Uses of Sulfur Compounds

Summary: Sulfuric acid is the most manufactured chemical: used in fertilisers, detergents, paints, plastics, metal processing, and as an electrolyte in car batteries. Tags: igcse chemistry environmental-chem Created: 2026-07-14 Last Updated: 2026-07-16

Sulfuric Acid: The World’s Most Important Industrial Chemical

Sulfuric acid (H₂SO₄) is the most manufactured chemical in the world, with global production exceeding 200 million tonnes per year. A country’s sulfuric acid production is often cited as a measure of its industrial development — the more H₂SO₄ a country produces, the more industrialised it typically is.

Sulfuric acid is not an end-product in itself for most applications; rather, it is a feedstock chemical — it is used to produce other chemicals and materials. Its importance stems from its unique combination of properties: it is a strong acid, a dehydrating agent, and an oxidising agent, and it is relatively cheap to manufacture from widely available raw materials.

The Contact Process (see Sulfur and Compounds) makes H₂SO₄ production economically viable at a massive scale.

Major Uses of Sulfuric Acid

Fertilisers (Largest Use — About 60% of Production)

The single largest consumer of sulfuric acid is the fertiliser industry. Sulfuric acid is used to manufacture several key fertilisers:

Ammonium sulfate, (NH₄)₂SO₄:

2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

Ammonium sulfate provides both nitrogen (from the ammonium ion) and sulfur (from the sulfate ion) to plants. Nitrogen is essential for protein synthesis and leaf growth; sulfur is needed for amino acid and enzyme formation.

Superphosphate fertilisers: Phosphate rock (calcium phosphate, Ca₃(PO₄)₂) is insoluble in water and cannot be absorbed by plants directly. Treating it with sulfuric acid converts it into soluble calcium dihydrogen phosphate (superphosphate):

Ca₃(PO₄)₂(s) + 2H₂SO₄(aq) → Ca(H₂PO₄)₂(aq) + 2CaSO₄(s)

The mixture of Ca(H₂PO₄)₂ and CaSO₄ is known as single superphosphate (SSP). This provides phosphorus, which is vital for root development and energy transfer (ATP) in plants.

Triple superphosphate (TSP) uses phosphoric acid (itself made from H₂SO₄) instead:

Ca₃(PO₄)₂ + 4H₃PO₄ → 3Ca(H₂PO₄)₂

Detergents and Soaps

Sulfuric acid is used in the sulfonation of organic compounds — a process that attaches sulfonic acid groups (−SO₃H) to long hydrocarbon chains. This produces sulfonates, which are the active cleaning agents in many household detergents, shampoos, and industrial cleaners. The sulfonate group makes one end of the molecule hydrophilic (water-attracting) while the hydrocarbon chain is hydrophobic (water-repelling), giving detergents their cleaning action.

Paints and Pigments

Sulfuric acid is essential in the production of titanium dioxide (TiO₂) — the most widely used white pigment in paints, coatings, plastics, paper, and sunscreen. TiO₂ is produced from ilmenite ore (FeTiO₃) using the sulfate process, where the ore is digested with concentrated sulfuric acid.

Titanium dioxide is valued for its:

• High opacity (covering power)
• Bright whiteness
• Non-toxicity
• UV-blocking properties

Plastics and Synthetic Fibres

H₂SO₄ is used as a catalyst and reactant in manufacturing:

• Nylon: Caprolactam (the monomer for nylon-6) is produced using sulfuric acid as a catalyst
• Rayon (viscose): Regenerated cellulose fibre is produced by treating wood pulp with sulfuric acid, which regenerates cellulose from a cellulose xanthate solution
• Various plastics: Sulfuric acid is involved in the production of phenol-formaldehyde resins, polycarbonates, and other polymers

Metal Processing

Pickling steel: Before steel can be galvanised, plated, or painted, surface rust and mill scale (iron oxides) must be removed. The steel is dipped in a bath of hot sulfuric acid (pickling):

FeO(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂O(l)
Fe₂O₃(s) + 3H₂SO₄(aq) → Fe₂(SO₄)₃(aq) + 3H₂O(l)

This removes surface oxides and leaves clean metal ready for further processing.

Extraction of metals from ores: Sulfuric acid is used to leach metals from low-grade ores. For example, copper can be extracted from copper oxide ores by leaching with dilute H₂SO₄:

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

The copper is then recovered from the CuSO₄ solution by electrolysis or displacement with scrap iron.

Lead-Acid Car Batteries

The lead-acid battery is the most common rechargeable battery, used in virtually all petrol and diesel vehicles to start the engine. Sulfuric acid (~35% concentration) serves as the electrolyte.

Discharge reaction (producing electricity): At the negative electrode (anode): Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻

At the positive electrode (cathode): PbO₂(s) + 4H⁺(aq) + SO₄²⁻(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l)

Overall discharge:

Pb(s) + PbO₂(s) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l)

During discharge, H₂SO₄ is consumed and water is produced, diluting the acid. The battery can be recharged by reversing the current, regenerating Pb, PbO₂, and H₂SO₄.

The density of the sulfuric acid electrolyte can be measured with a hydrometer to determine the state of charge of the battery — a fully charged battery has a higher acid concentration (and therefore higher density).

