How is NaOH or KOH manufactured?

The normal oxides of the alkali metals react with water forming the hydroxides, which are the strongest bases known. The special methods of manufacture of hydroxides are as follows:

1.         Gossage’s process or Sodalime Process : Slaked lime (Calcium Hydroxide) and dilute Sodium Carbonate solution are reacted in large tanks when the following reaction occurs.

                        Ca(OH)₂ + Na₂CO₃ >< CaCO₃¯ + 2NaOH

            Although this reaction is reversible, most of the sodium carbonate is converted to the hydroxide because calcium carbonate (insoluble) precipitates out. The sodium hydroxide is decanted from the sludge and concentrated (or evaporated) to dryness.

2.         Electrolysis of NaCI

This process is carried out either in Nelson cell (Diaphragm cell) or Castner – Kellner cell (mercury cathode process).

            In Nelson cell following reaction occur

                        NaCI >< Na⁺ + CI^–

            At cathode      Na⁺ + e^– ® Na

                                    2Na + 2H₂O ® 2NaOH + H₂­

            At anode         2CI^– – 2e^– ® CI₂­

In Castener – Kellner cell the sodium ions Na+ discharge on cathode (Hg) and form sodium amalgam; which reacts with water to form sodium hydroxide :

                        NaCI >< Na⁺ + CI^–

            At cathode      Na⁺ + e^– ® Na

                                    Na + Hg ® Na(Hg)

                                    2Na(Hg) + 2H₂O ® 2NaOH + H₂ + 2Hg­

            At anode         2CI^– – 2e^– ® CI₂­

The sodium hydroxide so obtained may contain the impurities of NaCI, Na₂SO₄ and Na₂CO₃. To obtain pure sodium hydroxide impure sample of NaOH is dissolved in alcohol and solution is filtered. The filterate on evaporation gives the pure flakes of NaOH.