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CAS number 30525-89-4 YesY
PubChem 24898648
Molecular formula OH(CH2O)nH (n = 8 - 100)
Appearance white crystalline solid
Density 1.42 g·cm−3 (25 °C)
Melting point 120 °C (248 °F; 393 K)
Solubility in water low
EU classification Toxic (T); Corrosive (C)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Paraformaldehyde (PFA) is the smallest polyoxymethylene, the polymerization product of formaldehyde with a typical degree of polymerization of 8–100 units. Paraformaldehyde commonly has a slight odor of formaldehyde due to decomposition. Paraformaldehyde is a poly-acetal.


Paraformaldehyde forms slowly in aqueous formaldehyde solutions as a white precipitate, especially if stored in the cold. Formalin actually contains very little monomeric formaldehyde; most of it forms short chains of Polyformaldehyde. A small amount of methanol is often added as a stabilizer to limit the extent of polymerization.


Paraformaldehyde can be depolymerized to formaldehyde gas by dry heating[1] and to form a formaldehyde solution by water in the presence of a base or heat. The very pure formaldehyde solutions obtained in this way are used as a fixative for microscopy and histology.

The resulting formaldehyde gas from dry heating paraformaldehyde is flammable.


Once paraformaldehyde is depolymerized, the resulting formaldehyde may be used as a fumigant, disinfectant, fungicide, and fixative. Longer chain-length (high molecular weight) polyoxymethylenes are used as a thermoplastic and are known as polyoxymethylene plastic (POM, Delrin). It was used in the past in the discredited Sargenti method of root canal treatment.[2]

Paraformaldehyde is not a fixative--it must be depolymerized to formaldehyde in solution. In cell culture, a typical formaldehyde fixing procedure would involve using a 4% formaldehyde solution in phosphate buffered saline (PBS) on ice for 10 minutes.


As a formaldehyde releasing agent, paraformaldehyde is a suspected carcinogen. Its acute oral LD50 in rats is 592 mg/kg.

See also[edit]


  1. ^ Yates, J (1973). "Adsorption and decomposition of formaldehyde on tungsten (100) and (111) crystal planes". Journal of Catalysis 30 (2): 260. doi:10.1016/0021-9517(73)90073-0. 
  2. ^