What is the composition of the brown ring formed in the nitrate test?

The brown ring test is a classical method used to detect the presence of nitrate ions (NO₃⁻) in an aqueous solution. The test involves adding freshly prepared ferrous sulfate (FeSO₄) solution to the sample, followed by carefully pouring concentrated sulfuric acid (H₂SO₄) along the sides of the test tube. A brown-colored ring appears at the junction of the two liquids, indicating the presence of nitrate ions.

The brown ring is formed due to the formation of a nitrosyl complex known as iron(II) nitrosyl sulfate, with the composition FeSO₄·NO. During the reaction, nitrate ions are first reduced to nitric oxide (NO) by ferrous sulfate. This nitric oxide then combines with Fe²⁺ ions to produce the brown-colored complex [Fe(H₂O)₅NO]SO₄. The color arises from charge transfer within the complex between Fe²⁺ and the nitric oxide ligand.

The reaction sequence can be summarized as follows:

1. 
   

   Nitrate ion is reduced by Fe²⁺ in an acidic medium to form NO gas.

   
   


2. 
   

   The NO gas formed does not escape but reacts with excess Fe²⁺ ions to produce the brown nitrosyl complex, FeSO₄·NO.

   
   

Other possible complexes such as FeSO₄·NO₂ or FeSO₄·NO₃ are not formed in this reaction. The product FeSO₄·NO₂ would involve nitrogen dioxide, which is not stable in the test conditions, and FeSO₄·NO₃ would simply represent the starting materials rather than the reaction product.

Therefore, the brown ring seen in the nitrate test is composed of the iron(II) nitrosyl complex, represented by FeSO₄·NO, which serves as a positive confirmation for nitrate ions.