Paraformaldehyde

Paraformaldehyde

Names
IUPAC name Polyoxymethylene
Identifiers
CAS Number	30525-89-4
PubChem	24898648
Properties
Chemical 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
Hazards
Safety data sheet	Oxford MSDS
EU classification (DSD)	Toxic (T); Corrosive (C)
Except where otherwise noted, 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.

Synthesis

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.

Reactions

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.

Uses

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.

Toxicity

As a formaldehyde releasing agent, paraformaldehyde is a potential carcinogen.^[3] Its acute oral LD₅₀ in rats is 592 mg/kg.^[4]

See also

• Polyoxymethylene plastic
• 1,3,5-Trioxane, the cyclic trimer of formaldehyde
• Polymer

References

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. http://www.dentalwatch.org/questionable/sargenti/overview.html
3. Cogliano, Vincent; Grosse, Yann; Baan, Robert; Straif, Kurt; Secretan, Béatrice; Ghissassi, Fatiha El (September 2004). "Advice on formaldehyde and glycol ethers". The Lancet Oncology 5 (9): 528. doi:10.1016/S1470-2045(04)01562-1. Retrieved 27 October 2015.
4. "MSDS - 158127 SAFETY DATA SHEET - Paraformaldehyde". SIGMA-ALDRICH. Retrieved 27 October 2015.