During electrolysis of NaCl, Sodium and chlorine exists as cation and anion respectively.There is competition between H+ and Na+. H+ being smaller in size moves faster towards the cathode ( negative) and liberated as Hydrogen gas.
Out of Cl- and OH- , Cl- being smaller in size migrates faster and form chlorine gas.
For the process should be efficient, acidic pH is necessary so that chlorine does not combines with OH- ion. Low pH means more H+ ions and hence H+ will combine more effectively with OH- ion. And hence, Cl- liberates as Cl2 gas.
Hence, chlorine will liberate and NaOH will be formed more effectively.
Nowadays, many new types of electrolysis are carried out such as Diaphragm cell , Membrane cell and many other to improve the production of NaOH.
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