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Chlorine or any of it's derivatives (i.e. salts of Hypochlorite, Chloramines) reacts with water to form Hypochlorous Acid.
CL2+H2O~HCL+HOCL (Hypochlorous Acid)
Hypochlorous Acid is a very unstable compound and decomposes quickly by releasing singlet Oxygen.
HOCL~HCL+O
This Oxygen is a very strong oxidising agent. Its actions on cellular components are indiscriminate; it oxidises protein and irreversibly denatures essential cellular enzymes. The destruction of cells by direct combination of Chlorine with the cell membrane or other protein is considered minor in the chlorination of water.
Another gas, Ozone, behaves in a similar way as it also releases singlet Oxygen.
O~O2+O
Some factors which influence chlorination are:
- Presence of suspended solids and organic matter. They may shield bacteria from the disinfectant action of Chlorine or react so that the disinfectant power is lost. The higher the pH and temperature of the water, the more Chlorine necessary per unit volume of water to be disinfected.
- Ammonia ions present. This will reduce the disinfecting power by forming Chloramine.
Presence of reduced ions or compounds such as Nitrite, H2S, Manganese or Iron, may reduce disinfecting properties of Chlorine because the singlet Oxygen will act as an oxidising agent of these ions rather than on bacteria.
- Ideally, enough Chlorine must be added to leave a residual of 0,2 to 1mg/l of free Chlorine after all microorganisms and extraneous organic matter have been saturated with Chlorine, i.e. break point chlorination has been reached.
 
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