Sodium Hydroxide and Chlorine 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:

1. Castner-Kellner Cell (Mercury Process)
2. Nelson Diaphragm Cell
3. Membrane Cell

Properties and Uses of the Electrolysis Products

Electrolytic Cells for the Production of Sodium Hydroxide

1. 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.

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) → 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.

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) → 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.