In brown ring test for nitrate ions, brown ring is formed due to complex

Options

A. `[Fe (H_(2)O)_(6)]^(2+)`
B. `[Fe(H_(2) O)_(5) NO]^(2+)`
C. `[Fe (H_(2) O)_(5) NO]^(3+)`
D. `[Fe(H_(2) O)_(5) NO_(2)]^(2+)`

Answer

### Step-by-Step Solution

1. **Understanding the Brown Ring Test**: The brown ring test is a qualitative analysis method used to detect the presence of nitrate ions (NO3⁻) in a solution. The test involves the reaction of ferrous sulfate (FeSO4) with nitrate ions in an acidic medium.

2. **Preparation of the Reaction Mixture**: To perform the test, a freshly prepared solution of ferrous sulfate is added to the solution suspected of containing nitrate ions. The solution should be acidic, typically achieved by adding dilute sulfuric acid (H2SO4).

3. **Reaction Mechanism**:
- The ferrous ions (Fe²⁺) react with the nitrate ions (NO3⁻) in the presence of protons (H⁺) from the acid.
- The reaction can be represented as:
\[ \text{Fe}^{2+} + \text{NO}_3^{-} + \text{H}^+ \rightarrow \text{Fe}^{3+} + \text{NO} + \text{H}_2\text{O} \]
- In this reaction, ferrous ions are oxidized to ferric ions (Fe³⁺), and nitrate ions are reduced to nitric oxide (NO).

4. **Formation of the Brown Ring Complex**:
- The nitric oxide (NO) produced in the reaction then reacts with the remaining ferrous ions to form a complex. The complex formed is:
\[ \text{[Fe(H}_2\text{O})_5\text{NO}]^{2+} \]
- This complex is responsible for the characteristic brown color of the ring observed at the interface of the two layers of the solution.

5. **Conclusion**: The brown ring formed in the test is due to the complex \(\text{[Fe(H}_2\text{O})_5\text{NO}]^{2+}\). Therefore, the correct answer to the question is option B: \(\text{[Fe(H}_2\text{O})_5\text{NO}]^{2+}\).