chlorine disinfecants ?

Norman Pilkington norman.pilkington at molsci.csiro.au
Thu Jul 15 18:14:35 EST 1999

In article <19990713184528.27929.00010783 at ng-fd1.aol.com>,
   adamporins at aol.com (Adamporins) wrote:
>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 
>chlorine". Another source says bleach works because the NaOCl breaks 
down and
>releases monatomic oxygen, which is the actual disinfectant.  Do 
>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

Elemental gaseous chlorine (supplied for water treatment in cylinders 
under pressure) reversibly hydrolyses in water to hypochlorous acid 
(HOCl) which, in turn, dissociates into its anions (hypochlrite 
negative ions). The proportion of each species depends on the pH 

'Chlorine' in commercial bleach is stabilised at high pH in sodium 
hydroxide (caustic soda) as its hypochlorite anion. It is also much 
safer to transport hypochlorite than the gas. As well, it is easier to 
dose into water than the gas. Accordingly, sodium hypochlorite is 
increasingly becoming the preferred form of chlorine even for use on 
the large scale.

However, the most active disinfectant species is the HOCl (free acid 
Accordingly, water disinfection is carried out at near neutral pH 
where there is still a significant proportion of HOCl. But the system 
cannot be allowed to be too acid (low pH) because it would corrode the 
treatment plant and the distribution system - as well as leading to 
increased cost due to the inefficiency of losing gaseous chlorine back 
to the atmosphere.

Norman Pilkington
CSIRO Molecular Science

Norman H. Pilkington (Mr)       	Phone: 	+61 3 9545 2389
Principal Research Scientist           	Fax:	+61 3 9545 2446
CSIRO Molecular Science
Bag 10

Email: norman.pilkington at molsci.csiro.au

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