Brown ring test is used for the identification of

Options

A. `PO_4^(-3) `
B. `NO_3^(-) `
C. `SO_4^(2-) `
D. `HCO_3^(-)`

Answer

### Step-by-Step Solution for the Question: "Brown ring test is used for the identification of"

1. **Understanding the Brown Ring Test**: The brown ring test is a qualitative analytical test used to detect the presence of nitrate ions (NO3^-) in a solution.

2. **Preparation of the Test**:
- Take a solution that is suspected to contain nitrate ions.
- Add iron(II) sulfate (FeSO4) to this solution. The iron(II) ions will react with the nitrate if present.

3. **Addition of Concentrated Sulfuric Acid**:
- Slowly add concentrated sulfuric acid (H2SO4) to the mixture. It is important to add the acid carefully so that it forms a separate layer below the aqueous solution containing the nitrate and iron(II) sulfate.

4. **Observation of the Brown Ring**:
- If nitrate ions are present in the solution, a brown ring will form at the interface between the aqueous layer and the sulfuric acid layer. This is due to the formation of a complex between iron(II) ions and the reduced form of nitrate.

5. **Chemical Reaction**:
- The overall reaction can be summarized as follows:
\[ 3 \text{FeSO}_4 + 2 \text{HNO}_3 + 6 \text{H}_2\text{SO}_4 \rightarrow 3 \text{Fe}_2(\text{SO}_4)_3 + 2 \text{NO} + 4 \text{H}_2\text{O} \]
- In this reaction, nitrate (NO3^-) is reduced to nitric oxide (NO), and iron(II) is oxidized to iron(III).

6. **Conclusion**:
- The formation of the brown ring indicates the presence of nitrate ions in the solution, confirming that the brown ring test is specifically used for the identification of nitrate ions.

### Final Answer:
The brown ring test is used for the identification of **nitrate ions (NO3^-)**.

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