131
Interferences
Summary of Method
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)
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, Correcting 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 correct chlorine concentration.
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, TOTAL, continued
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