Hydrochloric Acid
Properties
| State | Liquid (aqueous solution; pure HCl is a gas) |
| Color | Colorless (may appear slightly yellow when concentrated) |
| Solubility | Miscible with water |
| Melting Point | -27°C (37% solution) |
| Boiling Point | 110°C (20.2% azeotrope) |
About Hydrochloric Acid
Hydrochloric acid (HCl in water, 36.458 g/mol) is the default strong acid in chemistry — the one bottle that's always within arm's reach in any teaching lab and most research labs. It's the constant-boiling azeotrope at 20.2% (110 °C, 6.0 M) when boiled, and the concentrated commercial product is typically 37% (12.1 M, density 1.19 g/mL). HCl is a strong acid (pKa ~ -7) that's effectively fully dissociated in water, but unlike H2SO4 or HNO3, the conjugate base Cl- is non-oxidizing and non-coordinating, which makes HCl the cleanest choice for protonating organic substrates, dissolving metals where you don't want sulfate or nitrate side chemistry, or controlling pH in buffer prep. Industrially, the biggest use by tonnage is steel pickling — dipping rolled steel in 15% HCl removes the iron oxide scale formed during hot rolling without dissolving the underlying metal too aggressively. The pickling liquor is then regenerated by spray roasting, which recovers the HCl as gas and yields Fe2O3 as a byproduct. In organic chemistry, HCl converts amines to crystalline hydrochloride salts (the standard pharmaceutical formulation trick), generates carbocations from alcohols under SN1 conditions, and serves as the proton source in countless workups. In your own body, parietal cells in the gastric mucosa secrete HCl at 0.16 M (pH around 0.8) using a H+/K+ ATPase, which is why proton-pump inhibitors like omeprazole work the way they do.
Where you'll encounter it
If you've ever cleaned grout with muriatic acid, taken Tums for heartburn (neutralizing your own stomach HCl), or done any acid-base titration in a chemistry course, you've handled or generated HCl directly. The hardware-store muriatic acid is just 31-37% HCl rebranded for masonry work — pool owners use it weekly to drop pH after chlorine shocks raise it, and concrete contractors etch slabs with it before applying epoxy coatings to give the polymer something to grab onto. In your own gut, parietal cells pump out 1.5-2 liters per day of 0.16 M HCl, and the burning sensation of acid reflux is literally that fluid escaping past the lower esophageal sphincter. Every introductory chem lab on Earth keeps a 6 M HCl bottle on the shelf as the default acidification reagent for workups.
Common Uses
- Steel pickling to remove iron oxide scale before galvanizing or coating
- pH adjustment in swimming pool, water treatment, and food processing
- Acidification step in PVC monomer (vinyl chloride) and TDI/MDI synthesis
- Standard titrant for strong-acid/strong-base and weak-base titrations
- Conversion of free-base amines to crystalline hydrochloride salts
- Regeneration of cation-exchange resins in deionizer columns
- Catalyst and proton source in countless organic workup procedures
Safety Information
Corrosive — concentrated HCl causes severe burns to skin, eyes, and respiratory tract. The vapor is intensely irritating; concentrated HCl fumes visibly when opened in humid air because HCl gas reacts with water vapor to form aerosol droplets. OSHA PEL is 5 ppm (ceiling); ACGIH TLV is 2 ppm (ceiling). GHS: H290 (corrosive to metals), H314 (severe skin burns and eye damage), H335 (respiratory irritation). Always work in a fume hood for transfers above 100 mL. Wear nitrile gloves, splash goggles, and a face shield for the 37% concentrate. Add acid to water (never the reverse) when diluting. Keep separated from oxidizers (HCl + bleach releases Cl2 gas — a frequent cleaning-product accident).
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.