Paraformaldehyde (PFA) is the smallest
polyoxymethylene, the
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
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 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
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
.
Reactions
Paraformaldehyde can be
depolymerized to formaldehyde gas by dry heating and to formaldehyde solution by
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
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 subjects too small to 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 microscopy: optical mic ...
and
histology
Histology,
also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...
.
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
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 ...
,
fungicide
Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...
, 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.
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,
phenol
Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire.
The molecule consists of a phenyl group () ...
or other reactive agents in the manufacturing of
particle board, medium density
fiberboard
Fiberboard (American English) or fibreboard (Commonwealth 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 ...
and
plywood.
Toxicity
As a formaldehyde releasing agent, paraformaldehyde is a potential
carcinogen
A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...
. Its acute oral
median lethal dose in rats is 592 mg/kg.
See also
*
1,3,5-Trioxane (metaformaldehyde), the cyclic trimer of formaldehyde
References
{{Authority control
Polyethers
Hemiacetals