Uses of Sulfur Dioxide (SO₂)

Food Preservative

Sulfur dioxide is a widely used food preservative (E220 in the EU numbering system). It serves two purposes:

• Antimicrobial: Kills bacteria and fungi that would otherwise spoil food
• Antioxidant: Prevents enzymatic browning in cut fruits and vegetables

Common foods preserved with SO₂ or sulfites include:

• Dried fruits (apricots, raisins, sultanas, dried apples)
• Wine (SO₂ prevents oxidation and stops unwanted fermentation by wild yeasts)
• Fruit juices and soft drinks
• Sausages and processed meats
• Pickled foods

Sulfites (E221–E228): Sodium sulfite (Na₂SO₃, E221), sodium hydrogen sulfite (NaHSO₃, E222), sodium metabisulfite (Na₂S₂O₅, E223), and potassium metabisulfite (K₂S₂O₅, E224) are all salts that release SO₂ in acidic conditions and serve the same preservative function.

Some individuals are sensitive to sulfites, particularly asthmatics, and food labels must declare sulfite content above 10 mg/kg.

Bleaching Wood Pulp

SO₂ bleaches by reduction — it removes oxygen from coloured compounds, making them colourless (contrast with chlorine, which bleaches by oxidation). It is used to bleach wood pulp for paper production. The bleaching effect of SO₂ is partially reversible; paper bleached with SO₂ may slowly yellow with prolonged exposure to air as the reduced compounds are re-oxidised.

Other Uses of SO₂

• Water treatment: SO₂ is used to remove excess chlorine after chlorination (dechlorination): SO₂ + Cl₂ + 2H₂O → H₂SO₄ + 2HCl
• Refrigerant: Historically used as a refrigerant gas (now largely replaced)
• Solvent: Liquid SO₂ is a non-aqueous solvent used in some specialised chemical reactions

Environmental Considerations: Acid Rain

The burning of fossil fuels (coal, oil) in power stations releases SO₂ into the atmosphere. Sulfur dioxide is a major contributor to acid rain.

Formation of acid rain:

1. SO₂ is oxidised in the atmosphere to SO₃: 2SO₂ + O₂ → 2SO₃
2. SO₃ dissolves in rainwater to form sulfuric acid: SO₃ + H₂O → H₂SO₄
3. SO₂ can also dissolve directly in rainwater to form sulfurous acid (H₂SO₃), which slowly oxidises to H₂SO₄

Sulfurous acid: SO₂ + H₂O → H₂SO₃

Effects of acid rain:

• Acidifies lakes and rivers, killing aquatic life
• Leaches aluminium from soil, damaging tree roots
• Corrodes limestone buildings and statues: CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O
• Damages metal structures through accelerated corrosion

Control measures:

• Flue gas desulfurisation (FGD): Power stations pass exhaust gases through a slurry of limestone (CaCO₃) or lime (CaO), which reacts with SO₂: CaCO₃ + SO₂ → CaSO₃ + CO₂. The calcium sulfite (CaSO₃) is further oxidised to gypsum (CaSO₄), which is used in plasterboard.
• Low-sulfur fuels: Using natural gas (methane, CH₄) instead of coal, or removing sulfur from crude oil during refining
• Catalytic converters: In vehicles, reduce NOₓ emissions (the other major acid rain contributor)

This is covered in more detail in Air Pollution.

Key Concepts from Past Papers

• add water to anhydrous cobalt sulfate / add water to CoSO4
• colour changes (from blue) to red / pink
• any 2 from:
• improve growth of plants
• increase protein (in plants)
• fertilisers add nitrogen / nitrates / phosphorous / phosphates / potassium
• to put back nitrogen / nitrates / phosphorous / phosphates / potassium into the soil
• sulfuric acid + magnesium carbonate → magnesium sulfate + carbon dioxide + water IF full credit is not awarded, for either magnesium sulfate OR carbon dioxide + water

Keywords from Past Papers

sulfate, dioxide, sulfur, ammonia, water, sulfuric, acid, ammonium, oxygen, add, magnesium, plants, nitrogen, nitrates, phosphorous

Related Notes

• Sulfur and Compounds
• Air Pollution
• Making Salts
• Acids and Bases

Sources

• OpenStax Chemistry 2e, Chapter 18 (Representative Metals, Metalloids, and Nonmetals) — Section 18.10 on Sulfur — openstax.org/books/chemistry-2e/
• BBC Bitesize GCSE Chemistry, Sulfuric Acid and Its Uses — bbc.co.uk/bitesize/topics/
• Cambridge IGCSE Chemistry 0620 Syllabus, Topic 10: Chemistry of the Environment
• CK-12 Chemistry, The Uses of Sulfur and Its Compounds — ck12.org/chemistry/

Past Paper Sources

• 0620/31 May/June 2015: Q66(b)(i) (0m)
• 0620/32 Feb/March 2015: Q55(d)(ii) (2m)
• 0620/32 Feb/March 2018: Q66(b)(i) (1m), Q66(b)(iii) (1m), Q77(b)(iii) (1m)
• 0620/32 Feb/March 2022: Q44(a)(iii) (2m), Q22(c)(ii) (1m)
• 0620/32 Feb/March 2023: Q22(b)(ii) (1m)
• 0620/32 May/June 2018: Q11(a)(ii) (1m)
• 0620/32 May/June 2020: Q22(a)(i) (1m), Q22(a)(ii) (1m), Q22(a)(iv) (0m)
• 0620/33 May/June 2016: Q77(b)(i) (1m)
• 0620/33 May/June 2022: Q22(a)(ii) (1m)
• 0620/33 May/June 2023: Q33(b)(ii) (1m)
• 0620/33 October/November 2015: Q55(b)(ii) (1m), Q55(b)(iv) (1m)
• 0620/33 October/November 2016: Q22(d)(i) (2m), Q22(a)(ii) (1m), Q22(d)(ii) (1m)

Common Misconceptions