# Formic acid

Structural formula
General
Surname Formic acid
other names
• Methanoic acid ( IUPAC )
• Formylic acid
• Formalic acid
• Hydrocarboxylic acid
• FORMIC ACID ( INCI )
Molecular formula CH 2 O 2
Brief description

colorless, pungent smelling liquid

External identifiers / databases
 CAS number 64-18-6 EC number 200-579-1 ECHA InfoCard 100,000,527 PubChem 284 ChemSpider 278 DrugBank DB01942 Wikidata Q161233
properties
Molar mass 46.03 g mol −1
Physical state

liquid

density

1.22 g cm −3 (20 ° C)

Melting point

8 ° C

boiling point

101 ° C (decomposition)

Vapor pressure
• 44.6 h Pa (20 ° C)
• 72.5 hPa (30 ° C)
• 114 hPa (40 ° C)
• 174 hPa (50 ° C)
pK s value

3.77

solubility

miscible with water, ethanol , glycerine and diethyl ether

Refractive index

1.3714 (20 ° C)

safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary

danger

H and P phrases H: 226-302-314-331
EUH: 071
P: 210-280-303 + 361 + 353-304 + 340 + 310-305 + 351 + 338-403 + 233
MAK
• DFG : 5 ml m −3 or 9.5 mg m −3
• Switzerland: 5 ml m −3 or 9.5 mg m −3
Thermodynamic properties
ΔH f 0

−425.0 kJ / mol

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

Formic acid (according to the nomenclature of the IUPAC formic acid lat. Acidum formicum of formica , ant ') is a colorless, corrosive and water-soluble liquid, which is used in nature in many living beings for defense purposes. With the semi-structural formula HCOOH, it is the simplest carboxylic acid and the shortest-chain alkanoic acid ; the carboxy group  (–COOH) determines its properties particularly strongly. The carbon atom has an oxidation state of +2. It can therefore act as a hydride transfer analogous to the carbonyl compounds , hence its reducing effect. The salts of formic acid are called formates (systematically also methanoates ) and have the semi- structural formula (HCOO) n M, where n corresponds to the valence of the metal ion. Examples of formates are sodium formate , HCOONa, and aluminum formate , (HCOO) 3 Al. The esters of formic acid are also called formates.

## history

In the early 15th century, some alchemists and naturalists observed that certain ants (species belonging to the group of scale ants ) secrete an acidic liquid.

Red wood ant ( Formica rufa )
John Ray was the first to isolate formic acid in 1671.

The English naturalist John Ray was the first to isolate formic acid in 1671 by distilling large numbers of ants . In 1792 the doctor Christoph Girtanner wrote the following text about the extraction of formic acid:

“The formic acid is obtained by distillation from the large ants ( Formica rufa ). Ants are distilled over a gentle fire, and the formic acid is obtained in the template. It makes up about half the weight of the ants. Or you wash the ants in cold water, then lay them on a cloth and pour boiling water over them. If you squeeze the ants gently, the acid becomes stronger. To purify the acid, it is subjected to repeated distillation, and to concentrate it, it is allowed to freeze. "

- Christoph Girtanner

The French chemist Joseph Louis Gay-Lussac was the first to synthesize formic acid from hydrogen cyanide . In 1855 another French chemist, Marcellin Berthelot , invented the synthesis from carbon monoxide , which is still used today. For a long time, formic acid was of little technical importance. In the late 1960s, significant amounts of formic acid were produced as a by-product in the synthesis of acetic acid . Only later was formic acid used on a larger scale. It was now no longer just obtained as a by-product, but specifically produced synthetically.

## Occurrence

There is a liquid containing formic acid in the stinging hair of the
nettle

Formic acid is widespread in nature. It is used by many plant and animal species, especially by voices , as a component of poison mixtures for defense and attack purposes.

Formic acid is a natural component of bee honey ; Depending on the variety, 1 kilogram of honey contains 50 to over 1000 milligrams. Formic acid is also a component of tobacco smoke .

