On commercial scale, sodium hydroxide is prepared by :

Options

A. Dow's process
B. Solvay process
C. Castner-Kellner cell
D. Hall-Heroult process

Explanation

To solve the question of how sodium hydroxide is prepared on a commercial scale, we will analyze the given processes step by step.

### Step-by-Step Solution:

1. **Identify the Processes**: The question provides four processes:
- A) DAO's process
- B) Solvay process
- C) Kastner-Kelner cell
- D) Hall-Heroult process

2. **Evaluate DAO's Process**:
- The DAO's process is primarily used for the extraction of magnesium from seawater.
- The reactions involved do not produce sodium hydroxide.
- **Conclusion**: This process is not used for the preparation of sodium hydroxide.

3. **Evaluate Solvay Process**:
- The Solvay process is used for the manufacture of sodium carbonate (Na2CO3) from sodium chloride (NaCl).
- The reactions involve the formation of sodium bicarbonate and do not yield sodium hydroxide.
- **Conclusion**: This process is also not used for the preparation of sodium hydroxide.

4. **Evaluate Kastner-Kelner Cell**:
- The Kastner-Kelner cell is specifically designed for the electrolysis of brine (aqueous sodium chloride solution).
- During this process, sodium ions (Na+) are reduced at the cathode to form sodium metal, and chloride ions (Cl-) are oxidized at the anode to produce chlorine gas (Cl2).
- The overall reaction results in the formation of sodium hydroxide (NaOH).
- **Conclusion**: This process is indeed used for the commercial preparation of sodium hydroxide.

5. **Evaluate Hall-Heroult Process**:
- The Hall-Heroult process is used for the extraction of aluminum from bauxite ore.
- The reactions involved produce aluminum and carbon monoxide, not sodium hydroxide.
- **Conclusion**: This process is not used for the preparation of sodium hydroxide.

6. **Final Conclusion**:
- Based on the evaluations, the correct answer for the commercial preparation of sodium hydroxide is **Option C: Kastner-Kelner cell**.