The chemical properties of chlorine are
such that when liquid chlorine (Cl 2 ), and
hypochlorites are mixed with water, they
hydrolyze to form hypochlorous acid, which
will dissociate in water to form a hydrogen ion
(H+) and a hypochlorite ion (OCl−), according
to the reactions shown below. When sodium is
combined with hypochlorite to form sodium
hypochlorite, the following reactions would
apply.
Cl 2 +H 2 O→HOCL+H++Cl−
NaOCl+H 2 O→NaOH+HOCl
HOCL→H+OCl−Chlorine compounds are more effective
antimicrobial agents at a lower pH where the
presence of hypochlorous acid is dominant.
As the pH increases, the hypochlorite ion,
which is not as effective as a bactericide, pre-
dominates. Another chlorine compound,
chlorine dioxide, does not hydrolyze in aque-
ous solutions. Therefore, the intact molecule
appears to be the active agent.
Chlorine is known to be effective as a
sanitizer for mechanically polished stainless
steel, unabraded electropolished stainless
steel, and the polycarbonate surfaces,
reducing self-populations to less than 1.0
log CFU/cm^2. This sanitizer is less effective
on abraded electropolished stainless steel
and mineral resin surfaces, where popula-
tions exceed 1.0 log CFU/cm^2 (Frank and
Chmielewski, 1997).
Hypochlorites, the most active of the
chlorine compounds, are also the most
widely used. Calcium hypochlorite and
sodium hypochlorite are the major com-
pounds of the hypochlorites. These sanitiz-
ers are effective in deactivating microbial
cells in aqueous suspensions and require a
contact time of approximately 1.5 to 100
seconds. A 90% reduction in cell population
for most microorganisms can be attained in
less than 10 seconds, with relatively low lev-
els of free available chlorine (FAC). Bacter-
ial spores are more resistant than vegetative
cells to hypochlorites. The time required for
a 90% reduction in cell population can
range from approximately 7 seconds to
more than 20 minutes. The concentration of
free available chlorine needed for inactiva-
tion of bacterial spores is approximately 10
to 1,000 times as high (1,000 ppm, com-
pared with approximately 0.6 to 13 ppm) for
vegetative cells.Clostridiumspores are less
resistant to chlorine than Bacillus spores.
These data suggest that in sanitizing applica-
tions, where the concentration of hypochlor-
ous acid is low and the contact time is short,
there is limited effect on bacterial spores.
Although 200 ppm is effective for numerous
surfaces, 800 ppm is suggested for porous
areas.
The following example indicates how to
formulate a 200-ppm solution of chlorine in
a 200-L tank. This calculation assumes that
the chlorine contains 8.5% NaOCl.
8.5% NaOCl = 85,000 ppm (0.085
×1,000,000)
1 L = 1,000 mL
200 L = 200,000 mL
X 200 ppm
200,000 mL 85,000 ppm
85,000X= 40,000,000 mL
X= 470mL of 8.5% NaOClCalcium hypochlorite, sodium hypochlo-
rite, and brands of chlorinated trisodium
phosphate may be applied as sanitizers after
cleaning. The hypochlorites may also be
added to cleaning compound solutions to
provide a combination cleaner-sanitizer.
Organic chlorine-releasing agents, such as
sodium dichloroisocyanurate and dichloro-
dimethylhydantoin, can be formulated with
cleaning compounds.
Molecular hypochlorous acid is present in
highest concentration near pH 4, decreasing
rapidly as pH increases. At a pH higher than
5, hypochlorite (OCL−) increases; whereas,Sanitizers 171