Nitrates and Detection of Nitrate Radical in a Salt


The chemical compounds of nitric acid are referred to as nitrates. These nitrates are usually soluble in water. They are readily decomposed by heat giving rise to the formation of metallic oxide, nitrogen dioxide and oxygen.

2Cu(NO3)2 ———–> 2CuO + 4NO2 + O2

Some important nitrates of nitric acid are as under-

  • Silver nitrate (AgNO3)- It is also called as lunar caustic. It is extensively used in medicine and photography.
  • Sodium nitrate (NaNO3)- It is also referred to as chile saltpetre. It is largely used in making nitric acid and explosives. It is used as a valuable fertilizer.
  • Potassium nitrate (KNO3)- It is also known as saltpetre. It is commonly used in the manufacture of black gun powder and fire works. It is employed as a preservative for meat.

In addition to the above, there is a large variety of nitrates of other metals, which are put to innumerable uses.

Detection of Nitrate Radical in a Salt:

Since all common nitrates are soluble in water, they cannot be obtained as precipitates. Hence, it is not feasible to test for nitrate radicals by forming a characteristic precipitate. The following two tests are, usually, performed for detection of nitrate radical in salts-

Concentrated H2SO4 Test- Take a small amount of given nitrate salt in a test tube. Add to it a few cc. of concentrated H2SO4 and a small quantity of copper turnings or a few pieces of paper. Heat the contents of the tube. The evolution of reddish-brown fumes of nitrogen dioxide takes place. This indicates the presence of nitrate radical in the given salt.

NaNO3 + H2SO4 ———–> NaHSO4 + HNO3
4HNO3 + Cu ———–> Cu(NO3)2 + 2NO2 ↑ + 2H2O

Ring Test- Take an aqueous solution of nitrate salt in a test tube. Add to it, some freshly prepared solution of ferrous sulfate and shake. Hold the test tube in the slanting position and add concentrated H2SO4 along the side of the tube slowly and carefully. A dark brown ring is formed as the junction of two layers due to the formation of nitroso ferrous sulfate (FeSO4.NO). The reactions involved are-

NaNO3 + H2SO4 ———–> NaHSO4 + HNO3
6FeSO4 + 3H2SO4 + 2HNO3 ———–>3Fe2(SO4)3 + 4H2O + 2NO
FeSO4 + NO ———–> FeSO4.NO (bROWN RING)

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