Appendix A 9184Sc Theory Of Operation; Theory Of Operation; Principle Of Operation - HACH LANGE 9185sc User Manual

Appendix A 9184sc Theory of Operation

A.1 Theory of Operation
A.1.1 Principle of Operation
Figure 13 Dissociation Curve
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.
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 , HOCl and ClO
2
The dissociation reactions are as follow:
O ♦ H+ + Cl –
Cl + H + HOCl
2 2
HOCl ♦ H+ + ClO –
–
:
–
react as a function of pH
pK1 = 4.6 at 25°C
pK2 = 7.5 at 25°C
(Figure 13).
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