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9.5
Residual Chlorine Measurement
Residual chlorine
Residual chlorine is active chlorine remaining in water from the result of tap and pool water
disinfection or chlorine treatment for sterilization and deodorization in the food industry. Active
chlorine is used for industrial and household disinfectant or bleach.
In water, chlorine (Cl
according to a pH value and the compound formed from these three is called residual
chlorine. In Japan, the Waterworks Law requires it to disinfect tap water and maintain residual
chlorine in tap water.
Measurement principle
As shown in Fig. 1, when gold or platinum is a cathode (working electrode) and silver/silver
chloride is an anode (reference electrode) and voltage is applied between these electrodes, in
the cathode, the following reduction reaction occurs in an electrode soaked in residual
chlorine solution and a current flows in the external circuit.
HClO + e
In the anode, the following oxidation reaction occurs.
Ag → Ag
This current is proportional to the amount of residual chlorine in solution, so you can measure
the concentration of residual chlorine in sample water by measuring this current. This
measurement method is called the polarographic method and allows simple measurement of
residual chlorine differing from residual chlorine measurement methods using reagents which
require complicated preparation.
Temperature compensation
The current generated in the residual chlorine electrode varies depending on the residual
chlorine solution temperature. Compound movement is activated as the residual chlorine
solution temperature gets higher. The reaction in the cathode is also activated and the current
value is estimated high as a result. Temperature compensation means to compensate the
current change in this temperature.
pH influence
Residual chlorine is a compound that the abundance ratio of HClO and CIO
depending on pH in the solution. There is almost only HCIO around pH 5 and CIO
9.The amount of HClO, the reactive factor on the cathode side changes depending on pH in
the measuring solution, and the measuring current also changes in response. Therefore in
residual chlorine electrode calibration, making calibration solution close to the pH in the
measuring sample water is necessary.
), hypochlorous acid (HClO) and hypochlorite (ClO
2
→ ClO
+ 1/2 H
2
+
+ e
Cathode
Fig. 1 Residual chlorine electrode
Current
Anode
9 REFERENCE
) are balanced
changes
around pH
167
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