Formation of Brown Ring

The brown ring test is a confirmatory test for the presence of nitrate ions (NO3-) in a solution. When a solution containing nitrate ions is treated with a mixture of freshly prepared ferrous sulfate (FeSO4) and concentrated sulfuric acid (H2SO4), a brown ring forms at the junction of the two layers.

Explanation of the Formation

The formation of the brown ring is a result of the reaction between nitrate ions and ferrous ions (Fe2+) in the presence of sulfuric acid. The reaction occurs in two steps:

1. Oxidation of Ferrous Ions:
In the presence of sulfuric acid, the nitrate ions act as a strong oxidizing agent and oxidize the ferrous ions to ferric ions (Fe3+). This reaction is represented as follows:
2 Fe2+ + 2 H+ + NO3- → 2 Fe3+ + NO + H2O

2. Formation of Nitroso Complex:
The ferric ions formed in the previous step react further with nitric oxide (NO) to form a complex known as nitroso ferrous sulfate or brown ring complex. This complex has a brown color, which gives the characteristic brown ring at the junction of the two layers. The reaction is represented as follows:
Fe3+ + NO → Fe2+ + NO+ + H+

Explanation of the Correct Answer

The correct answer to the given question is option B) nitroso ferrous sulfate. This is because the brown ring formed in the brown ring test is indeed due to the formation of nitroso ferrous sulfate complex. The other options mentioned in the question (ferrous nitrite, ferrous nitrate, and FeSO4.NO2) are not correct as they do not correspond to the actual compound formed in the reaction.

To summarize, the brown color of the ring observed in the brown ring test for nitrates is due to the formation of nitroso ferrous sulfate complex. This reaction is specific to nitrate ions and can be used as a confirmatory test for their presence.