137
Pollution Prevention and Waste Management
Samples treated with sodium arsenite for manganese or
chromium interferences will be hazardous wastes as regulated by
Federal RCRA for arsenic (D004). See Section 3 for more
information on proper disposal of these materials.
Summary of Method
Chlorine in the sample as hypochlorous acid or hypochlorite ion
(free chlorine or free available chlorine) immediately reacts with
DPD
(N,N-diethyl-p-phenylenediamine) indicator to form a magenta
color which is proportional to the chlorine concentration.
Monochloramine
For conventional free chlorine disinfection (beyond the breakpoint), typical
monochloramine concentrations are very low. If monochloramine is present in the
sample, its interference in the free chlorine test depends on the sample temperature,
relative amount of monochloramine to free chlorine, and the time required to do the
analysis. Typical interference level of monochloramine in the free chlorine test are
listed below (as mg/L Cl
2
).
Ozone Interferes at all levels
Peroxides May interfere
Extreme sample pH and
highly buffered samples
Adjust to pH 6-7. See Interferences, Section 1.
Interfering Substance Interference Level and Treatment
NH
2
Cl Sample Temp. °C (°F)
as Cl
2
5 (40) 10 (50) 20 (68) 30 (83)
1.2 mg/L +0.15 +0.19 +0.30 +0.29
2.5 mg/L 0.35 0.38 0.55 0.61
3.5 mg/L 0.38 0.56 0.69 0.73
CHLORINE, FREE continued
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