The Chlor-Alkali Process: GCSE Chemistry

What is brine and why is it electrolysed?

Brine is a concentrated aqueous solution of sodium chloride (NaCl). Dissolved NaCl dissociates fully into Na⁺(aq) and Cl⁻(aq) ions; water also provides H⁺(aq) and OH⁻(aq) ions. When a direct current is passed through the solution using inert electrodes, selective discharge of these ions at each electrode produces valuable chemical products.

The chlor-alkali process is one of the largest-scale industrial electrolysis operations in the world. The United Kingdom produces hundreds of thousands of tonnes of chlorine per year by this method.

What happens at the cathode (negative electrode)?

The cathode attracts positive ions. Both Na⁺ and H⁺ ions are present in solution, but Na⁺ is much harder to discharge (sodium is a very reactive metal; its ion has a strong attraction for electrons but would react violently with water if produced). At the concentrations used, H⁺ ions from the ionisation of water are preferentially discharged:

Half-equation at the cathode:

2H⁺(aq) + 2e⁻ → H₂(g)

Hydrogen gas is produced as colourless bubbles at the cathode. It is collected and used commercially (see applications below).

What happens at the anode (positive electrode)?

The anode attracts negative ions. Both Cl⁻ and OH⁻ ions are present, but in concentrated brine the concentration of Cl⁻ is high enough that chloride ions are preferentially discharged over hydroxide ions:

Half-equation at the anode:

2Cl⁻(aq) → Cl₂(g) + 2e⁻

Chlorine gas is produced as yellow-green bubbles at the anode. Note: if the brine is dilute, OH⁻ would be discharged instead, producing oxygen — so concentration matters for product selection.

What remains in solution?

After Cl⁻ and H⁺ ions are removed at the electrodes, the Na⁺ and OH⁻ ions that remain combine to give sodium hydroxide solution (NaOH(aq)), also called caustic soda or lye. This is drained off and concentrated for sale.

Summary table: chlor-alkali process products

What are the industrial applications of chlorine?

Chlorine is one of the most widely used industrial chemicals:

• PVC (polyvinyl chloride): chlorine is combined with ethylene to make vinyl chloride monomer, which is then polymerised into PVC — used in pipes, window frames, electrical cable insulation, and medical equipment.
• Bleach: chlorine reacts with NaOH solution (both chlor-alkali products) to form sodium hypochlorite (NaClO), the active ingredient in household bleach. It kills bacteria and fungi and is used to whiten paper and textiles.
• Water purification: small controlled doses of chlorine or its compounds are added to drinking water and swimming pools to kill pathogens.
• Pharmaceuticals: chlorine is used to synthesise many medicines, including anaesthetics.

What are the industrial applications of sodium hydroxide?

Sodium hydroxide is a strong alkali with numerous uses:

• Soap and detergent manufacture: NaOH reacts with fats and oils in the saponification reaction, producing soap (sodium salts of fatty acids) and glycerol.
• Paper manufacture: NaOH (in the kraft process) dissolves lignin from wood pulp, leaving cellulose fibres for paper production.
• Aluminium production: NaOH dissolves aluminium ore (bauxite) to purify it before electrolysis.
• Oven and drain cleaners: the high pH of NaOH hydrolyses fats and proteins in blockages.

Frequently asked questions

Why is hydrogen gas produced at the cathode rather than sodium metal?

Sodium metal would react explosively with water. At the concentrations used in the chlor-alkali process, the energy required to discharge sodium ions (a very reactive metal ion) is much greater than that required to discharge hydrogen ions from water. Water spontaneously provides small concentrations of H⁺ ions (from H₂O ⇌ H⁺ + OH⁻), and these are selectively reduced at the cathode. The Na⁺ ions remain in solution and pair with OH⁻ to form the sodium hydroxide by-product.

What changes if dilute brine is used instead of concentrated brine?

In dilute brine, the concentration of Cl⁻ ions is low relative to OH⁻ ions. At the anode, OH⁻ ions are preferentially discharged rather than Cl⁻, and oxygen gas is produced instead of chlorine (4OH⁻ → 2H₂O + O₂ + 4e⁻). The cathode product (hydrogen) remains the same. This is why the industrial process uses concentrated brine — maintaining high Cl⁻ concentration ensures chlorine, not oxygen, is produced at the anode.

How is the chlor-alkali process different from the electrolysis of molten sodium chloride?

Electrolyis of molten NaCl produces sodium metal at the cathode (Na⁺ + e⁻ → Na) and chlorine at the anode — this is how sodium metal is manufactured industrially (the Downs cell). In the chlor-alkali process, NaCl is dissolved in water (aqueous), so water provides H⁺ ions that compete with Na⁺ at the cathode; H₂ is produced rather than sodium metal, and NaOH forms in solution. The presence of water completely changes the cathode product.

What are the environmental and safety concerns with chlorine production?

Chlorine gas is toxic — it was used as a chemical weapon in the First World War. Industrial plants must manage risks of leaks carefully, with neutralisation systems (alkaline scrubbers) and extensive monitoring. Older chlor-alkali plants used mercury cells (which produced purer NaOH) but mercury contamination of waterways was a serious problem. Modern plants use membrane cells or diaphragm cells, which keep the chlorine and NaOH streams physically separated and avoid mercury entirely. The environmental benefits of chlorine products (safe water, reduced infection rates from PVC medical equipment) are balanced against these hazards in lifecycle assessments.

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