Hach DR/700 Procedures Manual page 290

NITRATE, LR,
continued
Testing zero concentration samples, the limit of detection was 0.024 mg/L
NO N. The limit of detection was calculated as three times the standard
-
3
deviation when testing zero concentration samples (Adapted from
Analytical Chemistry, 1980, 52, 2242-2249).
INTERFERENCES
This method registers both the nitrate and nitrite nitrogen present in the
sample. If nitrite is present, the nitrite nitrogen test using program
number 50.08.1 should be done on the sample. The amount of nitrite
nitrogen found should be subtracted from the results of the nitrate
nitrogen test when the following pretreatment is used:
a) Add Bromine Water drop-wise to 30-mL of sample until a yellow
color persists. Mix after adding each drop.
b) Add one drop of Phenol Solution. Swirl to destroy the yellow color.
c) Continue with Step 4 of the nitrate procedure.
Calcium interferes in amounts over 100 mg/L as CaCO .
3
Chlorides in amounts over 100 mg/L cause low results. To determine
nitrate in high chloride samples or seawater, a manual calibration must
be performed. See section 3.2.4 in the DR/700 instrument manual.
Prepare nitrate standard solutions with the approximate chloride
concentration of the samples to be tested. Highly buffered samples or
extreme sample pH may exceed the buffering capacity of the reagents
and require sample pretreatment; see Interferences, pH (Section I).
SUMMARY OF METHOD
Cadmium metal reduces nitrates present in the sample to nitrite. The
nitrite ion reacts in an acidic medium with sulfanilic acid to form an
intermediate diazonium salt which couples to chromotropic acid to form
a pink-colored product.
50-40