Summary Of Method - Hach POCKET COLORIMETER II Instruction Manual

Summary of Method

Chlorine can be present in water as free chlorine and as combined chlorine. Both
forms can coexist in the same solution and can be determined together as total
chlorine. Free chlorine is present as hypochlorous acid or hypochlorite ion.
Combined chlorine represents a combination of chlorine-containing compunds
including but not limited to monochloramine, dichloramine, nitrogen trichloride,
–
and other chloro derivatives. The combined chlorine oxidizes triiodide ion (I ) to
3
iodine (I ). The iodine and free chlorine reacts with DPD
2
(N,N-diethyl-p-phenylenediamine) to form a red solution. The color intensity is
proportional to the total chlorine concentration. To determine the concentration of
combined chlorine, run a free chlorine test and a total chlorine test. Subtract the
results of the free chlorine test from the total chlorine test to obtain the combined
chlorine concentration.
The range of analysis using the DPD method for chlorine can be extended by
adding more indicator in proportion to sample volume. For example, two powder
pillows of DPD Chlorine Reagent are added to a 5-mL sample portion to extend the
range of analysis.
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