Hach DR/850 - Page 262

262
Interferences
Interfering Substances and Suggested Treatments.
Summary of Method
Ammonia compounds combine with chlorine to form
monochloramine. Monochloramine reacts with salicylate to form
5-aminosalicylate. The 5-aminosalicylate is oxidized in the
presence of a sodium nitroprusside catalyst to form a blue-
colored compound. The blue color is masked by the yellow color
from the excess reagent present to give a final green-colored
solution.
Interfering Substance Interference Level and Treatments
Calcium Greater than 1000 mg/L as CaCO
3
Glycine, hydrazine Less common. Will cause intensified colors in the prepared
sample.
Iron All levels. Correct for iron interference as follows:
1. Determine the amount of iron present in the sample using
one of the Total Iron procedures.
2. Prepare a deionized water sample containing the same
iron concentration as the original sample. Run the
procedure on this solution to determine the interference
due to iron. Subtract this value from the result in Step 12
obtained on the original sample.
Magnesium Greater than 6000 mg/L as CaCO
3
Nitrate Greater than 100 mg/L as NO
3
-
-N
Nitrite Greater than 12 mg/L as NO
2
-
-N
Phosphate Greater than 100 mg/L as PO
4
3-
-P
Sulfate Greater than 300 mg/L as SO
4
2-
Sulfide Sulfide will intensify the color. Eliminate sulfide
interference as follows:
1. Measure about 350 mL of sample in a 500-mL
erlenmeyer flask.
2. Add the contents of one Sulfide Inhibitor Reagent Powder
Pillow. Swirl to mix.
3. Filter the sample through a folded filter paper.
4. Use the filtered solution in Step 3.
Turbidity, sample color Turbidity and sample color will give erroneous high values.
Samples with severe interferences require distillation.
Albuminoid nitrogen samples also
require distillation.
Hach recommends the distillation procedure using the Hach
General Purpose Distillation Set. See the Optional Apparatus
list.
NITROGEN, AMMONIA, continued