Hydrogen chloride

Hydrochloric acid combines extreme molecular simplicity (one proton, one chlorine) and considerable industrial importance — about 20 Mt of commercial acid produced annually, plus 80-100 Mt-equivalent generated as a co-product of other processes, mostly hydrocarbon chlorination. It is the reference monoprotic strong acid: its pKa of -7 in aqueous medium places it among the most fully dissociated, making it the standard for acid-base titrations and the universal metal-pickling reagent.

At biological scale, HCl plays an unsuspected role through its extreme concentration: parietal cells in the human stomach actively secrete HCl at pH 1 (concentration ~0.1 mol/L) to denature dietary proteins, activate pepsin and sterilise the bolus of ingested microorganisms. The gastric wall itself is protected by an alkaline mucus layer that neutralises acidity at the cell surface — a fragile dynamic equilibrium whose breakdown causes ulcers. The Helicobacter pylori discovery (1982, Marshall and Warren's Nobel Prize) reversed the medical view: ulcers became a treatable infection rather than a psychosomatic disorder.

Industrially, steel pickling consumes ~50 % of the HCl produced: before zinc plating or painting, steel strips are dipped in HCl at 100-150 g/L to dissolve oxide scale and mill scale. PVC synthesis — the second most produced polymer worldwide — passes through HCl en route via 1,2-dichloroethane. A "cleaner" acid than H₂SO₄ (no residual SO₂) or HNO₃ (no NOₓ), HCl is often preferred when post-neutralisation with lime or soda is straightforward to manage.