1—49
Chlorine, Free and Total, HR, continued
Spec√ Secondary Standards
The DPD Chlorine Secondary Standards kit (Cat. No. 26353-00) cannot be used on the
high range (HR) chlorine channel.
Method Performance
Estimated Detection Limit (EDL) = 0.1 mg/L Cl
2
Typical Precision (95% confidence interval) = 5.0 ± 0.2 mg/L Cl
2
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 compounds including but not limited
to monochloramine, dichloramine, nitrogen trichloride, and other chloro derivatives.
The combined chlorine oxidizes triiodide ion (I
3
–
) to iodine (I
2
). The iodine and free
chlorine reacts with DPD (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.
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