The caterpillars of the great fork tail ( Cerura vinula ) - a species of butterfly  - as well as some species of ants (members of the subfamily Formicinae ) spray a liquid containing formic acid as a defense. Some species of ground beetles , scorpions and bees use secretions containing formic acid for both defense and attack purposes. In some species of jellyfish , formic acid is part of the poison in the nettle capsules.

In the stinging hairs of the nettles there is a nettle poison that contains, among other things, formic acid and sodium formate.

In the human body formic acid formed in addition to formaldehyde in the metabolism of methanol . Formic acid is easily biodegradable.

Traces of formic acid in space were detected spectroscopically. In the shell ( coma ) of the comet Hale-Bopp , in 2000 by D. Bockelée-Morvan and others, 0.09% formic acid (based on water = 100%) was used for the first time in addition to other organic compounds such as hydrocyanic acid, acetonitrile, methanol or methyl formate. found. There are two theories for the formation of these compounds: either the compounds were originally located in the nucleus of the comet and flow out of there, or they arise from gas phase reactions in the coma. According to simulations, however, the former is likely due to the distributions.

## Extraction, manufacture

Marcellin Berthelot discovered the synthesis of formic acid from carbon monoxide in 1855

The historical isolation of formic acid from dead ants is no longer carried out today. In the chemical industry, formic acid is usually produced using the process invented by Marcellin Berthelot in 1855. The synthesis is divided into two process steps:

${\ displaystyle \ mathrm {NaOH + CO \ longrightarrow HCOONa}}$
Sodium hydroxide reacts with carbon monoxide at about 6–8 bar and 130 ° C to form sodium formate .
${\ displaystyle \ mathrm {2 \ HCOONa + H_ {2} SO_ {4} \ longrightarrow 2 \ HCOOH + Na_ {2} SO_ {4}}}$
Sodium formate is reacted with sulfuric acid to form formic acid and sodium sulfate .

Formic acid is also produced from methanol with the help of carbon monoxide , among other things . Here, too, two procedural steps are carried out. Methyl formate is produced as an intermediate. In the end, methanol is recovered, which can be used again as a starting product for this synthesis :

${\ displaystyle \ mathrm {CH_ {3} OH + CO \ longrightarrow HCOOCH_ {3}}}$
At 80 ° C and 40 bar, methanol reacts with carbon monoxide to form methyl formate .
${\ displaystyle \ mathrm {HCOOCH_ {3} + H_ {2} O \ longrightarrow HCOOH + CH_ {3} OH}}$
Formic acid methyl ester reacts with water to form formic acid and methanol.

Because the hydrolysis of methyl formate would use a lot of water, some formic acid manufacturers use an indirect process with ammonia , which in turn requires two process steps. However, this indirect process has problems because the by-product ammonium sulfate is partially released:

${\ displaystyle \ mathrm {HCOOCH_ {3} + NH_ {3} \ longrightarrow HCONH_ {2} + CH_ {3} OH}}$
Formic acid methyl ester reacts with ammonia to form formamide and methanol.
${\ displaystyle \ mathrm {2 \ HCONH_ {2} +2 \ H_ {2} O + H_ {2} SO_ {4} \ longrightarrow 2 \ HCOOH + (NH_ {4}) _ {2} SO_ {4}}}$
Formamide reacts with sulfuric acid to form formic acid and ammonium sulfate .

Because of this problem, the manufacturers have developed a new process of direct hydrolysis, in which the formic acid can be separated from the large amounts of water in an energy-efficient manner ( liquid extraction ).

Formic acid occurs as a by-product in the production of acetic acid from light gasoline or n -butane and can also be produced with the help of hydrocyanic acid. There is a second method for production from methanol. Here, methanol is converted to formaldehyde and formic acid. However, these three processes are of little technical importance.

Biosynthetically, formic acid is released from glycine or serine and tetrahydrofolic acid via formyltetrahydrofolic acid in the poison glands of ants.

