chlorine disinfecants ?

John P. Hegarty jph10 at psu.edu
Thu Jul 15 10:45:03 EST 1999


Chlorine is highly reactive with a complex water chemistry.
Reactions with material in natural water generates many
substances (i.e. chloramines, THMs, HAAs) which have
varying cellular targets and toxicity.

Restricting comments to pure water with chlorine/bleach:

Chlorine gas (Cl2) and bleach (NaOCl) both form
hypochlorous acid (HOCl) when added to water:
Cl2 + H2O <==> HOCL + H+ + Cl-
NaOCl + H20 <==> HOCl + OH- + Cl- + Na+

HOCl is a MORE effective biocide than hypochlorite ion (OCl-).
HOCl dissociates to HOCl in a pH dependent manner:
HOCl <==> H+ + OCl-
At pH 7, about 73% will be in the HOCl form.
The relative concentrations of the hypochlorous acid (HOCl) and
hypochlorite ions (OCl-), are together termed "free chlorine"

The primary mechanism of HOCl killing is by direct oxidative
damage to the cell (i.e.. lipid peroxidation). Decomposition of
aqueous hypochlorite can evolve a molecular oxygen (singlet
spin state dioxygen) in the presence of hydrogen peroxide.
However, molecular oxygen does not participate in HOCl/OCl-
induced lipid peroxidation.


John P. Hegarty
Environmental Microbiology
Penn State Harrisburg


Adamporins <adamporins at aol.com> wrote in message
news:19990713184528.27929.00010783 at ng-fd1.aol.com...
Could someone tell me  what is the disinfectant active form of chlorine and
chlorine bleach ?  I have been reading two different versions. One says the
active disinfectant form is the OCl negative ion which is called "active
chlorine". Another source says bleach works because the NaOCl breaks down
releases monatomic oxygen, which is the actual disinfectant.  Do elemental
chlorine and bleach have different modes of action ?  I always thought that
elemental chlorine was the direct oxidizing agent in water purification, but
apparently not.  Some clarification would be greatly appreciated.
Thanks, Adam

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