposition of the gas, are toxic. If SF is exposed to electrical arcing, provision should be made to absorb the toxic components by passing the gas over activated alumina, soda-lime, or molecular sieves (qv) (56). 

T. J. Brice, R. 1. Coon, and W. A. Severson, "Properties of Some Fluorocarbon Derivatives of Sulfur Hexafluoride," paper presented at American Chemical Society, Minneapolis, Minn., 1955. 

One example of unimolecular dissociation is the breakup of gaseous sulfur hexafluoride [2551-62-4] 6 according to reaction 21 (106) ... 

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