Data
73
Technical data
What is residual chlorine?
Residual chlorine means hypochlorous acid that is produced when chlorine agent is dissolved
in water and chloroamine that is produced when hypochlorous acid is combined with
ammonia. The former is called free residual chlorine and the latter combined residual chlorine.
The two are collectively called residual chlorine. When chlorine gas or sodium hypochlorite is
injected into water, they are present as hypochlorous acid and ions (See equations 1 and 2).
Cl
2
+ H
2
O
HClO + HCl
Equation 1
HClO
H
+
+ ClO
Equation 2
This equilibrium is affected by the pH and temperature of water. On the acid side, the majority
is present as hypochlorous acid. Fig. 1 shows the percentage of hypochlorous acid
hypochlorous ions against pH changes. The disinfecting power changes with pH (i.e., it is
higher on the acid side and lower on the alkaline side) because the disinfecting power of HClO
is higher than that of ClO
. The high oxidizing power of hypochlorous acid/ions breaks the cell
membranes and walls of pathogenic organisms, such as microorganisms and viruses, and
denaturalizes the internal proteins and nucleic acid, thereby exerting a disinfecting effect.
Fig. 1 Changes in chemical form of hypochlorous acid due to pH of sample
SAMPLE pH
Distributio
n HClO
HCIO
CIO
5
7
8
9
5.5
6.5
7.5
8.5
6
100
90
0
80
70
60
50
40
30
20
10
h19136 | UTC 2020/03/30 3:34:51
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