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Formic acid



Name Formic acid


Chemical formula CH2O2
Formula weight 46.03 amu
Synonyms methanoic acid, aminic acid, hydrogencarboxylic acid
CAS number 64-18-6

Phase behavior

Melting point 281.5 K (8.4 °C)
Boiling point 373.9 K (100.8°C)
Triple point 281.4 K (8.3°C)
0.0236 bar
Liquid density 1.22 ×103 kg/m3

Acid-base properties

pKa 3.75

Liquid thermochemistry

ΔfH0liquid -425.1 kJ/mol
S0liquid 131.8 J/mol·K
ΔfusH 12.7 kJ/mol
ΔfusS 45.1 J/mol·K
Cp 99.0 J/mol·K

Gas thermochemistry

ΔfH0gas -378.6 kJ/mol
S0gas 248.7 J/mol·K
ΔvapH 22.7 kJ/mol
Cp 45.7 J/mol·K


Acute effects Corrosive. Contact with concentrated vapors or solution can cause severe chemical burns.
Chronic effects Repeated exposure can result in an allergic reaction. Suspected mutagen, may cause long-term liver and kidney damage.
Flash point 69°C
Autoignition temperature 601°C
Explosive limits 18-57%

More info

Properties NIST WebBook
MSDS Hazardous Chemical Database

SI units were used where possible. Unless otherwise stated, standard conditions were used.

Disclaimer and references

Formic acid (systematically called methanoic acid) is the simplest carboxylic acid. Its formula is CH2O2 or HCOOH. Its structure is shown at right.

In nature, it is found in the stings and bites of many insects of the order Hymenoptera, including bees and ants. It is also the principal irritant in the leaves of the stinging nettle. Its name come from the Latin word for ant, formica, referring to its early isolation by the distillation of ant bodies. A chemical compound such as a salt from the neutralization of formic acid with a base, or an ester derived from formic acid, is referred to as formate (or methanoate). The formate ion has the formula HCOO-.



As early as the 15th century, some alchemists and naturalists were aware that ant hills gave off an acidic vapor. The first person to describe the isolation of this substance (by the distillation of large numbers of dead ants) was the English naturalist John Ray, in 1671. It was first synthesized from hydrocyanic acid by the French chemist Joseph Gay-Lussac. In 1855, another French chemist, Marcellin Berthelot, developed a synthesis from carbon monoxide that is similar to that used today.

In the chemical industry, formic acid was long considered a chemical compound of only minor industrial interest. In the late 1960s, however, significant quantities of it became available as a byproduct of acetic acid production. With its growing use as a preservative and antibacterial in livestock feed, it is now produced for its own sake.


Formic acid is miscible with water and most polar organic solvents, and somewhat soluble in hydrocarbons. In hydrocarbons and in the vapor phase, is actually consists of hydrogen bonded dimers rather than individual molecules. In the gas phase, this hydrogen bonding results in severe deviations from the ideal gas law. Liquid and solid formic acid consists of an infinite network of hydrogen bonded formic acid molecules.

Formic acid shares most of the chemical properties of other carboxylic acids, although it will not form either an acyl chloride or an acid anhydride. Attempts to form either of these derivatives result in carbon monoxide instead. Heat can also cause formic acid to decompose to carbon monoxide. Formic acid shares some of the reducing properties of aldehydes.

Formic acid is unique among the carboxylic acids in its ability to participate in addition reactions with alkenes. Formic acids and alkenes readily react to form formate esters. In the presence of certain acids including sulfuric and hydrofluoric acids, however, a variant of the Koch reaction takes place instead, and formic acid adds to the alkene to produce a larger carboxylic acid.

Most simple formate salts are water soluble.


A significant amount of formic acid is produced as a byproduct in the manufacture of other chemicals, especially acetic acid. However, this production is insufficient to meet the present demand for formic acid, and some formic acid must be produced for its own sake.

When methanol and carbon monoxide are combined in the presence of a strong base, the formic acid derivative methyl formate results, according to the chemical equation


Industrially, this reaction is performed in the liquid phase at elevated pressure. Typical reaction conditions are 80°C and 40 atm. The most widely used base is sodium methoxide . Hydrolysis of the methyl formate produces formic acid:


However, direct hydrolysis of methanol requires a large excess of water to proceed efficiently, and so some producers perform it by an indirect route, first reacting the methyl formate with ammonia to produce formamide, and then hydrolyzing the formamide with sulfuric acid to produce formic acid:

HCONH2 + H2O + ½H2SO4 → HCOOH + ½ (NH4)2SO4

This technique has problems of its own, particularly disposing of the ammonium sulfate byproduct, and so recently, some manufacturers have developed energy-efficient means of separating formic acid from the large excess of water used in direct hydrolysis. In one of these processes, used by BASF, the formic acid is removed from the water by liquid extraction with an organic base.


The principal use of formic acid is as a preservative and antibacterial agent in livestock feed. When sprayed on fresh hay or other silage, it arrests certain decay processes and causes the feed to retain its nutritive value longer, and so it is widely use to preserve winter feed for cattle. In the poultry industry, it is sometimes added to feed to kill salmonella bacteria.

Some beekeepers use formic acid as a miticide against the Varroa mite.

Formic acid is of minor importance in the textile industry and in the tanning of leather. Some formate esters are artificial flavorings or perfumes.


The principal danger from formic acid is from skin or eye contact with liquid formic acid or with the concentrated vapors. Any of these exposure routes can cause severe chemical burns, and eye exposure can result in permanent eye damage. Inhaled vapors may similarly cause irritation or burns in the respiratory tract. Since carbon monoxide may be also be present in formic acid vapors, care should be taken wherever large quantities of formic acid fumes are present. The US OSHA Permissible Exposure Level (PEL ) of formic acid vapor in the work environment is 5 parts per million parts of air (ppm).

Formic acid is readily metabolized and eliminated by the body. Nonetheless, some chronic effects have been documented. Some animal experiments have demonstrated it to be a mutagen, and chronic exposure may cause liver or kidney damage. Another possibility with chronic exposure is development of a skin allergy that manifests upon re-exposure to the chemical.

See the MSDS (Material Safety Data Sheet)

Last updated: 06-01-2005 22:56:53
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