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
"active
>chlorine". Another source says bleach works because the NaOCl breaks
down and
>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
>
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
(acidity).
'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
form).
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
Australia
_____________________________________________________________________
Norman H. Pilkington (Mr) Phone: +61 3 9545 2389
Principal Research Scientist Fax: +61 3 9545 2446
CSIRO Molecular Science
Bag 10
CLAYTON SOUTH VIC 3169
AUSTRALIA
Email: norman.pilkington at molsci.csiro.auhttp://www.molsci.csiro.au
_____________________________________________________________________
