Method Of Analysis - Hach CL17 Instrument Manual

General Information
Figure 22 CL17 Chlorine Analyzer

Method of Analysis

Page 58
General Information
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Free available chlorine (hypochlorous acid and hypochlorite ions) oxidizes the
DPD indicator reagent at a pH between 6.3 and 6.6 to form a magenta-
colored compound. The depth or intensity of the resulting color is proportional
to the concentration of chlorine in the sample. A buffer solution specifically for
free chlorine maintains the proper pH.
Total available chlorine (free available chlorine plus combined chloramines)
is determined by adding potassium iodide to the reaction. Chloramines in
the sample oxidize iodide to iodine, which, along with any free available
chlorine, oxidizes DPD indicator to form the magenta color at a pH of 5.1.
A different buffer solution containing potassium iodide maintains reaction pH.
After the chemical reaction is complete, the optical absorbance at 510 nm is
compared to the absorbance measured through the sample before the
reagents were added. Chlorine concentration is calculated from the difference
in absorbance.
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