The chlor-alkali industry 

The chlor-alkali industry is a large worldwide multimillion industry which produces compounds that are essential to human existence. The industry is based on the electrolysis of sodium chloride solutions.

6.1 Summary

summary 1 jhgjaguyda

Chlor-Alkali industry involves the electrolysis of a sodium chloride solution which produces:

  • chlorine gas;
  • hydrogen gas and
  • sodium hydroxide (aq).

6.2 Definitions and terminology

The processes involved are electrolysis, oxidation and reduction.

  • is an endothermic reaction
  • involves a process during which electric energy is converted to chemical energy or brings about a chemical change
  • involves a non-spontaneous redox reaction.


  • takes place when electrons are given off
  • at the anode.


  • takes place when electrons are received
  • at the cathode.


  • Corrosive: Substance that damages and destroys substances (including living matter) it comes into contact with (touches).
  • Leaching: Movement of a substance (like NaOH (aq), pesticides or fertilisers) that are carried by water, down through soil (ground) to the underground water table. The water table is the level below which the ground is saturated with water.
  • Carbon footprint: The amount of carbon dioxide and other greenhouse gases released into the atmosphere due to the activities of a person, organisation or process.
  • Brine: A concentrated solution of NaCℓ(aq)
    warning for corrosive substanes jhajhdua
    You must know how to:
    • Identify the processes involved
    • Give a description of the concept or term
    • Give the correct definition
    • Describe the process and chemicals involved/ produced
    • State uses of the products
    • State the environmental issues

6.3 The Chlor-alkali industry – reactants and products

  • Sodium chloride solution, NaCℓ(aq)
  • Produced by evaporation from sea water, salt lakes


   Chlorine gas
 Hydrogen gas
Sodium hydroxide 
(caustic soda)
  •  Yellow-green gas
  • Poisonous: Do not inhale.
  •  Colourless gas
  • Explosive: No open flames or electronic sparks nearby.
  • Colourless  solution
  • Highly corrosive
  • Prevent leaching into the ground eye contact.

Reactant for production of:

  • hydrochloric acid, HCℓ(aq);
  • extraction of Cu, Au, Ti;
  • paint;
  • PVC plastics;
  • insecticides and weed killers;
  • bleaching agents.

Used to:

  •  disinfect water.

Reactant for production of:

  • hydrochloric acid, HCℓ(aq);
  • ammonia, NH3(g),
  • H2O2(g) (hydrogen peroxide);
  • nylon;
  • rocket fuel;
  • margarine from plant oils (hydrogenation of unsaturated hydrocarbons).


Reactant for production of:

  • soap and bleaching agents;
  • paper;
  • textiles;
  • oven cleaners.

Used to:

  • treat waste water (sewage water).

The membrane cell is preferred because the:

  • components of the cell are non-toxic;
  • products are more pure and
  • production costs are relatively low.
  • Sodium chloride (table sale) NaCℓ(s) is an ionic compound and is very soluble in water (a polar solvent).
  • NaCℓ (s) dissociates in an aqueous solution – the ions in the ionic crystal lattice separate.
  • NaCℓ(s) H2O (ℓ) Na+ (aq) + Cℓ (aq)
  • A sodium chloride solution, NaCℓ (aq), is known as brine.

6.4 The chlor-alkali industry – industrial process

The chlor-alkali industry products can be produced by using a:

  • membrane cell
  • mercury cell
  • diaphragm cell
  Membrane Cell  Diaphragm Cell  Mercury Cell
Purity and yield of Cℓ2 (g) produced 
  • Produces  relatively pure Cℓ2(g) that is mixed with very small amount of O2(g).
  • Produces  Cℓ2(g) that is mixed with O2(g) so the product is less pure.
  •  Low yield of Cℓ2(g) and of NaOH(aq).
 Purity of NaOH (aq) produced  High
  •  Medium
  • NaOH (aq) is mixed with NaCℓ (aq)
 Energy consumption (Affects costs) Lowest  Relatively low Very high
 Environmental impact
  • Lowest
  • Non-toxic, reusable components - low setup costs.
  • Lowest energy consumption therefore small carbon footprint.
  •  High
  • Diaphragm is made from asbestos.
  • Highest
  • Mercury reacts with the NaCℓ (aq) and leaches into the groundwater.
 Health risks
  • No potential  health risks / environmental risks due to components.
  • H2(g) is explosive and Cℓ2(g) is poisonous.
  • Diaphragm is made from asbestos – health risk for workers – asbestosis and cancer.
  • H2(g) is explosive and Cℓ2(g) is poisonous.
  • Mercury is toxic.
  • Risks due to contamination of groundwater. (Fish, animals and humans use the water.
  • H2(g) is explosive and Cℓ2(g) is poisonous.


