(OCl-). The hypochlorite ion is not effective for
disinfection, but it is in a free form. All of the total
free chlorine would be in the form of hypochlorous
acid if the pH was low enough.
Combined Chlorine refers to chlorine which is
not readily available. For example, chlorine
combined as chloramines or organic nitrogen is not
an effective disinfectant and will not readily
convert to hypochlorous acid or hypochlorite ion.
Total Residual Chlorine refers to the sum of total
free chlorine and combined chlorine. Low total
residual chlorine is of particular interest to ensure
there are no downstream consequences for aquatic
life.
Disinfectant Properties of Chlorine
Chlorine is known to be a good disinfectant; it is
able to kill living matter in water such as bacteria,
cysts, and spores. Studies show that certain forms
of chlorine are more effective than others.
Whatever the chemical reaction, the relative
efficiency of various disinfecting compounds is a
function of the rate of diffusion of the active agent
through the cell wall. Factors which affect the
efficiency of destruction are:
➔ Nature of disinfectant (type of chlorine residual
fraction)
➔ Concentration of disinfectant
➔ Length of contact time with disinfectant
➔ Temperature
➔ Type and concentration of organisms
➔ pH
Hypochlorous acid is the most effective of all the
chlorine forms, similar in structure to water. The
germicidal efficiency of HOCl is due to the relative
ease with which it can penetrate cell walls. This
penetration is comparable to that of water, and can
be attributed to both its modest size and to its
electrical neutrality. The concentration of
hypochlorous acid is dependent on the pH, which
establishes the amount of dissociation of HOCl to
H+ and OCl- ions. Lowering the temperature of the
reacting solution suppresses the dissociation;
conversely, raising the temperature increases the
amount of dissociation. The rate of dissociation of
HOCl is so rapid that equilibrium between HOCl
and the OCl- ion is maintained, even though the
HOCl is being continuously used up. The OCl- ion
form of chlorine is a relatively poor disinfectant
because of its inability to diffuse through the cell