The Water Cycle
Water in a Pool
Sodium hypochlorite tanks in the store
Sodium Hypochlorite is added to water as an alkali, and also to provide the water with a source of chlorine. The chlorine is a powerful oxidizing agent, as most of polluting matter is introduced to the pool water by bathers is capable of oxidation, it will destroy potential harmful micro-organisms.
Sodium Hypochlorite is corrosive.
There many micro-organisms which can cause harm to humans, and even with sufficient disinfection, they can remain in the water for a long time. E.coli has a residence time of less than a minute, Hepatitus A remains for 16 minutes, Giardia for 45 minutes and Cryptosporidum for 9600 minutes (6.7 days) It is therefore important to ensure all water is as clean as it can be.
The principle about adding sodium hypochlorite is that it dissolves in water to give hypochlorus acid, HOCl;
NaOCl + H20 <> HOCl + Na+ + OH-
Hypochlorus acid is a weak base, and reacts further;
HOCl <> H+ + OCl-
The hydrogen ion concentration will affect the pH of the water, which in turn will affect other important processes. If the pH is reduced, the hypochlorus acid concentration increases, and the hypochlorite decreases. Hypochlorous acid is the stronger disinfectant of the two, so the lower the pH, the stronger the disinfectant effect.
The sum of the hypochlorous and hypochlorite acids gives the free residual chlorine. This value must be strictly monitored, as there must be enough free chlorine to react with the pollutants. Generally, the free chlorine and the total combined chlorine are measured. The guidelines are that the minimum value for free chlorine is 1mg/l, the maximum value for combined chlorine is 1.5mg/l and the maximum value of free and combined chlorine should not exceed 8mg/l.