Understanding Chlorine

Because they are in such wide use, the hazards of common chemicals sometimes are taken for granted. Complacency can set in and improper procedures may be used by those who work with the chemicals regularly and by emergency responders who deal with the...


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Because they are in such wide use, the hazards of common chemicals sometimes are taken for granted. Complacency can set in and improper procedures may be used by those who work with the chemicals regularly and by emergency responders who deal with the materials during a release, resulting in injury and death. One of these chemicals is chlorine.

0304hazmat1.jpg
Photo by Robert Burke
One-ton chlorine containers are loaded on a flatbed truck bearing a poison gas placard and four-digit identification number.

Chlorine is a common hazardous material found in most communities in the United States as a gas or in compound with other chemicals that can release the chlorine when in contact with water or other chemicals. It is generally transported and stored as a liquefied compressed gas and will be found in 100- to 150-pound cylinders, one-ton containers and railroad cars.

Chlorine (elemental symbol Cl) is a nonmetallic element, a member of the halogen family of elements with an atomic number of 17 on the Periodic Table. Other halogens include fluorine, bromine and iodine. Chlorine was discovered in 1774 by Carl Scheele, who also discovered oxygen and several other important compounds. Scheele called his discovery “dephlogisticated marine acid.” Chlorine has an atomic weight of 35.453 and is a greenish-yellow diatomic gas with a pungent irritating odor, but does not exist freely as a gas in nature. Diatomic gases are elements that do not exist as a single molecule, in this case Cl, but rather as the diatomic molecule Cl2. Other elements that are diatomic are hydrogen, nitrogen, bromine, iodine, fluorine and oxygen. (Oxygen is often referred to as O2 because it is a diatomic element.)

The primary source of chlorine is in the minerals halite (rock salt), sylvite and carnallite and from the chloride ion (sodium chloride) in sea water. It can be liquefied for more economical shipping, storage and use.

Chlorine is toxic by inhalation (1 part per million in air), non-flammable, non-explosive and a strong oxidizer (stronger than oxygen). Because chlorine is a strong oxidizer, it will support combustion even though it is non-flammable. Chlorine has a National Institute for Occupational Safety and Health (NIOSH) immediately dangerous to life and health (IDLH) rating of 10 ppm and exposure limit time-waited average (TWA) of 1 ppm. The Occupational Safety and Health Administration (OSHA) ceiling for chlorine is 1 ppm. The maximum airborne concentration is 3 ppm. This is the amount to which a person could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms that could impair the ability to take protective action.

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Photo by Robert Burke
Chlorine railroad car with the name of the material stenciled on the container, an inhalation hazard warning and a bulk placard with a four-digit identification number.

Chlorine gas irritates the mucus membranes and the liquid burns the skin or causes irritation to the skin and may cause burning pain, inflammation, and blisters. Tissue contact with cryogenic liquid chlorine can cause frostbite injury. Chlorine’s odor threshold is about 3.5 ppm, although some report that odor can be detected below the 1 ppm OSHA ceiling and TWA. Short-term exposure to low concentrations of chlorine (1 to 10 ppm) can result in a sore throat, coughing, and eye and skin irritation. After a few breaths at 1,000 ppm, chlorine can be fatal. Exposures to chlorine should not exceed 0.5 ppm (an eight-hour time-weighted average over a 40-hour week).

Chlorine is not known to cause cancer. Reproductive and developmental effects are not known or documented. Chlorine has a boiling point of 29 degrees Fahrenheit, a freezing point of –150F, a gas density of 2.5 (making it heavier than air), a specific gravity of 1.56 (heavier than water) and a vapor pressure of 5,168 mm Hg at 68F. The vapor pressure of chlorine is 53.1 psi at 32F and 112.95 psi at 77F.

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