42-36
NITROGEN, AMMONIA, continued
SAMPLING AND STORAGE
Collect samples in clean glass or plastic bottles. If chlorine is present,
add one drop of sodium thiosulfate, 0.1 N, for each 0.3 mg/L Cl
2
in a
1-liter sample. Preserve the sample by reducing the pH to 2 or less with
sulfuric acid (at least 2 mL). Store at 4 °C (39 °F) or less. Preserved
samples may be stored up to 28 days. Before analysis, warm samples to
room temperature and neutralize with sodium hydroxide, 5 N, before
analysis. Correct the test result for volume additions: see Sampling and
Storage, Volume Additions, (Section I) for more information.
ACCURACY CHECK
Standard Additions Method
a) Snap the neck off a Nitrogen Ammonia Voluette Ampule Standard
Solution, 50 mg/L NH
3
-N.
b) Use the TenSette Pipet to add 0.1, 0.2, and 0.3 mL of standard to
three 25-mL samples. Mix each thoroughly.
c) Analyze each sample as described above. The nitrogen concentration
should increase 0.20 mg/L for each 0.1 mL of standard added.
d) If these increases do not occur, see Standard Additions (Section I) for
more information.
Standard Solution Method
To check accuracy, use a 1.0 mg/L Nitrogen Ammonia Standard
Solution listed under Optional Reagents. Or, this can be prepared by
diluting 1.00 mL of solution from a Voluette Ampule Standard for
Ammonium Nitrogen to 50.0 mL with demineralized water.
STATISTICAL EVALUATION
A single operator repetitively tested samples of two laboratory prepared
solutions, using one DR/700, matched cells and two representative lots
of testing reagents. Testing 1.50 mg/L NH
3
- concentration samples, the
standard deviation was ±0.013 mg/L NH
3
-N.
Testing zero concentration samples, the limit of detection was 0.015 mg/L
NH
3
-N. The limit of detection was calculated as three times the
standard deviation when testing zero concentration samples (Adapted
from Analytical Chemistry, 1980, 52, 2242-2249)
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