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Interferences
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.
Interfering Substance Interference Level and Treatment
Acidity Greater than 150 mg/L CaCO
3
. May not develop full color or color may fade
instantly. Neutralize to pH 6-7 with 1 N sodium hydroxide. Determine
amount to be added on separate sample aliquot, then add the same amount to
the sample being tested. Correct for volume addition (See Section 1, Correct-
ing for Volume Additions).
Alkalinity Greater than 250 mg/L CaCO
3
. May not develop full color or color may fade
instantly. Neutralize to pH 6-7 with 1 N sulfuric acid. Determine amount to
be added on separate sample aliquot, then add the same amount to the sample
being tested. Correct for
volume addition (See Section 1, Correcting for Volume Additions).
Bromine Interferes at all levels
Chlorine Dioxide Interferes at all levels
Chloramines, organic May interfere
Hardness No effect at less than 1,000 mg/L as CaCO
3
Iodine Interferes at all levels
Manganese, Oxidized
(Mn
4+
, Mn
7+
)
or
Chromium , Oxidized (Cr
6+
)
1. Adjust sample pH to 6-7.
2. Add 3 drops potassium iodide (30 g/L) to a 25-mL sample.
3. Mix and wait one minute.
4. Add 3 drops sodium arsenite (5 g/L) and mix.
5. Analyze 10 mL of the treated sample as described in the procedure.
6. Subtract the result from this test from the original analysis to obtain the cor-
rect chlorine concentration.
Monochloramine Causes a gradual drift to higher readings. When read within 1 minute after
reagent addition, 3 mg/L monochloramine causes less than a 0.1 mg/L increase
in the reading.
Ozone Interferes at all levels
Peroxides May interfere
Extreme sample pH and highly
buffered samples
Adjust to pH 6-7. See Interferences, Section 1.
CHLORINE, FREE continued
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