- Sulfuric acid, H₂SO₄, is a strong, corrosive mineral acid – the most widely produced chemical in the world (approx. 250–270 million tonnes a year).
- It is made by the contact process: sulfur or smelter gases → SO₂ → SO₃ over a vanadium catalyst → absorption in concentrated acid.
- More than half of all sulfuric acid goes into phosphate fertilisers; the rest is used by the chemical industry, metallurgy, battery manufacturing, papermaking and water treatment.
- Fuming sulfuric acid (oleum) is a solution of sulfur trioxide, SO₃, in concentrated sulfuric acid.
- When diluting, always add the acid to the water, never the other way round; concentrated acid causes severe burns (H314).
In short: sulfuric acid, H₂SO₄, is a strong, corrosive mineral acid – a colourless, oily liquid that is heavier than water (concentrated acid has a density of approx. 1.84 g/cm³). It is known as the “king of chemicals” because industry produces more of it than of any other substance. It is made by the contact process from sulfur or smelter gases. More than half of the output goes into phosphate fertilisers, and the rest is used by the chemical industry, metallurgy, refineries, papermaking, battery manufacturing and water treatment. Fuming sulfuric acid (oleum) is a solution of SO₃ in concentrated acid. Concentrated acid is a powerful dehydrating agent, causes severe burns and releases a great deal of heat on dilution – which is why the acid is always added to water.
What is sulfuric acid?
Sulfuric acid (systematic name: sulfuric(VI) acid) is an inorganic oxoacid of sulfur with the formula H₂SO₄. Sulfur is in the +6 oxidation state, which distinguishes it from sulfurous acid, H₂SO₃, in which sulfur is in the +4 state (in Polish the two are called sulfuric(VI) and sulfuric(IV) acid). It was historically known as oil of vitriol (or simply vitriol), because it was obtained by roasting iron and copper sulfates, which were called vitriols.
Sulfuric acid is known as the “king of chemicals”, and its consumption is sometimes used as a measure of a country’s industrial development. Approximately 250–270 million tonnes are produced worldwide each year – more than any other chemical substance.
In nature, sulfuric acid occurs rarely and in low concentrations: it forms in the atmosphere from sulfur dioxide (acid rain), in volcanic gases and during the oxidation of sulfide minerals such as pyrite. All of the acid used in industry is produced synthetically.
What are the properties of sulfuric acid?
Concentrated sulfuric acid is a colourless or slightly yellowish, odourless, oily liquid. Key parameters:
| Property | Sulfuric acid |
|---|---|
| Chemical formula | H₂SO₄ |
| Molar mass | 98.08 g/mol |
| CAS number | 7664-93-9 |
| Density (approx. 98%) | approx. 1.84 g/cm³ |
| Melting point (100%) | approx. 10°C |
| Boiling point (98.3%) | approx. 337°C |
| Acid strength | strong diprotic acid (pKa₁ approx. –3, pKa₂ approx. 1.99) |
| Typical commercial concentrations | 94–98% (concentrated), 37% (battery electrolyte), dilute solutions, oleum |
Sulfuric acid is strongly hygroscopic and dehydrating – it absorbs moisture from the air and draws water out of other compounds. This is why concentrated acid chars sugar, paper, wood and cotton fabrics. When hot and concentrated, it also acts as an oxidising agent and dissolves copper, releasing sulfur dioxide. Dilute acid reacts with many metals (e.g. iron and zinc) to release hydrogen, whereas concentrated acid passivates iron, so it can be stored in steel tanks.
Mixing sulfuric acid with water releases a very large amount of heat. If water were poured into concentrated acid, it could boil violently and spatter the corrosive liquid. Hence the rule: always add acid to water, slowly and while stirring.
What is fuming sulfuric acid (oleum)?
Fuming sulfuric acid, or oleum, is a solution of sulfur trioxide (SO₃) in concentrated sulfuric acid. Its composition is expressed as the free SO₃ content – commercial grades are typically 20%, 25%, 30% or 65% oleum. The simplified formula is H₂SO₄·xSO₃, and an equimolar composition gives pyrosulfuric (disulfuric) acid, H₂S₂O₇.
