↪ Sodium hydroxide is manufactured by the electrolysis of brine in which the electrolytic cell chosen in preference to mercury cathode cells is Diaphragm cell because it is relatively more efficient and no health hazard of mercury.

Principle

↪ Brine is an aqueous solution of NaCl

i.e., NaCl_(aq) → Na⁺_(aq) + Cl^-_(aq) 

↪ The following electrolytic reaction occurs in electrolysis of brine.

↪ At anode: Cl- is oxidized to chlorine gas as
2Cl^- + 2e^- → Cl_2(g)

↪ At cathode: Water molecules undergo reduction to hydrogen gas leaving OH- ion in the solution as,

2H₂O + 2e^- → H_2(g) + 2OH^-

↪ The overall cell reaction is

2NaCl_(aq) + 2H₂O_(l) → H_2(g) + Cl_2(g) + 2NaOH_(aq)

⁕ Flowsheet diagram of Manufacture of Sodium Hydroxide by Diaphragm Cell