Nitrogen Monoxide
Properties
| State | Gas |
| Color | Colorless |
| Solubility | Slightly soluble in water (1.94 mL/100 mL at 20°C) |
| Melting Point | -163.6°C |
| Boiling Point | -151.7°C |
About Nitrogen Monoxide
Nitrogen monoxide — almost always called nitric oxide, NO, in the lab — is a colorless diatomic gas with formula NO and molar mass 30.006 g/mol. It's a free radical with 11 valence electrons (one unpaired in a π* antibonding orbital), and unlike most radicals it's stable enough to bottle: the unpaired electron occupies an antibonding MO that doesn't favor dimerization. NO is one of the rare molecules that simultaneously matters in atmospheric chemistry, mammalian physiology, and transition-metal catalysis. Atmospherically, lightning and high-temperature combustion engines produce NO from N2 + O2 (the Zeldovich mechanism), and that NO oxidizes within minutes to NO2, then drives the photochemical smog cycle that builds ground-level ozone over cities. Physiologically, the 1998 Nobel Prize in Physiology or Medicine went to Furchgott, Ignarro, and Murad for discovering that mammalian endothelial cells synthesize NO from L-arginine via nitric oxide synthase (eNOS), and the diffusing NO relaxes vascular smooth muscle by activating soluble guanylate cyclase — that's the molecular mechanism by which nitroglycerin treats angina (it releases NO that dilates coronary arteries) and by which sildenafil enhances erectile function (PDE5 inhibition prolongs the cGMP signal NO triggers). Macrophages also use NO as a cytotoxic weapon against intracellular pathogens. In coordination chemistry, NO is a versatile ligand that binds metals as either NO+ (linear) or NO- (bent), and metal-nitrosyl complexes are key intermediates in industrial NOx removal catalysts.
Where you'll encounter it
If you've ever sat in a hospital NICU and seen a premature infant on inhaled NO therapy (INOmax, typically 20 ppm in N2), the dose works because inhaled NO selectively dilates pulmonary arterioles to improve oxygenation in persistent pulmonary hypertension of the newborn — without dropping systemic blood pressure, because NO is consumed by hemoglobin within seconds of crossing into the bloodstream. In a coordination-chemistry lab, when you add NO gas to a deep blue solution of [Fe(H2O)6]²⁺ in concentrated H2SO4, the brown ring you watch form at the interface is [Fe(H2O)5(NO)]²⁺ — that's the classical brown-ring test for nitrate, run by every analytical chemistry student. For atmospheric work, NO is the analyte detected by chemiluminescence in every roadside NOx monitor: NO + O3 → NO2* + O2 produces excited-state NO2 that emits red light proportional to the NO concentration.
Common Uses
- Inhaled vasodilator (INOmax, 20 ppm) for persistent pulmonary hypertension in neonates and ARDS adults
- Intermediate in the Ostwald nitric acid process (NH3 oxidation over Pt-Rh gauze at 850°C)
- Standard reagent in the brown-ring test for nitrate in classical qualitative analysis
- Calibration gas for chemiluminescence NOx monitors used in vehicle emissions and air quality stations
- Signaling-molecule probe in vascular biology and endothelial function research
- Cytotoxic effector studied in macrophage immunology and antimicrobial defense research
- Ligand precursor for transition-metal nitrosyl complexes used in homogeneous catalysis
Safety Information
GHS classifications: H270 (may cause or intensify fire; oxidizer, Category 1), H280 (gas under pressure), H314 (causes severe skin burns and eye damage), H330 (fatal if inhaled). NIOSH IDLH is 100 ppm; OSHA PEL is 25 ppm (30 mg/m3) as 8-hour TWA; ACGIH TLV is 25 ppm. Direct hazard is reaction with O2 to form NO2 within minutes — what kills people is usually the NO2 metabolite, not NO itself. Inhaled NO at therapeutic doses (5-80 ppm) requires precise monitoring because methemoglobin formation and rebound pulmonary hypertension on withdrawal are real risks. Cylinder NO is supplied as 100-1000 ppm in N2 to control oxidation. Always handle in stainless-steel or glass systems with O2-exclusion (purge to <1 ppm O2 before introducing NO); copper, brass, and rubber are degraded.
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.