The name comes from the fact that oleum gives off white fumes in contact with moist air – SO₃ vapour reacts with water vapour to form a sulfuric acid mist. Oleum is an intermediate in the contact process, and industry uses it for the sulfonation and nitration of organic compounds, e.g. in the production of detergents, dyes, pharmaceuticals and explosives. It is even more corrosive and reactive than concentrated sulfuric acid.
How is sulfuric acid produced?
Almost all sulfuric acid is manufactured by the contact process, which consists of three stages:
- Producing SO₂ – burning sulfur (often recovered from oil and natural gas refining) or using sulfur dioxide from smelter gases generated during copper, zinc and lead smelting.
- Oxidising SO₂ to SO₃ – over a vanadium(V) oxide (V₂O₅) catalyst at approx. 400–600°C.
- Absorbing SO₃ – the sulfur trioxide is absorbed in concentrated (98–99%) sulfuric acid rather than in water, because with water it would form an acid mist that is difficult to capture. The result is oleum or a more concentrated acid, which is then diluted to the required strength.
Modern plants use a double contact, double absorption configuration, which converts more than 99.7% of the SO₂ and reduces emissions. The heat of reaction is used to generate steam and electricity. A wet process (WSA) is also used, in which the acid is obtained from gases containing hydrogen sulfide or SO₂, e.g. in refineries. The historical lead chamber (nitrous) process produced a lower-concentration acid and is no longer used today.
In Poland, sulfuric acid is produced by, among others, the KGHM copper smelters (from smelter gases) and the Grupa Azoty chemical plants, which use it to make fertilisers.
What is sulfuric acid used for?
Sulfuric acid is a raw material for almost every branch of industry. Its main applications include:
- fertilisers – more than half of global production is used to digest phosphate rock and produce phosphoric acid, superphosphates and ammonium phosphates, as well as ammonium sulfate; you can read more about phosphoric acid in the article Phosphoric acid as an acidity regulator and versatile industrial agent,
- chemical industry – production of sulfates (e.g. of aluminium, iron, copper and sodium), hydrofluoric acid, titanium dioxide, viscose fibres, dyes and pharmaceuticals; for examples of sulfates, see our articles on copper sulfate and calcium sulfate,
- detergents – sulfonation in the production of anionic surfactants such as LAS and SLES (see: surfactants),
- metallurgy – leaching of copper, zinc and uranium ores, steel pickling (removal of rust and mill scale), electrorefining and electroplating,
- batteries – dilute sulfuric acid (approx. 30–37%) is the electrolyte in lead-acid batteries,
- refineries – an alkylation catalyst in the production of high-octane petrol components and an agent for purifying petroleum products,
- pulp and paper industry – pH control and production of chlorine dioxide for bleaching,
- water and wastewater treatment – lowering water pH, producing coagulants such as aluminium sulfate, and regenerating ion exchangers,
- agriculture – acidifying alkaline soils and irrigation water, and acidifying slurry to reduce ammonia emissions.
Is sulfuric acid used in the food industry?
Yes, but to a limited extent and usually as a processing aid rather than a typical additive present in the finished product. In the international numbering system, sulfuric acid is designated E513. It is used, among other things:
- to adjust the pH of process water and solutions in food plants,
- in the production of modified starch (acid-thinned starch) and syrups,
- in sugar refining and yeast production,
- to regenerate ion exchange resins and clean installations.
These applications require food-grade sulfuric acid, i.e. acid with a purity that meets food requirements – with low levels of heavy metals, arsenic and other impurities. Technical-grade sulfuric acid must not be used in food production.
Can sulfuric acid be used for cleaning?
Concentrated sulfuric acid is an ingredient of heavy-duty drain cleaners – it breaks down hair, grease, paper and other organic deposits. In industry, it is used for pickling and cleaning metal surfaces and for removing scale and deposits from installations.
At home, however, sulfuric acid is not a good cleaning agent. It is highly corrosive, releases heat on contact with water, damages many materials (including fabrics, wood, some plastics, concrete and metals) and can react violently with other products, e.g. hypochlorite-based bleaches, releasing toxic chlorine. For descaling, it is safer to use citric acid or sulfamic (amidosulfuric) acid, which is a solid, less aggressive and widely used in descalers.
Polyethylene, polypropylene and PTFE are resistant to sulfuric acid, so it is stored in tanks and jerrycans made of these plastics or in steel (concentrated acid).
