A brown ring is formed in the ring test for \(NO_3^-\) ion. It is due to the formation of

\([Fe(H_2O)_5(NO)]^{2+}\)

The correct answer is option 1. \([Fe(H_2O)_5(NO)]^{2+}\).

The brown ring test is a classic qualitative test used to detect the presence of nitrate ions \((NO_3^-)\) in a solution. It is particularly known for the formation of a distinct brown ring at the interface of two liquid layers, indicating the presence of nitrate ions.

Procedure for the Brown Ring Test

Preparation:

Take a small amount of the aqueous solution containing nitrate ions in a test tube. 

Addition of Iron(II) Sulfate \((FeSO_4)\):

Add freshly prepared iron(II) sulfate solution to the test tube containing the nitrate solution. This ensures that \(Fe^{2+}\) ions are present in the solution.

Addition of Concentrated Sulfuric Acid \((H_2SO_4)\):

Carefully add concentrated sulfuric acid by slowly pouring it down the side of the test tube. This creates a layer of acid at the bottom of the test tube, beneath the aqueous layer. The acid must be added carefully so that it forms a distinct layer without mixing with the upper aqueous solution.

Observation:

At the interface between the two layers (the aqueous nitrate/iron(II) sulfate solution and the concentrated sulfuric acid), a brown ring forms. This ring indicates the presence of nitrate ions.

Chemical Reactions Involved

Reduction of Nitrate Ions:

The nitrate ion \((NO_3^-)\) is reduced by the iron(II) sulfate \((FeSO_4)\) in an acidic environment to form nitric oxide \((NO)\). The reduction reaction can be represented as:

\(2 NO_3^- + 3 Fe^{2+} + 4 H^+ \rightarrow 2 NO + 3 Fe^{3+} + 2 H_2O\)

Here, the \(Fe^{2+}\) ions from iron(II) sulfate reduce the nitrate ions to nitric oxide.

Formation of the Brown Ring Complex:

The nitric oxide \((NO)\) formed in the reaction then reacts with more \(Fe^{2+}\) ions to form a complex ion:

\([Fe(H_2O)_6]^{2+} + NO \rightarrow [Fe(H_2O)_5(NO)]^{2+} + H_2O\)

In this reaction, one water molecule is displaced by the nitric oxide molecule, forming the complex \([Fe(H_2O)_5(NO)]^{2+}\).

Structure and Characteristics of the Brown Ring Complex

Complex Formation:

The complex formed is \([Fe(H_2O)_5(NO)]^{2+}\). It consists of an iron(II) ion \((Fe^{2+})\) surrounded by five water molecules \((H_2O)\) and one nitric oxide \((NO)\) ligand. The \(NO\) molecule acts as a ligand, coordinating to the iron center, replacing one of the water molecules.

Appearance: The complex \([Fe(H_2O)_5(NO)]^{2+}\) is responsible for the characteristic brown color observed in the ring, which gives the test its name.

Why the Brown Ring Forms at the Interface

Interface of Layers:

The brown ring forms specifically at the interface of the aqueous solution and the concentrated sulfuric acid because of the unique conditions at this boundary. The concentrated acid creates a highly acidic environment that promotes the reduction of nitrate to nitric oxide, while the aqueous layer contains the \(Fe^{2+}\) ions necessary for complex formation.

Selective Reaction:

The brown ring is highly localized to the interface because the reaction is dependent on both the presence of \(Fe^{2+}\) ions and the freshly generated \(NO\) gas, which are in the right proportions and conditions at this interface.

Conclusion

The brown ring test is a reliable and straightforward method to detect nitrate ions \((NO_3^-)\) in a solution. The formation of the brown ring is due to the formation of the complex ion \([Fe(H_2O)_5(NO)]^{2+}\), where nitric oxide \((NO)\) coordinates to \(Fe^{2+}\) in an acidic environment. This complex has a distinct brown color, which forms a ring at the interface between the aqueous solution and the concentrated sulfuric acid.

Thus, the correct answer is option 1: \([Fe(H_2O)_5(NO)]^{2+}\).