The Castner-Kellner cell process is an electrolytic method for the industrial production of sodium hydroxide, chlorine, and hydrogen from brine. Let's evaluate each statement: Option A: sodium hydroxide is prepared. In the Castner-Kellner cell, sodium ions are reduced at the mercury cathode to form sodium amalgam. This amalgam is then reacted with water in a separate decomposer unit to produce sodium hydroxide (NaOH) and hydrogen gas. So, this statement is correct.
Option B: brine solution is the electrolyte. The cell uses an aqueous solution of sodium chloride (NaCl), which is commonly known as brine, as the electrolyte. So, this statement is correct. Option C: mercury acts as anode and carbon rod acts as cathode. In the Castner-Kellner cell: Anode: Graphite (carbon) rods are used as the anode, where chloride ions are oxidized to chlorine gas ( 2Cl−(aq)⟶Cl2(g)+2e−). Cathode: A flowing layer of mercury acts as the cathode, where sodium ions are reduced to form sodium amalgam (Na+(aq)+e−⟶Na(. in Hg )). Therefore, the statement that mercury acts as the anode and carbon rod acts as the cathode is incorrect. The roles are reversed. Option D: chlorine gas liberates at anode. As mentioned above, at the graphite anode, chloride ions are oxidized to produce chlorine gas. So, this statement is correct. The incorrect statement is C . The final answer is C