Bromide and nitrates both give brown fumes with conc. `H_2SO_4`. Which of the following reagent will be used to distinguish between them?

Options

A. `K_2Cr_2O_7`
B. `BaCl_2`
C. `FeSO_4`
D. `K_4. [Fe(CN)_6]`

Answer

To distinguish between bromide and nitrate ions, we can use FeSO4 (Iron(II) sulfate) as a reagent. Here’s a step-by-step explanation of how to perform the test and the reactions involved:

### Step 1: Prepare the Samples
- Take two separate samples: one containing potassium bromide (KBr) and the other containing potassium nitrate (KNO3).

### Step 2: Add Concentrated Sulfuric Acid
- To both samples, carefully add concentrated sulfuric acid (H2SO4).
- **Observation:** Both samples will produce brown fumes. This is due to the formation of bromine (Br2) from bromides and nitrogen oxides (NOx) from nitrates.

### Step 3: Add Iron(II) Sulfate (FeSO4)
- To the sample containing potassium bromide, add a few drops of FeSO4.
- **Reaction:**
\[ 2 \text{KBr} + \text{FeSO}_4 \rightarrow \text{K}_2\text{SO}_4 + \text{FeBr}_2 \]
- **Observation:** The solution will remain yellowish, indicating the presence of bromide ions.

### Step 4: Test the Nitrate Sample
- Now, take the sample containing potassium nitrate and add concentrated sulfuric acid followed by FeSO4.
- **Reaction:**
\[ 6 \text{FeSO}_4 + 3 \text{H}_2\text{SO}_4 + 2 \text{HNO}_3 \rightarrow 3 \text{Fe}_2(\text{SO}_4)_3 + 4 \text{H}_2\text{O} + 2 \text{NO} \]
- The NO produced can react with FeSO4 to form a brown ring complex:
\[ \text{Fe}^{2+} + \text{NO} + \text{H}_2\text{O} \rightarrow \text{[Fe(NO)(H}_2\text{O)}_5]^{2+} + \text{SO}_4^{2-} \]
- **Observation:** A brown ring will form at the interface of the two layers, indicating the presence of nitrate ions.

### Conclusion
- The presence of a brown ring confirms the presence of nitrate ions, while a yellowish solution indicates bromide ions. Thus, FeSO4 is the reagent used to distinguish between bromides and nitrates.