Tetrachloroethylene

Tetrachloroethylene Cl₂C=CCl₂ is a manufactured chemical compound that is widely used for the dry cleaning of fabrics (often commonly called dry-cleaning fluid) and for metal-degreasing. It is also used to make other chemicals and is used in some consumer products.

Other names for tetrachloroethylene include perchloroethylene, perc, PCE, and tetrachloroethene. It is a nonflammable liquid at room temperature. It evaporates easily into the air and has a sharp, sweet odor. Most people can smell tetrachloroethylene when it is present in the air at a concentration of 1 part per million (1 ppm), although some can smell it at even lower levels.

Michael Faraday first synthesized tetrachloroethylene in 1821 by heating hexachloroethane until it decomposed into tetrachloroethylene and chlorine.

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Production

Most tetrachloroethylene is produced from ethylene via 1,2-dichloroethane. When 1,2-dichloroethane is heated to 400°C with chlorine, tetrachloroethylene is produced by the chemical reaction

ClCH₂CH₂Cl + 3 Cl₂ → Cl₂C=CCl₂ + 4 HCl

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.

Tetrachlorethylene can also be made from a mixture of partially chlorinated light hydrocarbons that would otherwise be hazardous waste products of several chemical processes. When these compounds are heated in the presence of excess chlorine, they are converted to a mixture of tetrachloroethylene, carbon tetrachloride, and hydrogen chloride.

Uses

Most applications of tetrachloroethylene use it for its solvent properties. Most organic materials dissolve in tetrachloroethylene. It is the most widely used solvent in dry cleaning. 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.

In the early 20th century, tetrachloroethylene was the most effective available treatment for hookworm.

Tetrachloroethylene is also extensively used as an intermediate in the manufacture of refrigerants such as HFC-134a.

Health and safety

Like many chlorinated hydrocarbons, tetrachloroethylene 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, tetrachloroethylene may dissolve fats from the skin, resulting in severe skin irritation in work environments where people have been exposed to high concentrations.

Tetrachloroethylene is a common soil contaminant. Such contamination most often results from spillage, overfilling, or sewer leakage 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 tetrachloroethylene 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 tetrachloroethylene can be quite high may have more menstrual problems and spontaneous abortions than women who are not exposed. However, it is not known if tetrachloroethylene was responsible for these problems because other possible causes were not considered.

Results of animal studies, conducted with amounts much higher than those that most people are exposed to, show that tetrachloroethylene can cause liver and kidney damage. Exposure to very high levels of tetrachloroethylene 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 tetrachloroethylene as a Group 2A carcinogen, which means that it is probably carcinogenic to humans.^[1]

Under a high temperature flame such as those produced by a TIG torch or other welding arcs, Phosgene gas is produced. The gas instantly attacks the central nervous system shutting down the ability to breathe. The odour of the converted chemical is similar to almonds and very sweet smelling. Care should be taken to allow the solvent to completely evaporate when welding porous castings recently cleaned with tetrachloroethylene.

Contamination at U.S. Marine base

On June 12, 2007, the U.S. Centers for Disease Control and Prevention reported that as many as 75,000 people may have drank water contaminated with tetrachloroethylene at Camp Lejeune in North Carolina. The affected area is the Tarawa Terrace family housing area, and the contamination lasted from November 1957 through February 1987, the agency said. An off-base dry cleaners leaked a maximum concentration of 200 micrograms per liter of the fluid into a septic system near the housing area's well.^[2]

Testing for PCE

One method of testing for tetrachloroethylene 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, tetrachloroethylene can be detected in the breath for weeks following a heavy exposure.

Tetrachloroethylene and trichloroacetic acid (TCA), a breakdown product of tetrachloroethylene, 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 tetrachloroethylene or the other chemicals.

Popular culture

• Tetrachloroethylene contamination was used as a plot focus in a story arc of the TV show Ghostwriter.

• It has been featured in an episode of Stargate SG1.

References

1. ^ IARC monograph. "TETRACHLOROETHYLENE" Vol. 63, p. 159. Last Updated May 20, 1997. Last retrieved June 22, 2007.
2. ^ "Marines drank tainted water for 30 years: CDC", Reuters, Jun 12, 2007. 

Further reading

Agency for Toxic Substances and Disease Registry (ATSDR). 1997. Toxicological Profile for Tetrachloroethylene. link

Doherty, R.E. 2000. A History of the Production and Use of Carbon Tetrachloride, Tetrachloroethylene, Trichloroethylene and 1,1,1-Trichloroethane in the United States: Part 1 - Historical Background; Carbon Tetrachloride and Tetrachloroethylene. Journal of Environmental Forensics (2000) 1, 69-81. link