Electrolysis of Brine

Summary: Electrolysis of brine (concentrated aqueous sodium chloride) is a key industrial process that produces hydrogen gas, chlorine gas, and sodium hydroxide solution — the foundational products of the chlor-alkali industry. Tags: electrolysis brine chlorine hydrogen sodium-hydroxide chlor-alkali igcse-chemistry electrochemistry Created: 2026-07-13

Electrolysis of brine refers to the electrolytic decomposition of concentrated aqueous sodium chloride (NaCl(aq)), a process central to the chlor-alkali industry. When an electric current is passed through brine using inert electrodes (typically graphite or titanium), competing ions migrate to the electrodes: Na⁺ and H⁺ cations move toward the cathode, while Cl⁻ and OH⁻ anions move toward the anode. At the cathode, H⁺ ions are preferentially discharged over Na⁺ ions — despite Na⁺ being present at far higher concentration — because hydrogen is lower in the reactivity series, meaning H⁺ is a much weaker reducing agent and gains electrons more readily; this produces hydrogen gas (H₂) and leaves Na⁺ and OH⁻ ions in solution, forming sodium hydroxide (NaOH). At the anode, chloride ions (Cl⁻) are discharged in preference to hydroxide ions (OH⁻) when the brine is concentrated, yielding chlorine gas (Cl₂); this selectivity arises because the high concentration of Cl⁻ shifts the competition in its favour, whereas in dilute sodium chloride solution oxygen gas (O₂) would form instead from the oxidation of OH⁻. The overall outcome makes this one of the most economically significant electrolysis reactions: chlorine for water treatment and PVC production, hydrogen for ammonia synthesis and fuel, and sodium hydroxide for soap, paper, and textile manufacturing.

Ions Present in Brine

Brine is concentrated aqueous sodium chloride. In solution, NaCl dissociates completely, and water itself undergoes slight self‑ionisation:

• From NaCl: Na⁺(aq) and Cl⁻(aq)
• From water: H⁺(aq) and OH⁻(aq) (present at very low concentration, ~10⁻⁷ mol dm⁻³ each)

Thus the four ions present in the electrolyte are: Na⁺, Cl⁻, H⁺, and OH⁻.

Electrode Reactions

At the Cathode (–)

The cathode is the negative electrode. Cations (Na⁺ and H⁺) are attracted to it. The ion that is lower in the reactivity series (i.e. the weaker reducing agent) is preferentially discharged:

2H⁺(aq) + 2e⁻ → H₂(g)

Hydrogen gas bubbles off at the cathode. Sodium ions are not discharged because sodium is far more reactive than hydrogen; Na⁺ remains in solution.

Test for product: A lighted splint gives a “squeaky pop” — confirming hydrogen gas.

At the Anode (+)

The anode is the positive electrode. Anions (Cl⁻ and OH⁻) are attracted to it. In concentrated NaCl solution, chloride ions are discharged preferentially:

2Cl⁻(aq) → Cl₂(g) + 2e⁻

Chlorine gas bubbles off at the anode. In dilute NaCl solution, OH⁻ would discharge instead, producing oxygen. The high concentration of Cl⁻ in brine is essential for this selectivity.

Test for product: Damp blue litmus paper turns red then is bleached white — confirming chlorine gas.

Overall Reaction

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

Sodium hydroxide (NaOH) forms and remains dissolved in the solution left behind.

Summary of Products

Concentrated vs. Dilute NaCl Electrolysis

This distinction is a common examination focus. The outcome at the anode depends on chloride ion concentration:

The cathode product (hydrogen) is the same in both cases, because H⁺ is always discharged in preference to Na⁺ regardless of concentration.

Industrial Importance: The Chlor‑Alkali Industry

The electrolysis of brine is the foundation of the chlor‑alkali industry, which produces three commodities of enormous economic value:

1. Chlorine (Cl₂) — used in water treatment, disinfectants, bleaches, and the manufacture of PVC (polyvinyl chloride) plastics.
2. Hydrogen (H₂) — used in ammonia production (Haber process), hydrogenation of oils (margarine manufacture), and increasingly as a clean fuel.
3. Sodium hydroxide (NaOH) — used in soap making, paper production, textile processing, and drain cleaners.

Industrial cells (membrane cells, diaphragm cells, or mercury cells) are designed to keep the products separate: chlorine and sodium hydroxide must not mix, as they react to form sodium chlorate(I) (NaClO, bleach), which would contaminate the desired products. Modern plants predominantly use membrane cells, which employ a cation‑exchange membrane to isolate the anolyte and catholyte compartments while permitting Na⁺ migration.

Key Exam Points

• Memorise the four ions present: Na⁺, Cl⁻, H⁺, OH⁻.
• Cathode product is always hydrogen (not sodium), because hydrogen is below sodium in the reactivity series.
• Anode product is chlorine only when brine is concentrated. If the question says “dilute sodium chloride”, the anode product is oxygen.
• The solution remaining after electrolysis of brine is sodium hydroxide (NaOH) — an alkali.
• Be able to write half‑equations for both electrodes.

Related Pages

• Electrolysis — foundational principles of electrolysis
• Electrolysis of Aqueous Solutions — general rules for predicting products
• Anode and Cathode — electrode definitions and conventions
• Electrolysis of Halide Compounds — halide ion discharge at the anode
• Reactivity Series — basis for preferential discharge at the cathode
• Chlor-Alkali Industry — industrial applications
• Haber Process — downstream use of hydrogen produced from brine electrolysis

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus, Topic 4.1: Electrolysis
• Gallagher, R. & Ingram, P., Complete Chemistry for Cambridge IGCSE, 3rd Edition, Oxford University Press, 2016
• Harwood, R. & Lodge, I., Cambridge IGCSE Chemistry Coursebook, 5th Edition, Cambridge University Press, 2021
• Clegg, A. et al., Cambridge IGCSE Chemistry Study and Revision Guide, Hodder Education, 2017
• Royal Society of Chemistry, “The Chlor‑Alkali Industry,” rsc.org

Common Misconceptions