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Tetrachloroethylene, also known under its systematic name tetrachloroethene and as perchloroethylene, perchloroethene, perc, and PCE, is a chlorocarbon with the formula Cl2C=CCl2. It is a colourless liquid widely used for dry cleaning of fabrics, hence it is sometimes called "dry-cleaning fluid." It has a sweet odor detectable by most people at a concentration of 1 part per million (1 ppm). Worldwide production was about 1 megatons in 1985.[1]
[edit] ProductionMichael Faraday first synthesized tetrachloroethene in 1821 by thermal decomposition of hexachloroethane into tetrachloroethene and chlorine. Most tetrachloroethene is produced by high temperature chlorinolysis of light hydrocarbons. The method is related to Faraday's discovery since hexachloroethane is generated and thermally decomposes.[1] Side products include carbon tetrachloride, hydrogen chloride, and hexachlorobutadiene. Several other methods have been developed. When 1,2-dichloroethane is heated to 400 °C with chlorine, tetrachloroethene is produced by the chemical reaction:
This reaction can be catalyzed by a mixture of potassium chloride and aluminium chloride, or by activated carbon. Trichloroethylene is a major byproduct of the reaction, and since both are salable commercial chemicals, typical industrial practice is to collect both products and then separate them by distillation. [edit] UsesMost tetrachloroethylene is consumed as a solvent used in dry cleaning. It is desirable because it dissolves many organic materials, it is volatile, highly stable, and nonflammable. Usually as a mixture with other chlorocarbons, it is also used to degrease metal parts in the automotive and other metalworking industries. It appears in a few consumer products including paint strippers and spot removers. It is used as a solvent for proton NMR scans. [edit] Historical applicationsTetrachloroethene was also extensively used as an intermediate in the manufacture of HFC-134a and related refrigerants. In the early 20th century, tetrachloroethene was the most effective available treatment for hookworm. [edit] Health and safetyLike many chlorinated hydrocarbons, tetrachloroethene is a central nervous system depressant, and inhaling its vapors (particularly in closed, poorly ventilated areas) can cause dizziness, headache, sleepiness, confusion, nausea, difficulty in speaking and walking, unconsciousness, and death. After repeated or extended skin contact, tetrachloroethene may dissolve fats from the skin, resulting in severe skin irritation in work environments where people have been exposed to high concentrations. Tetrachloroethene is a common soil contaminant. Such contamination most often results from spillage, overfilling, sewer leakage, or the illegal disposal into UIC wells (e.g. septic systems, drywells) at commercial dry cleaning facilities. Because of the mobility of PCE in groundwater, its toxicity at low levels, and its density (which causes it to sink below the water table), cleanup activities tend to be especially problematic compared to cleanups of oil spills. In industry, most workers are exposed to levels lower than those causing obvious nervous system effects. The health effects of tetrachloroethene at levels typically encountered in occupational or environmental exposures have not been well established. Results from some studies suggest that women who work in dry cleaning industries where exposures to tetrachloroethene can be high; may have more menstrual problems and spontaneous abortions than women who are not exposed. However, it is not known if tetrachloroethene was responsible for these problems because other possible causes were not considered.[citation needed] Results of animal studies, conducted with amounts much higher than those that most people are exposed to, show that tetrachloroethene can cause liver and kidney damage. Exposure to very high levels of tetrachloroethene can be toxic to the unborn pups of pregnant rats and mice. Changes in behavior were observed in the offspring of rats that breathed high levels of the chemical while they were pregnant. The International Agency for Research on Cancer has classified tetrachloroethene as a Group 2A carcinogen, which means that it is probably carcinogenic to humans.[2] Under a high temperature flame such as those produced by a TIG torch or other welding arcs, phosgene gas is produced. [edit] Testing for exposureOne method of testing for tetrachloroethene exposure is to measure the amount of the chemical in the breath, much the same way breath-alcohol measurements are used to determine the amount of alcohol in the blood. Because it is stored in the body's fat and slowly released into the bloodstream, tetrachloroethene can be detected in the breath for weeks following a heavy exposure. Tetrachloroethene and trichloroacetic acid (TCA), a breakdown product of tetrachloroethene, can be detected in the blood. These tests are relatively simple to perform. These tests are not available at most doctors' offices, but can be performed at laboratories with the necessary equipment. Because exposure to other chemicals can produce the same breakdown products in the urine and blood, the tests for breakdown products cannot determine if one has been exposed to tetrachloroethene or the other chemicals. [edit] References
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