(1) Chlorine is the most commonly used chemical to kill bacteria in pool
water. At room temperature and pressure, chlorine is a heavy green gas with a
characteristic odor. Chlorine is extremely poisonous, and safety precautions must be
followed when handling it.
(2) When chlorine is added to water, two acids are formed: hydrochloric
acid (HCl) and hypochlorous acid (HOCl). Hydrochloric acid is a useless by-product of
chlorination. It is neutralized by adding soda ash (1.25 to 1.5 pounds of soda ash for
each pound of chlorine). Hypochlorous acid, on the other hand, is the active
disinfecting agent. Hypochlorite (OCl-) is also formed and serves as a disinfecting
b. Free Available Chlorine.
(1) Hypochlorous acid is extremely effective in killing bacteria. Hypochlorite
ion kills bacteria more slowly. HOCl is 80 to 100 times more effective as a bactericide
than is OCl-. The pH of the pool water determines the proportion of the HOCl to OCl-.
The higher the pH of the pool water (the more basic), the less there is of hydrochlorous
acid. For example:
At a pH of 7.0, the acid is 72 percent molecular.
At a pH of 7.5, the acid is 50 percent molecular.
At a pH of 8.0, the acid is 21 percent molecular.
This is why chlorine is much less effective at killing bacteria at a high pH. Figure 1-4
shows the relationship between pH values and the presence of hypochlorous acid in
(2) Hypochlorous acid and hypochlorite ion are called "free" chlorine. Some
of this chlorine combines with matter in the water and some of it remains uncombined.
The portion of hypochlorous acid and hypochlorite ion that remains uncombined is
called "free available chlorine" or sometimes "free residual chlorine." It is the free
available chlorine that is critical for killing bacteria. Free available chlorine disperses in
bright sunlight, in high temperatures, and when water is agitated.