Liquisys M CCM223/253
Commissioning
Hauser
41
Total chlorine
The sum of free and combined chlorine is referred to as total chlorine.
Measurement of free chlorine with the sensors CCS140 and CCS141
Molecular chlorine (Cl
2
) has pH values of < 4. Hypochlorous acid (HOCl) and hypochlorite
(OCl
–
) remain within the range of pH 4 to 11 as components of free chlorine. As
hypochlorous acid dissociates with an increasing pH value to form hypochlorite ions (OCl
–
)
and hydrogen ions (H
+
), the amounts of the individual components in free chlorine change
with the pH value (
å
31). For example, if the amount of hypochlorous acid is 97% at
pH 6, it drops to about 3% at pH 9.
Hypochlorous acid has a highly disinfecting effect in an aqueous solution but the disinfecting
effect of hypochlorite is extremely low. Therefore, chlorine is not suitable as a disinfecting
agent at high pH values.
a0002017
Fig. 31:
Schematic diagram of pH compensation
A
Measured value
with
pH compensation
B
Measured value
without
pH compensation
C
pH compensation
The CCS140 or CCS141 chlorine sensors selectively measure the amount of hypochlorous
acid in amperometric measurement. They do not measure the amount of hypochlorite ions
because these ions do not permeate through the sensor membrane.
Measurement of total chlorine with the sensor CCS120
If ammonium is present in the medium - preferably water - in addition to free chlorine,
chloramines are formed (Cl
n
NH
m
). The chloramine compounds form in various dimerization
degrees. They are called "combined chlorine". Combined chlorine has a lower disinfecting
effect but it forms larger depots compared to free chlorine. This means:
• Slower elimination of existing germs
• Disinfecting effect over considerably longer periods
• Disinfecting effect over considerably longer transport distances
The amperometric sensor CCS120 measures the total chlorine content, i.e. free chlorine as
well as chloramine components.
This type of measurement is only slightly pH dependent.
pH compensation of chlorine sensor signal for free chlorine
(only on ES and EP versions, for CCS140/141 sensors)
To calibrate and inspect the chlorine measuring system, a colorimetric reference
measurement must be carried out using the DPD method. Free chlorine reacts with diethyl-
p-phenylenediamine and forms a red dye. The intensity of the red pigmentation increases
%
100
80
60
40
20
0
4
5
6
7
8
9
10
11
100
%
80
60
40
20
0
A
B
C
pH
OCl
-
HOCl
Summary of Contents for Liquisys M CCM223
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