Hach-Lange 9187 sc - Manual (Page 39)

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Appendix A 9184sc Theory of Operation
A.1 Theory of Operation
The 9184sc Chlorine Analyzer is an on-line, single-channel industrial analyzer that
measures free chlorine in drinking water treatment plants, distribution networks, and other
applications that require monitoring free chlorine at the ppb and ppm levels.
This instrument uses an amperometric method to measure HOCl concentration.
A membrane allows the selective diffusion of HOCl molecules to the amperometric sensor
(
Figure 1 on page 6 ). The measurement is compensated for pH and temperature.
A.1.1 Principle of Operation
Specific terms exist to mention the different species of chlorine:
• Active Chlorine HOCl (hypochlorous acid)
It is the most powerful disinfectant, up to 100 times more efficient than hypochlorite
• Total Free Chlorine (TFC): HOCl + ClO – :
It is composed of dissolved chlorine (at low pH values), hypochlorous acid gas and
hypochlorite ion. These species coexist, their relative proportion is depending on pH
and temperature (see curve below for a dissociation at 25°C).
• Total Combinated Chlorine (TCC):
It results from the addition of total free chlorine and chloramines (mono-, di- and
trichloramine). The 9184 sc does not measure this parameter.
Fraction of Cl
2
, HOCl and ClO – react as a function of pH ( Figure 13 ).
Figure 13 Dissociation Curve
The dissociation reactions are as follow:
Cl
2
+ H
2
O ♦ H+ + Cl – + HOCl pK1 = 4.6 at 25°C
HOCl ♦ H+ + ClO – pK2 = 7.5 at 25°C