The germicidal strength of different forms of chlorine in water are ranked as follows:
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Caution
In an automated watering system, do not acidify chlorinated water below pH 5.0. At lower pH, chlorine will be present as dissolved chlorine gas (Cl2) which may cause swelling of the silicon o-ring in animal drinking valves.
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HOCl > OCl- > inorganic chloramines > organic chloramines
HOCl is 100 times more powerful an oxidant and disinfectant than is the hypochlorite ion. Consequently, free chlorine is most effective at pH 5 to 7 where HOCl is the predominant form. The effectiveness declines with increased pH.
Although inorganic chloramines are weaker disinfecting agents, they are more stable than free chlorine and can provide disinfection over long exposure periods. Some municipalities use chloramines for disinfection because they do not form trihalomethanes.
Since free chlorine is a stronger disinfectant than combined chlorine, the free chlorine test gives the best measure of disinfection at low concentrations (below 2 ppm). At higher concentrations, the difference between free and total chlorine will be insignificant so the total chlorine test can be used.
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