Presence of peroxide in ethers of old stock can be tested by first treating them with `FeSO_4` solution and then adding an aqueous solution of ………&from red colour solution

Options

A. `SnCl_2`
B. `HgCl_2`
C. `Kl`
D. KCNS

Answer

To determine the presence of peroxide in ethers of old stock, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Formation of Peroxides:**
- Ethers, when exposed to air over time, can form peroxides. For example, diethyl ether (C2H5) can form diethyl ether peroxide (C2H5O-O-C2H5).

2. **Treatment with FeSO4:**
- The first step in the test is to treat the ether with a solution of ferrous sulfate (FeSO4). The ferrous ions (Fe²⁺) in FeSO4 react with the ether peroxide.

3. **Oxidation of Ferrous Ion:**
- During this reaction, the ferrous ions (Fe²⁺) are oxidized to ferric ions (Fe³⁺). This is a key step because it indicates that peroxides are present.

4. **Addition of Aqueous Solution of KCN:**
- After the treatment with FeSO4, an aqueous solution of potassium thiocyanate (KCN) is added. This solution contains thiocyanate ions (CN⁻).

5. **Formation of Red-Colored Complex:**
- The ferric ions (Fe³⁺) react with the thiocyanate ions (CN⁻) to form a complex ion, [Fe(CN)₆]³⁻, which is red in color. This red-colored solution confirms the presence of peroxides in the ether.

### Final Answer:
The aqueous solution that should be added after treating the ether with FeSO4 is **potassium thiocyanate (KCN)**.