Organic peroxides are a special class of compounds which have unusual stability problems. These peroxides are among the most hazardous chemicals normally handled in chemical laboratories and in manufacturing. As a class, organic peroxides are low power explosives. They are hazardous because of their extreme sensitivity to shock, sparks or other forms of mild ignition. Many peroxides that are routinely handled in laboratories are far more sensitive to shock than most primary explosives such as TNT. Peroxides have a specific half-life, or rate of decomposition, under any given set of conditions. A low rate of decomposition may auto-accelerate into a violent explosion, especially in bulk quantities of peroxide. They are sensitive to heat, friction, impact, and light as well as to strong oxidizing and reducing agents. All organic peroxides are extremely flammable and fires involving bulk quantities of peroxides should be approached with extreme caution.
As with all chemicals, be sure to label the container with the date they are opened. Limit the shelf life of these containers to six months except where noted. When these chemicals are purchased from the Chemistry Stock Room, return the empty serialized containers to the stockroom.
Compounds of the following types are known to form peroxides:
- Ethers, especially cyclic ethers, and ethers containing primary and secondary alcohol groups. The contents of the container should be destroyed by the user within one month
- Compounds containing benzylic hydrogen atoms. Such compounds are especially susceptible to peroxide formation if the hydrogens are on tertiary carbon atoms, e.g., cumene (isopropyl benzene)
- Compounds containing the allylic (CH2 = CHCH2R) structure, including most olefins
- Ketones, especially cyclic ketones
- Vinyl and vinylidene compounds, e.g. vinyl acetate and vinylidene chloride
Specific examples of chemicals that can form dangerous concentrations of peroxides on long exposure to air:
- decalin (decahydronaphthalene)
- ethyl ether
- isopropyl ether
- tetrahydrofuran (THF)
- tetralin (tetrahydronaphthalene)
One particularly dangerous common solvent is benzene. It is considered a Category I Carcinogen by OSHA. It should never be used as a solvent without specific authorization by an instructor or supervisor. Chronic poisoning can occur by inhalation of relatively small amounts of benzene over a long period of time. The toxic action is primarily on the blood-forming organs. Benzene has been documented to cause leukemia. Contact of benzene with the skin should be avoided, partly because, like other strong solvents, it causes dryness and cracking of the skin, but primarily because of potential skin absorption. Benzene is also a very flammable solvent and should be handled and disposed of with extreme care. Whenever possible, toluene should be substituted for benzene as a reagent. Your instructor should advise you when this is possible. The current OSHA permissible exposure limit is 1 ppm.
Another dangerous common solvent is carbon tetrachloride. Care must be taken to keep exposure to the liquid and its vapors to an absolute minimum. High concentration in the air can rapidly lead to death from respiratory failure. Less severe exposure can lead to kidney and liver damage. Carbon tetrachloride is also absorbed through the skin. Another chlorinated hydrocarbon, such as dichloromethane, should be substituted for carbon tetrachloride whenever possible, upon the instructor's direction. It should be noted, however, that almost all chlorinated hydrocarbons have been found to be toxic to some degree. The current threshold limit value for carbon tetrachloride is 5 ppm.
Formaldehyde is a colorless, pungent, irritant gas that is water soluble and most frequently is marketed as 37-56% aqueous solutions, commonly known as formalin. It is also provided as a solid polymer that decomposes to its monomer upon heating. It is a suspected human carcinogen and inhalation of the vapors may result in severe irritation of the upper respiratory tract and edema. Formaldehyde is a severe eye irritant, causing delayed effects that are not appreciably eased by eye washing. Skin sensitization can result from repeated exposure. Laboratory operations with formalin in open vessels should be carried out in a fume hood. Because repeated exposure to formaldehyde can lead to a formaldehyde allergy, it is well to avoid skin contact with aqueous solutions by appropriate use of neoprene, butyl rubber, or polyvinyl chloride gloves. Splash-proof goggles must be worn to avoid any possibility of splashing formaldehyde in the eyes. The current threshold limit value for formaldehyde is 1 ppm.
Perchloric acid is a water white liquid, with no odor. Perchloric acid can be dangerously reactive. At ordinary temperatures, 72% perchloric acid solution reacts as a strong non-oxidizing acid. Contact with combustible material at elevated temperatures may cause fire or explosion. Perchloric acid should only be used in a fume hood specially designed for that purpose. "Wash down" hoods are mandatory when using this material