Which of the following complexes is responsible for the brown colour of the ring formed in the ring test for the nitrates?

Options

A. `[Fe(H_(2)O)_(5)NO]^(2+)`
B. `[Fe(CN)_(5)NO]^(2-)`
C. `[Fe(NO_(2))_(6)]^(4-)`
D. `[Fe(H_(2)O)_(5)NO_(2)]^(+)`

Answer

To determine which complex is responsible for the brown color of the ring formed in the ring test for nitrates, we can follow these steps:

### Step-by-step Solution:

1. **Understanding the Ring Test for Nitrates**: The ring test is a qualitative analysis method used to detect the presence of nitrates in a solution. When a nitrate ion is present, it can react with certain metal ions to form a colored complex.

2. **Identifying the Key Reactants**: In the ring test, ferrous sulfate (FeSO4) is commonly used. When ferrous ions (Fe²⁺) are present, they can react with nitrate ions (NO3⁻) in the solution.

3. **Formation of the Brown Ring Complex**: When the ferrous sulfate solution is treated with concentrated sulfuric acid and then with a nitrate solution, a brown ring complex is formed. The reaction involves the formation of a complex ion.

4. **Determining the Complex**: The complex responsible for the brown color is [Fe(H2O)5(NO)]²⁺. This complex is formed when the ferrous ion coordinates with water molecules and a nitrosyl group (NO).

5. **Conclusion**: The brown color of the ring in the ring test for nitrates is due to the formation of the complex [Fe(H2O)5(NO)]²⁺, which is also known as the Brown-Ring Complex.

### Final Answer:
The complex responsible for the brown color of the ring formed in the ring test for nitrates is [Fe(H2O)5(NO)]²⁺.

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