Testing for nitrate anions in solutions is more difficult than for other types of anions, because even after precipitation, nitrates remain water soluble. Sulphates give off a barium precipitate when tested for anions in the same solution, for example -- and barium settles nicely to the bottom instead of dissolving back into the water. However, by adding iron (II) sulphate and sulphuric acid to an aqueous nitrate solution, you can determine whether nitrate anions are present.
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Things you need
- Iron (II) sulphate
- Sulphuric acid
- Test tube
Add 10 drops of a solution to a test tube.
Add in some concentrated sulphuric acid.
Pour iron (II) sulphate into a nitrate solution and move the test tube in a gentle circle to mix.
Add five more drops of sulphuric acid. Do not stir -- instead, hold the test tube at an angle and let each drop go down the side of the tube and form a layer on top of the first solution.
Look for a brown ring at the bottom of the test tube. If there are nitrate ions in the original solution, this brown ring will form relatively quickly. This ring comes from the oxidation of the iron.
Tips and warnings
- The formula of the reaction in this "brown ring test" goes as follows:
- NO3 + 3Fe + 4H --> 3Fe + NO + 2H2O
- The iron sulphate reduces the nitrate ion, resulting in oxidation and a nitrosyl complex.
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