|Molecular formula||OH(CH2O)nH (n = 8 - 100)|
|Appearance||white crystalline solid|
|Density||1.42 g·cm−3 (25 °C)|
|Solubility in water||low|
|EU classification||Toxic (T); Corrosive (C)|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Paraformaldehyde (PFA) is the smallest polyoxymethylene, the polymerization product of formaldehyde with a typical degree of polymerization of 8–10 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 and to 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 thermoplastic and are known as polyoxymethylene plastic (POM, Delrin). It was used in the past in the discredited Sargenti method of root canal treatment.
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.
- 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.