Chlorine, Free
Chlorine, Free
Page 2 of 4
ChlorineFre_TNT_Other_DPD_Eng_Ody.fm
Interferences
5. Remove the cap from
a Free Chlorine DPD Test
‘N Tube. Add 10 mL of
sample to the tube. (Fill
the vial to the top of the
Hach label.
6. Cap and invert at
least 10 times to dissolve
the powder. (This is the
prepared sample).
7. Wipe the sample and
place it in the adapter.
8. Touch Read.
Results will appear in
mg/L Cl
2
.
Interfering Substance Interference Levels and Treatments
Acidity
Greater than 150 mg/L CaCO
3
. May not develop full color or color may fade instantly. Neutralize
to pH 6–7 with1 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
3.1.3 Correcting for Volume Additions on page 29).
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 3.1.3
Correcting for Volume Additions on page 29).
Bromine, Br
2
Interferes at all levels
Chlorine Dioxide, ClO
2
Interferes at all levels
Chloramines, organic May interfere
Hardness No effect at less than 1,000 mg/L as CaCO
3
Iodine, I
2
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 1 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 in the sample.
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