  • Very few companies still use mercury cells but many still use diaphragm cells. When new industrial sites are set up today, membrane cells are mostly used.

the membrane cell jkhgjyhgad

Related Items

e.g. Worked example 1
1. Chlorine is produced industrially by electrolysis in an electrolytic cell and can be represented as follows:

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

2H2 + 2e–   →  H2  + 2OH–
II   Cℓ2 2e–  → 2Cℓ–     
III  2Cℓ–      → Cℓ2 + 2e–
IV  H2 + 2OH–  → 2H2O + 2e–

The correct statement(s) is/are:

  1. I and III
  2. I only
  3. II and IV
  4. III only

2. Chlorine is a poisonous gas commonly used as a bleaching agent. Chlorine is produced in industry by _____________.


  1. A
  2. Electrolysis of brine

Activity 1

The electrolysis of saturated sodium chloride can be illustrated as follows:
Hydrogen and chlorine bubble off at the electrodes.

activity 1 jygjjyauydg

  1. Give an equation to show how chlorine bubbles are formed at the electrode. (2)
  2.  At which electrode are the chlorine bubbles formed? (1)
  3. At which electrode does hydrogen gas form? (1)
  4. What is the name of the product which leaves the cell? (1)
  5. Give TWO applications of the product formed in 2.4. (2)
  6. What purpose does the porous diaphragm serve? (2)
  7. The chlorine gas produced, dissolves in water to form chlorine water. Write down a balanced equation for the reaction that takes place. (2) [11]


  1. 2Cℓ(aq) → Cℓ2(g) + 2e  (2)
  2. Anode  (1)
  3. Cathode  (1)
  4. Sodium hydroxide  (1)
  5. Making soap and detergents; paper; rayon and other fibres; dyeing textiles  (any 2) (2)
  6. Stops chlorine passing through; helps to separate sodium hydroxide from NaCℓ(aq).  (2)
  7. Cℓ2 + H2O → HCℓ + HOCℓ (2)  [11]

e.g.      Worked example 2
The diagram below shows a type of membrane cell used in the chlor-alkali industry.

worked example 2 kjgauygudyag

  1. Name the gases A and B
  2. Why is the membrane called a cationic membrane?
  3. Write down the half-reaction that takes place at electrode N.
  4. Apart from its use in household products, name ONE industrial use of chlorine.
  5. Explain why this electrolytic process cannot be done in one large container without a membrane.


  1. A: Chlorine
    B: Hydrogen
  2. Allows only the positive ions (cations) to pass through it.
  3. 2H2O + 2e → H2 + 2OH
  4. Manufacture of PVC, paper, drugs, etc
    Disinfectant for water
  5. In a single pot the chlorine will react with water to form chlorine water/or the chlorine will react with the OH– ions to form bleach OR the products formed will be contaminated

Activity 2

The simplified diagram of a cell used in the chlor-alkali industry is shown below.

activity 2 jhgauygduyag

  1. Write down the CHEMICAL FORMULA of brine. (1)
  2. At which electrode, X or Y is chlorine gas formed? (1)
  3. Write down a half-reaction that explains the formation of hydrogen gas at one of the electrodes. (4)
  4. The purity of the sodium hydroxide produced in the chlor-alkali industry depends on the extent to which it is separated from the chlorine gas produced by this cell. Briefly describe how chlorine gas and sodium hydroxide are prevented from mixing in this cell. (2)
  5. Apart from the advantages and disadvantages of products produced, write down for this process:
    5.1 ONE positive impact on humans. (1)
    5.2 ONE negative impact on humans (1)  [10]


  1.  NaCℓ (aq)  (1)
  2.  Y  (1)
  3. 2H2O + 2e →H2+2OH (4)
  4. The membrane prevents chloride ions from moving to the cathode, only allows positive ions through. (2)
  5. 5.1 Job creation resulting in more people having a better life.  (1)
    5.2 Uses huge amounts of electricity resulting in load shedding. 
    Chemical plant uses a lot of space that could have been used for housing/gardens, etc.  (any one) (1)
Last modified on Wednesday, 29 September 2021 12:49