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These are used to control the growth of algae and bacteria in swimming pools. A range of materials is available for this purpose. The rate of application is usually indicated on the label and should be adhered to, but often the instructions are inadequate and no expiry date is given even though some of the reagents are unstable. Most of the chemicals contain chlorine which is released more or less slowly, killing organisms in the water.

The most common chemical is calcium hypochlorite and this has to be used every few days. Sodium hypochlorite is also in common use. If the treatment is done in sunlight a considerable portion of the active hypochlorite is lost to the air as chlorine gas. Isocyanuric acid can be used as a stabiliser to reduce this loss, or dichloroisocyanuric acid can be used directly as a more stable, but more expensive, source of chlorine.

Most pool treatments gradually make the water more alkaline and the correct acidity has to be restored by adding an acid or acid salts. Test kits are available to measure the acidity and the amount of free chlorine in the pool to help maintain the correct levels. Some of these kits contain very toxic reagents and must be treated with care. The solutions should never be poured into the pool. The chemistry of pool treatment is quite involved but there is a good description in Chemistry in the Marketplace by Ben Selinger (see Resource list).

Health effects
All pool chemicals should be handled with care---see Calcium hypochlorite for precautions to be taken when handling pool chemicals. Most are strong oxidisers and many are corrosive. The majority of these chemicals are toxic when ingested. The pool water can be an irritant to eyes, nose, throat and lungs because of the chlorine. Chlorine and chloroform can be liberated from the pool into the air on hot days---sunny days by the poolside can be dangerous!

Environmental effects
These chemicals should not be released into sewers or waterways as they can kill the bacteria that help to break down sewage. To dispose of unwanted or out-of-date chemicals, contact your local council or state environment protection agency.

Alternatives
Electrolytic chlorinators are available which generate the required chlorine directly from the water to which salt has to be added; other chlorinators use chemicals contained within a cell so handling risks are minimal. There are also ultraviolet and ozone systems. Chemical use can be reduced by keeping the pool clean with an automatic pool cleaner and efficient filter system. Use salt-water pools.

 
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