Castner-Kellner cell is used for the preparation of

Solution:

The Castner-Kellner cell is an industrial method used for the electrolysis of brine (aqueous sodium chloride, NaCl solution) to produce sodium hydroxide (NaOH), chlorine gas (Cl₂), and hydrogen gas (H₂). The overall reaction is:

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Therefore, the Castner-Kellner cell is used for the preparation of NaOH.