Hach DR/850, Procedures Manual, Page: 123 / 460

123
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