Hach DR/850 Procedures Manual page 145

CHLORINE, TOTAL,
Summary of Method
Pollution Prevention and Waste Management
continued
Chlorine can be present in water as free available chlorine and as
combined available chlorine. Both forms can exist in the same
water and be determined together as the total available chlorine.
Free chlorine is present as hypochlorous acid and/or hypochlorite
ion. Combined chlorine exists as monochloramine, dichloramine,
nitrogen trichloride and other chloro derivatives.
The combined chlorine oxidizes iodide in the reagent to iodine.
The iodine reacts with DPD (N, N-diethyl-p-phenylenediamine)
along with free chlorine present in the sample to form a red color
which is proportional to the total chlorine concentration. To
determine the concentration of combined chlorine, run free
chlorine and total chlorine tests. Subtract the results of the free
chlorine test from the results of the total chlorine test to obtain
combined chlorine.
Samples treated with sodium arsenite for manganese or
chromium interferences will be hazardous wastes as regulated by
Federal RCRA for arsenic (D004).
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