Sulfuric Acid
Properties
| State | Liquid (oily, viscous; viscosity 24 cP at 25°C, similar to motor oil) |
| Color | Colorless when pure; technical grade is pale yellow-brown |
| Solubility | Miscible with water in all proportions; mixing releases ~75 kJ/mol of dilution heat |
| Melting Point | 10°C (100% acid); −32°C for 98% azeotrope; eutectic at −74°C around 38% |
| Boiling Point | 337°C (98% azeotrope; pure H2SO4 boils at 290°C with decomposition) |
About Sulfuric Acid
Sulfuric acid, H2SO4 (98.079 g/mol), is the largest-tonnage industrial chemical on the planet — about 280 million tonnes per year, more than chlorine, ammonia, or sodium hydroxide. It is a strong diprotic acid: the first dissociation is essentially complete in water (pKa1 ≈ −3), the second has pKa2 = 1.99, so at any concentration above 0.1 M the bisulfate ion HSO4⁻ is the dominant species rather than free SO4²⁻. Concentrated H2SO4 (96–98%) is a viscous oily liquid that does three different things at once depending on what it touches: it is a strong acid (protonates), a powerful dehydrating agent (yanks H and O out of organics in a 2:1 ratio — drop concentrated acid on sugar and you get a column of black carbon foam), and a moderately strong oxidizer when hot (above 200°C it will oxidize copper and even some noble metals). About 60% of global output goes into making phosphate fertilizer (H2SO4 + Ca5(PO4)3F → phosphoric acid + CaSO4·2H2O, the wet-process route that makes diammonium and monoammonium phosphate); other major uses are petroleum alkylation (concentrated H2SO4 catalyzes isobutane + butene → isooctane for gasoline blending), pickling steel before galvanizing, lead-acid battery electrolyte (37% by mass aqueous), and as the primary mineral acid in nearly every nitration, sulfonation, and esterification reaction. Manufactured almost exclusively by the Contact process: burn S to SO2, oxidize SO2 to SO3 over V2O5, absorb SO3 into 98% H2SO4.
Where you'll encounter it
If you've ever popped the hood on an older car and noticed the corroded white-blue crust around the battery terminals, that's the lead sulfate and copper sulfate that forms when 37% sulfuric acid electrolyte vapor escapes through the vent caps and reacts with the metal lugs. In a chemistry teaching lab, the dramatic 'sugar snake' demo where concentrated H2SO4 is poured onto a beaker of granulated sucrose and a tall column of black carbon foam erupts is sucrose dehydration: C12H22O11 + H2SO4 → 12 C + 11 H2O + heat, with the steam blowing the carbon up into the snake shape. In the petrochemical industry, every refinery making premium gasoline runs an alkylation unit on concentrated sulfuric acid (or its competitor HF), and the spent acid sludge is regenerated by burning back to SO2 and recycled through the Contact loop. And in mining country, acid rock drainage from oxidizing pyrite at abandoned coal mines produces dilute sulfuric acid that has acidified entire watersheds — Pennsylvania alone has roughly 4,000 miles of streams below pH 5 from this source.
Common Uses
- Phosphate fertilizer manufacture (60% of global consumption) via wet-process phosphoric acid
- Alkylation catalyst at refineries combining isobutane and butenes into high-octane gasoline blendstock
- Lead-acid battery electrolyte at 37% by mass for automotive starting and stationary backup batteries
- Pickling steel coil before galvanizing, tinplating, or cold rolling to remove mill scale
- Strong-acid catalyst in esterification, nitration of toluene to TNT, and sulfonation of dyes and detergents
- Wastewater pH adjustment and metal precipitation in mining and electronics manufacturing effluent treatment
- Drying agent in desiccators (98% H2SO4) and dehydrating agent in carbohydrate chemistry
Safety Information
Sulfuric acid mist carries a very low occupational exposure limit, and the advisory value for the thoracic fraction is lower still — strong inorganic acid mists are an IARC Group 1 carcinogen. GHS: H314 causes severe skin burns and eye damage Category 1A, H290 may be corrosive to metals. Mixing with water is strongly exothermic — the iron rule is acid-into-water (AAA: Always Add Acid), never water-into-acid, because adding a small amount of water to concentrated acid floats on top, locally boils, and ejects a spray of hot acid. Concentrated acid will char skin to the bone within seconds; first aid is immediate brushoff of any solid contamination followed by 30-minute water flush, then medical care. Contact with chlorates, permanganates, or carbides gives explosive reactions. Dilute acid below 10% is much safer to handle but will still etch concrete, eat through stainless steel, and burn skin on prolonged contact.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.