Data
68
Technical data
J
What is residual chlorine?
Residual chlorine refers to hypochlorous acid, which is generated when a chlorine agent is
dissolved with water, and chloramine, which is generated when hypochlorous acid is
combined with ammonia. The former one is called “free residual chlorine”, and the latter one is
called “combined residual chlorine”, and they are collectively called “residual chlorine”. When
chlorine gas or sodium hypochlorite is injected into water, they will exist as hypochlorous acid
and hypochlorite ion (refer to equations 1 and 2 below).
Cl
2
+
H
2
O
⇔
HClO
+
HCl
…
Equation 1
HClO
⇔
H
+
+
ClO
-
…
Equation 2
This equilibrium is affected by pH or the temperature of the water, and hypochlorous acid
mainly exists in acid liquid. Fig. 1 indicates the abundance ratio of hypochlorous acid and
hypochlorite ion affected by pH variations. The sterilizing power varies depending on pH (the
power increases in acid and it decreases in alkali) because the sterilizing power of HCIO is
stronger than that of CIO
-
. With its strong oxidizing power, hypochlorous acid destroys the cell
membrane or cell wall of pathogenic organisms such as microorganisms or viruses, and
denatures the contained protein or nucleic acid. By doing this, it has a sterilization and
disinfection effect.
Fig. 1 Chemical change of hypochlorous acid affected by pH of sample water
SAMPLE pH
5 5.5 6 6.5 7 7.5 8 8.5 9
%
Distri
bution
H
C
IO
100
90
80
70
60
50
40
30
20
10
0