CHLORINE

CYNTHIA BURROWS, UNIVERSITY OF UTAH

As an anion, chlorine is rather ordinary. Sodium grabs all the press from NaCl as the dietary culprit in hypertension, while silver's the key to AgCl's action in photography. Arguably, chloride plays a more important role in the antitumor drug cisplatin, Pt(NH₃)₂Cl₂, although its function there is to get lost (that is, hydrolyze) so that platinum has room to bind to DNA. Sadly, the –1 oxidation state of chlorine is often glossed over as the necessary counterion to an exotic metal ion or complex cation, the necessary yin to complement the yang.

Conversely, molecular chlorine, Cl₂, has held a starring role in history, both for its benefits to human health and for its detrimental effects on the environment. Carl Wilhem Scheele, a Swedish pharmacist, first described the greenish yellow gas in 1774 after dropping hydrochloric acid onto manganese dioxide. Sir Humphry Davy recognized the gas as an element in 1810 and named it based on the Greek word for its color, khloros.

Chlorine was by this time already in use. In the small town of Javelle, France, chlorine added to alkaline water created l'eau Javelle ("bleach" in English, NaOCl in chemspeak) that was used in the fabric industry in the late-18th century.

CHLORINE AT A GLANCE Name: From the Greek khloros, greenish yellow. Atomic mass: 35.45. History: Discovered, yet misidentified as a compound, by Swedish chemist Carl Wilhelm Scheele in 1774. Identified as an element by Sir Humphry Davy in 1810. Occurrence: Found in nature dissolved in salts in seawater and in the deposits of salt mines. Today, most chlorine is produced through the electrolysis of aqueous sodium chloride. Appearance: Yellowish-green, dense, sharp-smelling gas. Behavior: Liquid chlorine burns skin, and gaseous chlorine irritates mucous membranes. Breathing high concentration of the gas can be fatal; chlorine was used as a poison gas during World War I. Uses: Chlorine is essential to living systems and is also one of the top chemicals manufactured in the U.S. for commercial uses. It is an excellent disinfectant for swimming pools and water supplies, and its compounds are used in plastics such as polyvinyl chloride (PVC), stain removers, and bleach. Sodium chloride is common table salt.

SHORE LEAVE Morton Salt maintains a plant near the south end of Utah's Great Salt Lake. PHOTO BY CYNTHIA BURROWS

The mid-19th century saw one of the most dramatic improvements in human health: Bleach began to be used as a disinfectant in hospitals, and chlorination of the water supply in London during a cholera outbreak in 1850 saved many lives. Chlorine continues to be the disinfectant of choice in the food industry, in swimming pools, and in the drinking water supply in most developed countries.

The Chlorine Chemistry Council argues that the element also has an enormous economic impact, contributing 2 million jobs and around $50 billion to the annual U.S. economy in one way or another. Only a fraction of that is in the form most consumers would easily recognize: household bleach and swimming pool chemicals. We tend to overlook the fact that the C in PVC (polyvinyl chloride) pipes is chloride, and without chlorine we wouldn't have Saran wrap, nylon, microprocessors, soccer balls, or plastic toys. Even less obvious to most is the role of chlorine in the wood and paper industry (as a bleach) and in the processing of metals and the production of other materials such as titanium dioxide. Chlorocarbon compounds range from the good (chloroquine, an antimalarial) to the bad (DDT and chlorofluorocarbons) to the downright ugly (polychlorinated biphenyls). All are synthesized by chlorination of hydrocarbon precursors.

Where does all this chlorine come from? I can literally see tons of it out my window. Elemental chlorine does not exist naturally on our planet but is manufactured by electrolysis of seawater. The vast deposits of salt created during millions of years of continental upheaval and slow evaporation of the ancient Lake Bonneville are mined on the shores of the present-day Great Salt Lake. Through elaborate extraction procedures, the various chloride salts can be separated. Some of this salt ends up on your french fries (NaCl), and some you throw on your sidewalk in the winter (CaCl₂). The MgCl₂ is electrolyzed to produce Mg⁰, a lightweight metal used in the auto industry. Of course, the by-product of magnesium production is elemental chlorine, which can be responsibly used for all of the above-mentioned health and manufacturing applications.

The dark side of Cl₂ production is that too much of it is released directly into the atmosphere. According to the Environmental Protection Agency's Toxics Release Inventory, the biggest U.S. point source of atmospheric Cl₂ is 50 miles upwind of my house on the western shore of Great Salt Lake. Magnesium Corp. of America released 42 million lb of Cl₂ into the skies of Utah's West Desert in 2000, about 90% of the U.S. total for that year.

A little bit of chlorine is a great way to kill bacteria, but higher concentrations turn Dr. Jekyll to Mr. Hyde. More than twice as dense as air, chlorine can settle to the ground as it did in Ypres, France, in April 1915, accounting for thousands of fatalities. Responsible use of chlorine will ensure its continued applications toward improvement of human health and lifestyle without waging war on the environment.

Cynthia Burrows is a professor of chemistry at the University of Utah in Salt Lake City and senior editor of ACS's Journal of Organic Chemistry. She and her family enjoy camping and rockhounding in Utah's mineral-rich West Desert.

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