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37% solution of formaldehyde
IUPAC name
Other names
Carbonyl hydride
Formalin (aqueous solution)
Methyl aldehyde
Methylene glycol
Methylene oxide
Molar mass 30.03 g/mol
Appearance Colorless gas
Odor Pungent, unpleasant
Density 0.8153 g/cm3 (−20 °C)
Melting point −92 °C (−134 °F; 181 K)
Boiling point −19 °C (−2 °F; 254 K)
40 g/ 100 ml (20 °C)
Solubility Reacts with ammonia, amines, nitromethane
Miscible with acetone, benzene, CCl4, diethyl ether, ethyl acetate, THF, toluene
Soluble in chloroform, ethanol
Acidity (pKa) 13.27
Safety data sheet MATHESON (anhydrous)
Sigma-Aldrich (37% aq. sol.)
Flash point 64 °C (147.2 °F; 337 K)
Lethal dose or concentration (LD, LC):
100 mg/kg (oral, rat)
333 ppm (mouse, 2 hr)
815 ppm (rat, 30 min)
Related compounds
Related compounds
Formic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Formaldehyde, also known as methanal (or formalin which is a solution of the gas in water/methanol), is the simplest aldehyde with the formula CH2O. It used to be found in most biology labs used as a preservative for dissection specimens, but this use has fallen into disfavor due to its carcinogenic nature.



Formaldehyde is a good electrophile. It is relatively easily oxidized into formic acid and thus formic acid is often a contaminant in fomaldehyde solutions.

Formaldehyde reacts exothermically with aq. ammonia producing hexamine.

Formaldehyde reacts violently with a variety of reagents, such as aniline, nitromethane, perchloric acid, performic acid, magnesium carbonate, and hydrogen peroxide.[1]

In solution, most of the formaldehyde reacts with water to form methylene glycol CH2(OH)2. This is a very unstable compound that exists in equilibrium with formaldehyde and easily releases it.


Formaldehyde is a gas at STP and is usually supplied in a 37% aqueous solution. Formaldehyde has an extremely characteristic and unpleasant pungent odor, which may be perceived differently by each person; Sciencemadness user ave369 for example, characterizes the smell of formaldehyde as "stinging".


A 37% solution of formaldehyde is cheaply available from suppliers such as Elemental Scientific.

The same solution can also be purchased from agricultural and veterinary stores, as a disinfectant. Most often, there will be small amounts of paraformaldehyde, appearing like a white suspension. The solution can only be kept stable for a few months, or up to a year before the concentration of dissolved formaldehyde becomes very low and the solution becomes opaque.

Some countries limit the sale of formaldehyde to the general public due to its carcinogenic effects. Most often, formalin is sold early in spring or late in autumn, so you should search the vet stores during those periods.


The decomposition of 1,2,3-trioxane in strong acidic conditions yields formaldehyde.

However a better option exists, which involves the thermal decomposition of calcium formate which gives off formaldehyde as a gas which must be led into water to form a solution.

Formaldehyde can also be prepared by placing red hot copper into methanol, however this method is not economically viable for the amateur and produces lower quality formaldehyde. You also need large amounts of copper.

Pyrolysis of wood gives off formaldehyde, but many other side products are also produced, making purification difficult.


Formaldehyde can be used along with ammonia to produce hexamine, a solid fuel used for camping and a precursor to many explosives including RDX. The mixture of concentrated sulfuric acid and concentrated (37%) formaldehyde is known as Marquis reagent, a versatile drug testing reagent. Paraformaldehyde can also be formed although the conditions of this reaction are usually out of reach of the amateur, although it does slowly form when left in air for long periods of time.



Formaldehyde is toxic, with a LD50 of 100 mg/kg and is carcinogenic. It will damage the optic nerves and cause blindness (methanol, being metabolized to formaldehyde, will also do this). Pure gaseous formaldehyde is highly flammable. Care must be taken to ensure formaldehyde is not mixed and heated with acetaldehyde as this will produce the dangerous substance acrolein. Formaldehyde will react with hydrochloric acid to create highly carcinogenic chloromethyl ethers.

Skin contact may cause dermatitis or irritation. Formaldehyde readily absorbs through skin, and contact with large amounts is harmful.


Formaldehyde should be kept in closed bottles, away from any heat source. Since it polymerizes at low temperatures, it should be kept at near room temperature. For aq. formaldehyde (formaline), methanol is added as stabilizer.

Due to its volatility and carcinogenity, it's best to keep it in a separate cabinet.


Formaldehyde can be neutralized with sodium hydroxide, reaction that generates sodium formate and methanol. Another convenient route is to add aq. ammonia, reaction which produces hexamine, but also gives off lots of heat.


  1. Sax, 1989; HSDB, 1995

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