Answer to Question #17379 in Physical Chemistry for Mr Jenkins
Cl2 + H2O ⇋ HOCl + H+ + Cl-
The equilibrium constant is pH dependent and at the pH values of tap water and pool water (pH = 6.5-8.5) there is virtually no Cl2 present. Hypochlorous acid is a weak acid:
HOCl + H2O ⇋ OCl- + H3O+
such that at pH 7.5 [HOCl] ≈ [OCl]-
Both HOCl and OCl- are powerful oxidants destroying bacteria cells in seconds. HClO is however a significantly more powerful oxidant than OCl-. This is why sodium hypochlorite is used in pools as it is milder and easier to handle than Cl2 (which is also used with adjustment of pH):
OCl- + H2O ⇋ HOCl- + OH-
So hypochlorite ion does not react with Cl- at pH ≈ 7.5
Our bodies/bacteria give off cmpds with -NH2 groups (eg amino acids). These are converted by HOCl/OCl- to volatile NH2Cl and NHCl2 and give the pool water the distinctive "chlorine" smell. (I have smelled Cl2 and it is pretty much like heavily chlorinated pool water,however). OCl- slowly decomposes to chlorate ClO3- which is not a disinfectant:
3OCl- ⇋ 2Cl- + ClO3-
Anyway the OCl- levels gradually disappear over say three days.
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