Chlorine Cl

Discovery

In 1774 the Swedish chemist Carl Wilhelm Scheele heated muriatic acid (hydrochloric acid) with the mineral pyrolusite (manganese dioxide) and obtained a greenish-yellow gas with a suffocating smell. He noticed that it bleached litmus paper and flowers and attacked metals, but like most chemists of the time he assumed it was a compound containing oxygen, later called oxymuriatic acid.

In 1810 Humphry Davy tried hard to extract oxygen from the gas and failed. He concluded that it was an element and named it chlorine. Soon after, chlorine compounds became important in bleaching textiles and paper, building on work by Claude Louis Berthollet in the 1780s.

The twentieth century saw both sides of chlorine. Continuous chlorination of public water supplies, introduced in the early 1900s, sharply reduced deaths from cholera and typhoid. At the same time chlorine gas was released as a weapon at Ypres in 1915 during the First World War.

Origin of the name

Davy took the name from the Greek chloros, meaning pale green or yellow-green, after the colour of the gas. The same root appears in chlorophyll. The symbol is Cl.

Main uses

• Plastics: roughly a third or more of chlorine production goes into PVC for pipes, window frames, cables and flooring.
• Water treatment: chlorine and its compounds disinfect drinking water and swimming pools.
• Bleach: sodium hypochlorite is household bleach, and chlorine compounds bleach pulp and textiles.
• Chemical industry: chlorine is used to make hydrochloric acid, solvents, pharmaceuticals and titanium dioxide pigment.

Isotopes

Chlorine has two stable isotopes: chlorine-35 (about 76 percent) and chlorine-37 (about 24 percent). This mixture is why chlorine's atomic weight, about 35.45, is far from a whole number, and it produces a distinctive 3:1 pattern in mass spectra. The radioactive chlorine-36, with a half-life of about 301,000 years, is formed by cosmic rays and was also released by nuclear tests in the 1950s; it is used to date very old groundwater.

In everyday life

Chloride ions help maintain the body's fluid balance and, as hydrochloric acid, make stomach juices strongly acidic to digest food. Most chloride in the diet comes from salt.

Chlorine gas is highly irritating to the eyes and lungs. Household bleach should never be mixed with ammonia-based cleaners or acids, because the combination releases toxic gases. The typical 'swimming pool smell' is mostly caused by chloramines, formed when chlorine reacts with sweat and urine.