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Sodium Hydroxide Production by Electrolysis

Key Concepts

  • Sodium hydroxide, NaOH, is also known as lye or caustic soda.

  • Sodium hydroxide is a commonly used base.

  • Electrolysis of concentrated sodium chloride solutions (brine) produces chlorine gas, hydrogen gas and aqueous sodium hydroxide.

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

  • Cl2(g) is produced at the anode (positive electrode).

  • H2(g) and NaOH(aq) are produced at the cathode (negative electrode).

  • Three types of electrolytic cell are used to produce sodium hydroxide from brine:

  1. Castner-Kellner Cell (Mercury Process)

  2. Nelson Diaphragm Cell

  3. Membrane Cell

Properties and Uses of the Electrolysis Products

sodium hydroxide
NaOH
hydrogen
H2
chlorine
Cl2
Properties
  • strong base
  • water soluble
  • dissolving NaOH in water is an exothermic reaction
  • deliquescent : absorbs moisture from the air
  • absorbs carbon dioxide from the air
  • flammable gas
  • poisonous yellow gas

Uses
  • domestic oven and drain cleaner
  • pulp and paper production
  • manufacture of rayon and other synthetic fibres
  • soap and detergent production
  • extraction of alumina from bauxite (Bayer Process)
  • removal of sulfur dioxide and hydrogen sulfide in petroleum refining
  • margarine
  • nylon
  • HCl (gas and acid)
  • PVC
  • dry-cleaning solvent
  • HCl (gas and acid)
  • bleaches
  • weed-killers
  • paints and dyes
  • anti-bacterial agent in water

Electrolytic Cells for the Production of Sodium Hydroxide

  1. Castner-Kellner Cell (Mercury Process)

    • Anode (positive electrode): titanium
      Anode reaction (oxidation):
      2Cl-(aq) -----> Cl2(g) + 2e

    • Cathode (negative electrode): mercury flowing along bottom of cell
      Cathode reaction (reduction):
      Na+(aq) + e -----> Na(s)

    • Na(s) dissolves in the liquid mercury to form an amalgam which is removed to the decomposer.

    • In the decomposer the amalgam reacts with water to form sodium hydroxide, hydrogen gas and mercury.
      2Na/Hg + 2H2O(l) -----> 2Na+ + 2OH- + H2(g) + 2Hg(l)
      Sodium hydroxide solution and hydrogen gas are collected.
      Mercury is recycled through the electrolytic cell.

    • Mercury is toxic so care must be taken to prevent mercury losses.

  2. Nelson Diaphragm Cell

    • Porous diaphragm of asbestos or metal oxide with polymer separates anode and cathode compartments.
    • Diaphragm prevents hydroxide ions entering anode compartment and prevents chloride ions entering cathode compartment.

    • Saturated brine enters anode compartment where chlorine gas is produced.
    • Anode (positive electrode):
      carbon (graphite) or titanium coated with Ru-Ti oxide.
    • Anode reaction (oxidation):
      2Cl-(aq) -----> Cl2(g) + 2e

    • Cathode (negative electrode):
      steel mesh
    • Cathode reaction (reduction):
      2H2O(l) + 2e -----> H2(g) + 2OH-(aq)

    • Na+ migrates across diaphragm to cathode compartment combining with OH- to form NaOH.
    • Overall cell reaction (showing Na+ spectator ions):
      2H2O(l) + 2Cl-(aq) + 2Na+(aq) -----> 2Na+(aq) + 2OH-(aq) + H2(g) + Cl2(g)

    • Product contains sodium chloride and sodium hydroxide. NaOH(s) can be crystallised out.

  3. Membrane Cell

    • Favoured method for producing sodium hydroxide as there is no hazardous waste, very pure sodium hydroxide is produced and it has the lowest energy requirements.

    • Ion-exchange membrane selectively allows Na+ and water to flow to the cathode compartment but prevents products from moving between compartments.

    • Saturated brine enters the anode compartment where chlorine gas is formed.
    • Anode (positive electrode):
      titanium
    • Anode reaction (oxidation):
      2Cl-(aq) -----> Cl2(g) + 2e

    • Cathode (negative electrode):
      nickel
    • Cathode reaction (reduction):
      2H2O(l) + 2e -----> H2(g) + 2OH-(aq)

    • Na+ migrates across the membrane to cathode compartment combining with OH- to form NaOH.
    • Overall cell reaction (showing Na+ spectator ions):
      2H2O(l) + 2Cl-(aq) + 2Na+(aq) -----> 2Na+(aq) + 2OH-(aq) + H2(g) + Cl2(g)

    • Product is concentrated sodium hydroxide.


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