How does sulfuric acid differ from sulfurous acid and hydrosulfuric acid?
Sulfur forms several acids with similar-sounding names that are easy to confuse:
| Acid | Formula | Characteristics | Uses |
|---|---|---|---|
| Sulfuric acid (sulfuric(VI) acid) | H₂SO₄ | strong, stable oxoacid, powerful dehydrating agent | fertilisers, chemicals, metallurgy, batteries |
| Sulfurous acid (sulfuric(IV) acid) | H₂SO₃ | weak, unstable acid – a solution of SO₂ in water; readily decomposes and oxidises | bleaching, preservation (sulfites), disinfection |
| Hydrosulfuric acid (aqueous hydrogen sulfide) | H₂S (aqueous solution) | very weak binary (oxygen-free) acid smelling of rotten eggs; hydrogen sulfide is highly toxic | chemical analysis, production of sulfur and sulfides |
| Sulfamic (amidosulfuric) acid | H₃NSO₃ | solid, crystalline acid, easy to store | descalers, cleaning installations |
| Thiosulfuric acid | H₂S₂O₃ | unstable; its salts – thiosulfates – are used | photography, analytical chemistry, water dechlorination |
Among the mineral acids, sulfuric acid is most often compared with hydrochloric acid – we cover it in the article Hydrochloric acid – applications in the food and pharmaceutical industries.
Is sulfuric acid dangerous?
Yes. Concentrated sulfuric acid is classified as a substance that causes severe skin burns and eye damage (H314) and may be corrosive to metals (H290). Hazards:
- skin and eyes – deep chemical and thermal burns (due to the heat of dehydration), with a risk of blindness if the eyes are affected,
- respiratory tract – acid mist and vapours irritate the nose, throat and lungs; occupational exposure to strong inorganic acid mists containing sulfuric acid is classified as carcinogenic to humans,
- ingestion – severe burns to the oesophagus and stomach, potentially life-threatening,
- reactions – violent reactions with water, bases, metals (hydrogen), organic substances and oxidising agents.
When working with the acid, use acid-resistant gloves (e.g. butyl or neoprene), goggles or a face shield, protective clothing and good ventilation. After contact with the skin or eyes, rinse immediately with plenty of water for at least 15 minutes, remove contaminated clothing and seek medical help.
Sulfuric acid is also an explosives precursor regulated under EU law – suspicious transactions, disappearances and thefts of this substance must be reported to the relevant authorities.
We explain what the GHS05 pictogram and the other hazard symbols on chemical labels mean in the article Chemical pictograms – what are they and what do they mean?.
How do you neutralise and dispose of sulfuric acid?
Spilled sulfuric acid is neutralised with alkaline substances:
- sodium carbonate or sodium bicarbonate (soda ash, baking soda) – the reaction is mild, but it releases CO₂ and foam,
- lime (calcium hydroxide or calcium carbonate) – this produces gypsum, i.e. calcium sulfate,
- sodium hydroxide – effective but highly exothermic; it is mainly used under controlled conditions in industrial plants, as we explain in the article What is sodium hydroxide and what is it used for?
Larger spills are first covered with an inert absorbent (sand, diatomaceous earth), and the neutralising agent is added gradually while monitoring the pH. Sulfuric acid must never be poured into drains or onto the ground. Spent acid and neutralised solutions are hazardous waste – they must be handed over to a company licensed to collect them. Used batteries should be taken to collection points, where the electrolyte is recovered or neutralised.
How can your business buy sulfuric acid?
When buying sulfuric acid for industrial use, it is worth specifying:
- concentration – most commonly 94–98% (concentrated), dilute solutions or oleum,
- purity grade – technical, battery, food or analytical grade,
- packaging – jerrycans, drums, 1000 l IBC containers or tanker deliveries,
- transport – sulfuric acid is carried as class 8 dangerous goods (UN 1830, and solutions of up to 51% as UN 2796),
- documentation – safety data sheet, certificate of analysis and specification.
Sulfuric acid is sold mainly to businesses; because of the regulations on explosives precursors, the seller may ask about its intended use. Foodcom supplies sulfuric acid in a range of concentrations and packaging sizes, as well as other industrial chemicals such as sodium hydroxide. For specifications, prices and availability, please contact our sales team.
