Nitric Acid
Properties
| State | Liquid (fuming in concentrated form) |
| Color | Colorless (pure); yellow-brown (concentrated, due to dissolved NO2) |
| Solubility | Miscible with water |
| Melting Point | -42°C |
| Boiling Point | 83°C |
About Nitric Acid
Nitric acid is one of the three big mineral acids alongside H2SO4 and HCl, with formula HNO3 and a molar mass of 63.012 g/mol. What sets it apart is the dual character: it's both a strong acid (pKa ≈ -1.4, so fully dissociated in dilute aqueous solution) and a powerful oxidizer thanks to the +5 nitrogen, which is why HNO3 dissolves copper, silver, and mercury — metals that don't react with non-oxidizing acids — by oxidizing them while H+ is still around to consume the electrons. Concentrated HNO3 (68% w/w, 15.7 M) is the classic 'fuming' liquid that turns yellow on standing as dissolved NO2 builds up, and it's the acid in aqua regia (1 part HNO3 to 3 parts HCl) that dissolves gold and platinum by combining oxidation with chloride complexation. Industrially, the Ostwald process (developed by Wilhelm Ostwald in 1902) makes HNO3 by oxidizing ammonia over a Pt-Rh gauze catalyst at 850°C to NO, then NO2, then absorbing in water — and over 80% of the resulting acid goes into ammonium nitrate fertilizer. The remainder feeds explosives manufacturing (TNT via toluene nitration, nitroglycerin from glycerol), nylon precursors (adipic acid from cyclohexanone), and dye intermediates. In the lab, dilute HNO3 dissolves metal samples for ICP-MS, etches stainless steel for metallography, and runs the silver mirror test on aldehydes when paired with AgNO3.
Where you'll encounter it
If you've ever pulled a fuming nitric bottle out of an acid cabinet, the brown vapor curling out of the cap is real NO2 from photochemical decomposition — leave the bottle in sunlight and the contents go from clear to yellow to amber within weeks. In a metallography lab, 'nital' (1-5% HNO3 in ethanol) is the everyday etchant for revealing the grain structure of carbon steels — swab it on a polished sample for 5-15 seconds and the pearlite colonies and ferrite grains pop into visible contrast under the microscope. In a teaching lab, the xanthoproteic test is the universal cautionary tale: a drop of concentrated HNO3 on skin nitrates aromatic amino acids (tyrosine, tryptophan, phenylalanine) in keratin and the affected patch turns bright yellow within minutes — it doesn't wash off, you have to wait for the skin to slough.
Common Uses
- Manufacturing ammonium nitrate fertilizer via the AN reactor route (~80% of all HNO3 production)
- Toluene nitration to TNT and glycerol nitration to nitroglycerin in explosives plants
- Adipic acid synthesis from cyclohexanone for nylon-6,6 polymerization
- Stainless steel passivation and metallography etching (nital, 1-5% HNO3 in ethanol)
- Sample digestion for ICP-MS, ICP-OES, and AA trace metal analysis
- Component of aqua regia (1:3 HNO3:HCl) for dissolving gold, platinum, and palladium
- Tollens reagent preparation (with AgNO3 and NH3) for the silver mirror aldehyde test
Safety Information
GHS classifications: H272 (may intensify fire; oxidizer, Category 3), H290 (may be corrosive to metals), H300 (fatal if swallowed), H314 (causes severe skin burns and eye damage), H330 (fatal if inhaled). OSHA PEL is 2 ppm (5 mg/m3) as 8-hour TWA; ACGIH TLV is 2 ppm with 4 ppm STEL. Concentrated HNO3 reacts violently with organic materials, alcohols, ketones, and many metal powders — keep separate storage. NO2 fumes from the bottle cause delayed pulmonary edema (symptoms can appear 6-24 hours after exposure with no early warning). Always work in a fume hood with face shield, neoprene or butyl gloves (nitrile is attacked), and apron. Spills are neutralized with sodium bicarbonate. Never mix with HCl in a closed container — toxic NOCl and Cl2 gases evolve.
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.