How to Test for Nitrogen in Water

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You can test for nitrates in water with the diphenylamine test.
You can test for nitrates in water with the diphenylamine test. (Image: Jupiterimages/Photos.com/Getty Images)

Nitrogen in water will typically be found in the form of nitrates, water-soluble salts containing the negatively charged nitrate ion. One of the simplest tests to confirm the presence of nitrates is the diphenylamine test. It does involve the use of sulfuric acid, however, which is extremely toxic and corrosive, so do not perform this experiment if you have no experience working with these kinds of chemicals; even if you do, be extremely careful.

Things You'll Need

  • Goggles, gloves, long-sleeved coat
  • 200 mL graduated cylinder
  • Diphenylamine
  • Spatula and scale
  • Distilled water
  • Concentrated sulfuric acid
  • Plastic corrosion-resistant bottle (e.g., HDPE)
  • Glass container (e.g. ashtray, glass)
  • Pasteur pipette

Wear long pants, closed-toed shoes, gloves, chemical splash goggles and a long-sleeved coat. Concentrated sulfuric acid can cause severe burns if it comes in contact with your skin. Handle it carefully.

Weigh out 0.5 grams of diphenylamine and add it to the graduated cylinder, followed by 20 mL of distilled water. Stir or swirl until it dissolves.

Add sulfuric acid to bring the total volume up to 100 mL. Add it slowly and carefully. Concentrated sulfuric acid releases considerable heat on contact with water. Do not add any additional water or any other chemicals.

Store this solution in a corrosion-resistant plastic bottle away from heat in a cool dark place. Place this container inside a plastic tub so that if it leaks, the spill will be contained. Do not keep it above eye level or within reach of children.

Place 10 or 15 drops of the water you want to test in your glass container.

Add a few drops of the reagent you prepared earlier and watch the color of the solution. If the solution turns a deep blue in 30 minutes or less, nitrates are present at high levels. Do not assume just because you get a negative result, however, that nitrates are not present -- if they are present at trace levels, they will be more difficult to detect with this test. Some other oxidizing agents can occasionally cause this test to give a false-positive, although this is unusual.

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