Electrolysis of brine

• Electrolysis of brine


  Produces chlorine, hydrogen and sodium hydroxide
• Electrolysis of brine


  1. Chlorine is found at the anode

  2. Hydrogen is found at the cathode

  3. Sodium is found in the solution surrounding the cathode
• Equation at the anode

  2Cl- (aq) --> Cl2(g) + 2e-
• Equation at the cathode

  2H+ (aq) + 2e- --> H2 (g)
• Chlorine gas reaction


  Oxidation (each chloride ion has lost an electron)
• Hydrogen gas reaction


  Reduction (Hydrogen ion has gained an electron)
• Caustic soda


  Sodium hydroxide
• Specialised cells used in the electrolysis of brine


  • Diaphragm Cell
  • Membrane Cell
• Diaphragm cell


  1. Divides the cell

  2. Allows brine to pass from one side to the other

  3. Prevents chlorine and hydrogen gaseous products from passing through

  4. Sodium hydroxide formed is mixed with brine(Sodium chloride)
• Membrane cell


  1. Membrane divides the cell

  2. Allows positive sodium ions to pass from one side to the other

  3. Prevents negative chloride ions passing from anode to cathode

  4. Prevents chlorine and hydrogen gaseous products from passing through

  5. Sodium hydroxide formed is not mixed brine(sodium chloride)
• Diaphragm cell


  • Cheaper to construct
  • Diaphragm needs replacing regularly
  • Purity of sodium hydroxide is lower
  • Uses slightly more energy per tonne of chlorine produced
• Membrane cell


  • More expensive to construct
  • Membrane needs little maintenance
  • Purity of sodium hydroxide is high
  • Uses slightly less energy per tonne of chlorine produced
• Diaphragm

  • Divides the cell
  • Allows brine to pass from one side to the other
  • Prevents chlorine and hydrogen gaseous products from passing through
• Membrane cell

  • Divides the cell
  • Allows positive sodium ions to pass from one side to the other
  • Prevents negative chloride ions passing from anode to cathode
  • Prevents chlorine and hydrogen gaseous products from passing through
• Chloride ions can pass through the diaphragm


  But not the membrane
• Sodium hydroxide from the diaphragm cell


  Is contaminated with sodium chloride
• To allow the ions to move


  The ions can then carry the electrical current during electrolysis
• Large amounts of electricity required for electrolysis
• Graphites anodes need to be replaced regularly
• Hydrogen is formed at the cathode
• Chlorine is formed at the anode
• Purification and extraction steps


  1. Bayer process is used

  2. Bauxite is ground up

  3. Conditions are concentrated alkali, under pressure, high temperature
• Alumina


  • Is amphoteric
  • Reacts with alkali, basic oxides do not react in alkali
  • Filtration to remove soluble impurities
• Brine


  • Sodium chloride solution
  • Ions are free to move in solution and carry electric charge
  • Negative chloride ions are attracted to the positive anode
  • Positive hydrogen ions are attracted to the negative cathode
• Chlorine gas is formed at the anode
• Hydrogen gas is formed at the cathode
• Membrane


  • Keeps chlorine gas separate from hydrogen gas
  • Chlorine will not be able to react with hydrogen
  • Allows Na+ to pass through
  • Does not allow Cl- to pass through
• Desired product forms at cathode in electrolysis of alumina


  But at anode in electrolysis of brine
• Alumina is an oxide


  But brine is a chloride
• Alumina is a molten liquid


  But brine is an aqueous solution
• Hydrogen is formed at the cathode, chlorine is formed at the anode and sodium hydroxide is left behind
• Brine is sodium chloride solution
• Ions are free to move in solution and carry electric charge
• Negative chloride ions are attracted to the positive anode
• Positive hydrogen ions are attracted to the negative cathode
• Hydrogen can react with chlorine to form HCl
• Diaphragm separates sodium hydroxide from chlorine so that they do not react together
• Membrane


  • Keeps chlorine gas separate from hydrogen gas
  • Chlorine will not be able to react with hydrogen
  • Allows Na+
  • The sodium hydroxide solution is not contaminated with chloride ions
  • This gives sodium hydroxide solution that it has higher purity
• Large amount of electricity required for electrolysis
• sodium chloride is melted. current passed through the electrolyte. chloride ions travel to anode. chloride ions lose electrons.