GY-Formaldehyde

Appearance

 Colourless gas (pure substance); industrially a clear colourless liquid (37 %/44 %/50 % solutions) or a white solid (paraformaldehyde)

Odour

Strong pungent, suffocating odour ; odour threshold 0.03–0.6 mg/m³ (detectable by humans at about 0.1 mg/m³)

Melting point

−92 °C (pure substance; −117 °C is also reported under a different measurement basis)

Boiling point

−19 to −21 °C (pure); about 98–101 °C for the 37 % solution and about 98 °C for the 37 % methanol-free solution

Density

 0.8153 g/cm³ (pure, liquid at −20 °C); 1.075–1.114 g/cm³ for the 37 % solution at 20 °C; 1.125–1.135 for 44 %; 1.147–1.152 for 50 %

Relative vapour density

1.03–1.08 (air = 1; the vapour is slightly heavier than air and can accumulate in low-lying areas and pits)

Vapour pressure

About 2 hPa (37 % solution, 20 °C); 0.2 kPa (37 % methanol-free solution, 20 °C); >1 atm for the pure substance

Flash point

⚠️ Varies with methanol content : about 50–64 °C for the methanol-containing commercial product; about 83–85 °C for the methanol-free 37 % solution

Auto-ignition temperature

424 °C (formalin containing 15 % methanol); 430 °C (GESTIS value)

Explosion limits

⚠️ 7 %–73 % (v/v) — an unusually high upper limit, so vapour/air mixtures readily fall within the explosive range

Solubility

Fully miscible with water, ethanol, acetone and diethyl ether (about 400 g/L for the pure substance in water); commercial solutions mix with water in all proportions

pH value

 About 2.8–4.0 (37 % solution) — acidic because of formic acid formed by oxidation, with acidity increasing on storage

Partition coefficient

log Kow = 0.35 (octanol/water; not lipophilic and not bioaccumulative)

Other constants

Ionisation potential 10.88 eV; critical temperature 137.2–141.2 °C; critical pressure 6.64–6.78 MPa; pKa (hydrate) 13.27; vapour pressure of the pure substance >1 atm at 20 °C

Stability and reactivity

Stable in closed containers at ambient temperature; ⚠️ polymerises readily (to paraformaldehyde, accelerated at low temperature and in alkaline conditions, exothermic); oxidises to formic acid on contact with air and light; reacts violently with strong oxidants, strong acids and strong bases ; ⚠️ reacts with hydrochloric acid to form bis(chloromethyl) ether (an IARC Group 1 carcinogen) ; exothermic condensation with phenol, urea, melamine and amines; violent or explosive reactions with hydrogen peroxide, nitric acid, perchloric acid, nitromethane and magnesium

Fire hazard rating

 NFPA 704: health 3 / flammability 2 / reactivity 0 (the flammability rating depends on the methanol content)