Hach-Lange 9187 sc - Manual (Page 43)

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Appendix C 9187sc Theory of Operation
C.1 Theory of Operation
The 9187sc Chlorine Dioxide Analyzer is an on-line, single-channel industrial analyzer
that measures chlorine dioxide in drinking water treatment plants, distribution networks,
and other applications that require monitoring chlorine dioxide at the ppb and ppm levels.
This instrument uses an amperometric method to measure chlorine dioxide concentration.
A membrane allows the selective diffusion of
ClO2 molecules to the amperometric sensor
( Figure 1 on page 6 ). The measurement is compensated for temperature.
C.1.1 Principle of Operation
Measurement is carried out using an amperometric method after diffusion of the chlorine
dioxide molecules through a membrane. The molecules of chlorine dioxide contained in
the sample diffuse through the membrane and are then found in an electrolyte zone of
very slight thickness between the membrane and the cathode. A constant work potential is
applied to the work electrode (cathode) where CIO2 is reduced according to the reaction:
ClO
2
+ 5e – + 4H+ ♦ Cl – + 2H
2
O
At the silver electrode (anode) the silver is oxidized to:
Ag+ : Cl – + Ag ♦ AgCl + e –
The reduction in chlorine dioxide at the cathode generates a current which is directly
proportional to the partial pressure of it in the sample. The electrochemical reaction and
diffusion through the membrane are dependent upon temperature, consequently the
measurement cell is fitted with a temperature sensor which enables the automatic
compensation of measurement variations according to temperature.