Swimming pools and spas require continuous sanitation to prevent the growth of algae and kill harmful bacteria. The most common type of sanitizer used to treat pool and spa water is chlorine.
Chlorine is a strong oxidizer, but other oxidizers such as bromine and iodine are also used for pool and spa disinfection. Depending on the pH level, chlorine will be present as hypochlorous acid (HOCl), the most effective form for sanitization, or as the less effective hypochlorite ion (OCl¯).
Chlorine can also combine with other molecules present in the water; when it combines with ammonia, chloramines are formed, which are virtually ineffective at sanitizing and responsible for producing the familiar and unpleasant chlorine odor.

Chlorine is added to swimming pools in various forms including calcium hypochlorite, sodium hypochlorite, or in some instances, chlorine gas.
At first, after the addition to water, chlorine is available as free chlorine. The measurement of free chlorine signifies the amount available for disinfection.
Once chlorine begins to sanitize bacteria and pathogens present in the water, it becomes combined chlorine; combined chlorine is no longer available to act as a disinfectant.
The measurement of total chlorine represents the sum of both free chlorine and combined chlorine. With both free and total chlorine measurements, a drinking water operator or pool owner can determine if there is enough chlorine available for disinfection.