Hach-Lange 9187 sc - Manual (Page 40)

38
9184sc Theory of Operation
It is also important to notice that the dissociation constants are temperature-dependent
(the equipment takes into account this element).
The amperometric sensor consists of:
• a gold working electrode (cathode) where the main reaction occurs
• a silver counter-reference electrode (anode)
• KCl electrolyte
• a microporous membrane selective to HOCl
The HOCl molecules in the sample diffuse through the membrane to a thin region between
the membrane and the cathode that contains the electrolyte.
A constant potential is applied to the working electrode where HOCl is reduced according
to the reaction:
At the silver electrode (anode) the silver is oxidized to Ag + :
The reduction in HOCl at the cathode generates a current that is directly proportional to its
partial pressure in the sample.
The electrochemical reaction and diffusion through the membrane are dependent upon
temperature. Consequently, the measurement cell contains a temperature sensor that
allows for automatic temperature compensation.
In the acidification version an additional analyzer version offers the possibility to measure
samples with a high pH value. The sample pH is kept constant between 5.5 and 6.5 by
continuously adding acid solution. At such pH levels, all ClO
– ions turn into HOCl, thus
enabling the sensor to measure TFC.
HOCl H
+
2e
–
Cl
– → H
2
O + + +
2Cl
–
2Ag
+
+ 2AgCl 2e
–
+
→