The Most Important Industrial Acids

Economists once suggested that a country’s sulfuric acid production was a good measure of its industrial strength. That might sound odd, but it reflects a real truth: acids sit near the start of almost every supply chain. The fertiliser that grows our food, the steel in our cars, the plastics in our phones and the medicines in our cupboards all depend on a handful of acids produced in enormous quantities.

Here are the most important industrial acids, how they’re made and what they’re used for. Production figures are rounded, recent estimates and vary from year to year.

1. Sulfuric acid, H₂SO₄

Production: roughly 250–280 million tonnes per year worldwide, more than any other chemical by mass.

How it’s made: the Contact process

1. Sulfur is burned to make sulfur dioxide: S + O₂ → SO₂. (Sulfur comes mostly from cleaning crude oil and natural gas; some SO₂ comes from smelting metal sulfide ores.)
2. Sulfur dioxide is oxidised to sulfur trioxide over a vanadium(V) oxide catalyst at about 400–450 °C: 2SO₂ + O₂ ⇌ 2SO₃.
3. SO₃ is absorbed into concentrated sulfuric acid to form oleum, which is diluted with water to give sulfuric acid.

SO₃ isn’t added directly to water because the reaction is so exothermic that it forms a fine mist of acid that’s hard to capture.

Main uses:

• Phosphate fertilisers (about half of all production): phosphate rock is treated with sulfuric acid to make phosphoric acid and superphosphates.
• Metal processing: leaching copper and other metals from ores, pickling steel.
• Making other chemicals: detergents, dyes, pigments, hydrofluoric acid, explosives.
• Car batteries (lead–acid).
• Petroleum refining (alkylation) and paper making.

2. Phosphoric acid, H₃PO₄

Production: tens of millions of tonnes per year (usually reported as P₂O₅ equivalent).

How it’s made:

• The wet process (most common): phosphate rock (mainly calcium phosphate) reacts with sulfuric acid, producing phosphoric acid and gypsum (calcium sulfate) as a by-product.
• The thermal process: phosphorus is burned to P₄O₁₀, which is dissolved in water. This gives purer acid for food and electronics.

Main uses:

• Fertilisers, especially ammonium phosphates (by far the biggest use).
• Food additive (E338): gives colas their sharp taste.
• Rust removal and metal treatment.
• Detergents and water treatment (as phosphates).
• Animal feed supplements.

3. Nitric acid, HNO₃

Production: roughly 60–70 million tonnes per year.

How it’s made: the Ostwald process

1. Ammonia is oxidised over a platinum–rhodium catalyst at about 850–900 °C: 4NH₃ + 5O₂ → 4NO + 6H₂O.
2. Nitrogen monoxide is oxidised further: 2NO + O₂ → 2NO₂.
3. Nitrogen dioxide is absorbed in water: 3NO₂ + H₂O → 2HNO₃ + NO.

The ammonia comes from the Haber process, so nitric acid ultimately comes from nitrogen in the air.

Main uses:

• Ammonium nitrate fertiliser (the majority of production).
• Making nylon precursors (adipic acid) and polyurethane precursors.
• Explosives for mining and quarrying.
• Etching and cleaning metals; refining precious metals.

4. Hydrochloric acid, HCl

Production: around 20 million tonnes per year of HCl (much of it produced as a by-product and used on site).

How it’s made:

• Mostly as a by-product of chlorinating organic compounds (for example, in making PVC and polyurethane precursors).
• By direct combination of hydrogen and chlorine: H₂ + Cl₂ → 2HCl, then dissolving in water.

Main uses:

• Pickling steel: removing rust and scale before galvanising or coating.
• Making PVC precursors and other chemicals.
• pH control and regeneration of ion-exchange resins in water treatment.
• Oil well acidising: dissolving rock to increase oil and gas flow.
• Food processing, for example in making gelatin and some sweeteners.

5. Hydrofluoric acid, HF

Production: a few million tonnes per year.

How it’s made: fluorite (calcium fluoride) is heated with concentrated sulfuric acid: CaF₂ + H₂SO₄ → CaSO₄ + 2HF.

Main uses:

• Making fluorocarbons: refrigerants and fluoropolymers such as PTFE.
• Aluminium production (making cryolite and aluminium fluoride).
• Uranium processing.
• Etching glass and silicon chips.
• Oil refining (alkylation).

HF is extremely hazardous; see hydrofluoric acid.

6. Ethanoic (acetic) acid, CH₃COOH

Production: roughly 15–20 million tonnes per year.

How it’s made: mostly by carbonylation of methanol (reacting methanol with carbon monoxide) using rhodium or iridium catalysts, in processes such as the Cativa process. Vinegar for food is still made by bacterial fermentation of ethanol.

Main uses:

• Making vinyl acetate (for paints, adhesives and PVA glue).
• Making terephthalic acid (a solvent in its production), used for PET bottles and polyester.
• Making cellulose acetate (for film and fibres) and ethyl ethanoate (a solvent).
• Food: vinegar and as a preservative (E260).

7. Other important industrial acids

Acid	Main uses
Adipic acid	nylon-6,6 production
Terephthalic acid	PET plastic and polyester fibres
Citric acid	food and drink, cleaning products, metal cleaning; made by fermentation using the mould Aspergillus niger
Lactic acid	food, and polylactic acid (PLA) biodegradable plastic
Formic acid	leather tanning, animal feed preservation, de-icing
Boric acid	glass fibre, antiseptics, flame retardants
Perchloric acid	making perchlorates for rocket propellant

A worked example: fertiliser chemistry

The link between acids and food is easiest to see in one reaction. Phosphate rock, mainly calcium phosphate, is almost insoluble, so plants can’t use it directly. Treating it with sulfuric acid produces phosphoric acid (and calcium sulfate):

Ca₃(PO₄)₂ + 3H₂SO₄ → 2H₃PO₄ + 3CaSO₄

Phosphoric acid is then neutralised with ammonia (made by the Haber process) to make ammonium phosphates, which supply both nitrogen and phosphorus in a water-soluble form:

H₃PO₄ + NH₃ → NH₄H₂PO₄ (monoammonium phosphate)

A single bag of fertiliser can therefore carry the products of three major industrial processes: sulfuric acid, phosphoric acid and ammonia.

Why acids matter so much

Look at the list of uses and three themes stand out:

1. Feeding the world. Sulfuric, phosphoric and nitric acids are mostly used to make fertilisers. Without them, global food production would fall dramatically.
2. Processing materials. Acids dissolve ores, clean metal surfaces, and build the chemical intermediates for plastics and fibres.
3. Chains of production. Many acids are made from others: sulfuric acid is used to make phosphoric acid and hydrofluoric acid, and the ammonia from the Haber process becomes nitric acid.

Safety and environment

Industrial acids are handled in huge quantities, so plants use corrosion-resistant materials, spill containment, scrubbers to capture acid gases, and strict worker protection. Environmental controls matter too: the gypsum by-product of phosphoric acid, for instance, can contain small amounts of radioactive and heavy-metal impurities from phosphate rock and must be managed carefully.

Key takeaways

• Sulfuric acid is the world’s most-produced chemical, made by the Contact process and used mainly for fertilisers.
• Phosphoric and nitric acids are also largely used for fertilisers; nitric acid comes from ammonia via the Ostwald process.
• Hydrochloric acid is mostly a by-product and is widely used for pickling steel.
• Acetic acid, made from methanol and CO, feeds the plastics, paints and adhesives industries.
• For the bases on the other side of industry, see industrial bases.