Hach-Lange CL17 USER MANUAL - Manual (Page 9)

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General information
2.2.2 Method of analysis
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.
Figure 1 Chlorine analyzer