Paraformaldehyde (PFA) is the smallest

polyoxymethylene Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic pol ...

, 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 In organic chemistry, an acetal is a functional group with the connectivity . Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragments n ...

Synthesis

Paraformaldehyde forms slowly in aqueous formaldehyde solutions as a white precipitate, especially if stored in the cold.

Formalin Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...

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 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 Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of mic ...

and

histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...

. The resulting formaldehyde gas from dry heating paraformaldehyde is flammable.

Uses

Once paraformaldehyde is depolymerized, the resulting formaldehyde may be used as a

fumigant Fumigation is a method of pest control or the removal of harmful micro-organisms by completely filling an area with gaseous pesticides—or fumigants—to suffocate or poison the pests within. It is used to control pests in buildings (s ...

disinfectant A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than st ...

fungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...

, and fixative. Longer chain-length (high molecular weight) polyoxymethylenes are used as a thermoplastic and are known as

plastic (POM, ''Delrin''). It was used in the past in the discredited Sargenti method of

root canal treatment Root canal treatment (also known as endodontic therapy, endodontic treatment, or root canal therapy) is a treatment sequence for the infected pulp of a tooth which is intended to result in the elimination of infection and the protection o ...

. 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 Chromatin immunoprecipitation (ChIP) is a type of immunoprecipitation experimental technique used to investigate the interaction between proteins and DNA in the cell. It aims to determine whether specific proteins are associated with specific genom ...

) 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 Melamine is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millio ...

, phenol or other reactive agents in the manufacturing of

particle board Particle board, also known as chipboard or low-density fiberboard, is an engineered wood product manufactured from wood chips and a synthetic resin or other suitable binder, which is pressed and extruded. Particle board is often confused with o ...

, medium density

fiberboard Fiberboard (American English) or fibreboard (British English) is a type of engineered wood product that is made out of wood fibers. Types of fiberboard (in order of increasing density) include particle board or low-density fiberboard (LDF), medi ...

and plywood.

Toxicity

As a formaldehyde releasing agent, paraformaldehyde is a potential

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substa ...

. Its acute oral

median lethal dose In toxicology, the median lethal dose, LD50 (abbreviation for "lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 is a toxic unit that measures the lethal dose of a toxin, radiation, or pathogen. The value of LD50 for a substance is t ...

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

References

Polyethers Hemiacetals

Synthesis

Reactions

Uses

Toxicity

See also

References