In Castner-Kellner cell for production of sodium hydroxide :

Options

A. Brine is electrolyzed with Pt electrodes
B. Brine is electrolyzed using graphite electrodes
C. Molten sodium chloride is electrolysed
D. Sodium amalgam is formed at mercury cathode

Answer

To solve the question regarding the Castner-Kellner cell for the production of sodium hydroxide, we will analyze each of the given options step by step.

### Step-by-Step Solution:

1. **Understanding the Process**:
The Castner-Kellner process is an electrolytic method used to produce sodium hydroxide (NaOH) from sodium chloride (NaCl) solution, commonly referred to as brine.

2. **Electrolyte Used**:
The electrolyte in this process is a concentrated solution of sodium chloride (brine). This is the medium through which the electrolysis occurs.

3. **Electrodes**:
- The anode (positive electrode) is made of graphite.
- The cathode (negative electrode) is made of mercury.

4. **Electrolysis Reaction**:
When an electric current is passed through the brine solution:
- Sodium ions (Na⁺) migrate to the mercury cathode where they are discharged and form sodium amalgam (Na-Hg).
- Chloride ions (Cl⁻) migrate to the graphite anode where they are oxidized to produce chlorine gas (Cl₂).

5. **Formation of Sodium Amalgam**:
At the mercury cathode, sodium ions are reduced to form sodium amalgam:
\[ \text{Na}^+ + \text{Hg} \rightarrow \text{Na-Hg (sodium amalgam)} \]

6. **Chlorine Production**:
At the graphite anode, chlorine gas is produced:
\[ 2 \text{Cl}^- \rightarrow \text{Cl}_2 + 2 \text{e}^- \]

7. **Final Product**:
The sodium amalgam is then treated with water in a separate chamber (denuder) to produce sodium hydroxide:
\[ \text{Na-Hg} + \text{H}_2\text{O} \rightarrow \text{NaOH} + \text{Hg} \]

8. **Evaluating the Options**:
- **Option 1**: Brine is electrolyzed with platinum electrode - **Incorrect** (uses graphite and mercury).
- **Option 2**: Brine is electrolyzed using graphite electrode - **Partially Correct** (but mercury is also used).
- **Option 3**: Molten sodium chloride is electrolyzed - **Incorrect** (uses brine solution).
- **Option 4**: Sodium amalgam is formed at mercury cathode - **Correct**.

### Conclusion:
The correct answer is **Option 4: Sodium amalgam is formed at mercury cathode**.