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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) -----> H_2(g) + Cl_2(g) + 2NaOH_(aq)
Cl_2(g) is produced at the anode (positive electrode).
H_2(g) and NaOH_(aq) are produced at the cathode (negative electrode).
Three types of electrolytic cell are used to produce sodium hydroxide from brine:

Castner-Kellner Cell (Mercury Process)
Nelson Diaphragm Cell
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

Castner-Kellner Cell (Mercury Process)
- Anode (positive electrode): titanium
  Anode reaction (oxidation):
  2Cl^-_(aq) -----> Cl_2(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^- + H_2(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.
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) -----> Cl_2(g) + 2e
- Cathode (negative electrode):
  steel mesh
- Cathode reaction (reduction):
  2H₂O_(l) + 2e -----> H_2(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) + H_2(g) + Cl_2(g)
- Product contains sodium chloride and sodium hydroxide. NaOH_(s) can be crystallised out.
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) -----> Cl_2(g) + 2e
- Cathode (negative electrode):
  nickel
- Cathode reaction (reduction):
  2H₂O_(l) + 2e -----> H_2(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) + H_2(g) + Cl_2(g)
- Product is concentrated sodium hydroxide.