Sulfuric acid

Sulfuric acid was, for a long time, used as an economic indicator before GDP: its per-capita consumption tracked a country's level of industrialisation closely. This centrality stems from its ubiquity — it intervenes at one stage or another in about half of all modern chemical production chains, without ever appearing as a visible consumer product. About 60 % of world production (~270 Mt/yr) is consumed in superphosphate and ammonium-nitrate fertiliser manufacture, with the rest split between sulfide ore refining, petrochemistry, lead-acid batteries and organic synthesis.

Chemically, H₂SO₄ is a strong diprotic acid with an exceptional affinity for water — so much that it dehydrates sugar, wood or skin in seconds with violent exotherm, leaving a black carbonaceous residue. This property is leveraged as a dehydrating agent in organic synthesis but also explains why handling remains dangerous even at moderate concentration. Concentrated solutions (>96 %) are fuming (oleum) because of dissolved SO₃.

Industrially, the contact process (1831, patented by Peregrine Phillips) remains the global standard. It catalytically oxidises SO₂ to SO₃ over vanadium pentoxide, then absorbs SO₃ in concentrated H₂SO₄ to give oleum, diluted afterwards to the desired strength. The process is globally exothermic: a modern plant recovers 4-6 GJ per tonne of acid as HP steam — making it one of the few industrial processes that is a net energy producer while making a chemical.