:Paraformaldehyde
{{Distinguish|Paraldehyde}}
{{chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 446640299
| ImageFile = Paraformaldehyd.svg
| ImageSize = 150px
| IUPACName = Polyoxymethylene
| OtherNames =
|Section1={{Chembox Identifiers
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 30525-89-4
| EINECS = 608-494-5
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = Y19UC83H8E
| UNNumber = 2213
| PubChem =
| ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}
| ChemSpiderID = none
| SMILES = }}
|Section2={{Chembox Properties
| Formula = OH(CH2O)nH (n = 8 - 100)
| MolarMass =
| Appearance = White powder with formaldehyde-like odour
| Density = 1.46 g/cm3
| MeltingPtC = 120
| BoilingPt =
| VaporPressure = 1.579 kPa
| Solubility = slightly soluble}}
|Section3={{Chembox Hazards
| GHSPictograms = {{GHS02}}{{GHS08}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|228|302|315|317|318|332|334|351}}
| PPhrases = {{P-phrases|}}
| ExternalSDS = [https://www.fishersci.com/store/msds?partNumber=O4042500&productDescription=PARAFORMALDEHYDE+R+500G&vendorId=VN00033897&countryCode=US&language=en Fisher Scientific]
| FlashPtC = 71
| AutoignitionPtC = 300
| LD50 = 800 mg/kg (rat, oral)
| LC50 = 1070 mg/m3 (rat, 4h)
| ExploLimits = 7.0% (low), 73% (high)
| NFPA-H = 3
| NFPA-F = 2
| NFPA-I = 1
}}
}}
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{{cite journal | doi = 10.1016/0021-9517(73)90073-0 | title = Adsorption and decomposition of formaldehyde on tungsten (100) and (111) crystal planes | year = 1973 | last1 = Yates | first1 = J | journal = Journal of Catalysis | volume = 30 | page = 260 | issue = 2 }} and to formaldehyde solution by water in the presence of a base, an acid or heat. The high purity 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.{{Cite web|url=http://www.dentalwatch.org/questionable/sargenti/overview.html|title = Be Wary of Sargenti Root Canal Treatment | Quackwatch|date = May 2018}}
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. In histology and pathology specimens preparation, usually, the fixation step is performed using 10% Neutral Buffered Formalin (4% formaldehyde) for, at least, 24 hours.
Paraformaldehyde is also used to crosslink proteins to DNA, as used in ChIP (chromatin immunoprecipitation) which is a technique to determine which part of DNA certain proteins are binding to.
Paraformaldehyde can be used as a substitute of aqueous formaldehyde to produce the resinous binding material, which is commonly used together with melamine, phenol or other reactive agents in the manufacturing of particle board, medium density fiberboard and plywood.{{Cite web|url=http://dovechem.co.id/en/paraformaldehyde-3|title = Paraformaldehyde - Dover Chemical}}
Toxicity
As a formaldehyde releasing agent, paraformaldehyde is a potential carcinogen.{{cite journal|last1=Cogliano|first1=Vincent|last2=Grosse|first2=Yann|last3=Baan|first3=Robert|last4=Straif|first4=Kurt|last5=Secretan|first5=Béatrice|last6=Ghissassi|first6=Fatiha El|title=Advice on formaldehyde and glycol ethers|journal=The Lancet Oncology|date=September 2004|volume=5|issue=9|page=528|doi=10.1016/S1470-2045(04)01562-1|pmid=15384217}} Its acute oral median lethal dose in rats is 592 mg/kg.{{cite web|title=MSDS - 158127 SAFETY DATA SHEET - Paraformaldehyde|url=https://www.sigmaaldrich.com/US/en/product/sial/158127
|publisher=SIGMA-ALDRICH|access-date=15 February 2022}}
See also
- 1,3,5-Trioxane (metaformaldehyde), the cyclic trimer of formaldehyde