## properties

### Physical Properties

Dimer formation in formic acid due to hydrogen bonds shown in dashed green .

Formic acid is a relatively unstable, colorless, clear and highly volatile liquid. At 8 ° C it solidifies to a colorless solid; it boils at 100.7 ° C. Melting and boiling points are significantly higher than those of organic compounds with similar molar masses ( e.g. propane ), since hydrogen bonds between the individual molecules also have to be broken when they melt and boil . Some of these persist in the gaseous state, which is why formic acid deviates significantly from the behavior of an ideal gas . It forms an azeotrope with water .

Formic acid has a density of 1.22 g · cm −3 at 20 ° C. To melt the formic acid, 12.7 kJ / mol are required, and 22.7 kJ / mol to evaporate. The triple point is 8.3 ° C and 0.0236 bar.

Formic acid has a strong and pungent odor. The odor threshold is 1 ml / m 3 . Formic acid can be mixed with water, ethanol and glycol in any ratio. It is also soluble in most other polar organic substances, but only in small amounts in hydrocarbons .

The acid constant (p K s value) is 3.77. It is the strongest unsubstituted monocarboxylic acid . For comparison: hydrochloric acid has a p K s value of −7, sulfuric acid of −3.

The detection of formic acid vapors (e.g. to determine the concentration in the workplace ) can be done with the help of gas detection devices . Otherwise, the formic acid is detected through its reducing effect, usually because it can reduce an ammoniacal silver nitrate solution to silver .

### Thermodynamic properties

The standard enthalpy of formation Δ f H 0 liquid is −424.72 kJ mol −1 , Δ f H 0 gas is −378.6 kJ mol −1 .

The standard entropy S 0 liquid is 128.95 J mol −1 K −1 , S 0 gas 248.7 J mol −1 K −1 .

The heat capacity of the liquid is given as 99.04 J mol −1 K −1 (25 ° C), that of the gas as 45.7 J mol −1 K −1 (25 ° C).

According to Antoine, the vapor pressure function results from log 10 (P) = A− (B / (T + C)) (P in bar, T in K) with A = 2.00121, B = 515.000 K and C = −139.408 K. in the temperature range from 273.6 K to 307.3 K.

### Chemical properties

Copper (II) formate hydrate

In the presence of oxygen, formic acid burns to carbon dioxide and water. Formic acid is a strong reducing agent because it is also an aldehyde (hydroxy formaldehyde ).

${\ displaystyle \ mathrm {2 \ HCOOH + O_ {2} \ longrightarrow 2 \ CO_ {2} +2 \ H_ {2} O}}$
Formic acid burns with oxygen to form carbon dioxide and water.
${\ displaystyle \ mathrm {HCOOH \ longrightarrow CO + H_ {2} O}}$
Formic acid decomposes in the presence of conc. Sulfuric acid to water and carbon monoxide .
${\ displaystyle \ mathrm {HCOOH \ longrightarrow CO_ {2} + H_ {2}}}$
At higher temperatures and in the presence of a catalyst ( platinum , palladium ) it breaks down to carbon dioxide and hydrogen .

Formic acid reacts with metals to form metal formates and hydrogen:

${\ displaystyle \ mathrm {2 \ Na + 2 \ HCOOH \ longrightarrow 2 \ HCOONa + H_ {2}}}$

Sodium reacts with formic acid to form sodium formate, forming hydrogen .

${\ displaystyle \ mathrm {HCOOH + 3 \ OH ^ {-} + 2 \ Ag ^ {+} \ longrightarrow HCO_ {3} ^ {-} + 2 \ Ag + 2 \ H_ {2} O}}$
Formic acid reduces silver ions in the alkaline.

Formic acid reacts with alcohols in the presence of a catalyst (usually sulfuric acid) to form water and alkyl formates.

${\ displaystyle \ mathrm {HCOOH + CH_ {3} OH \ longrightarrow H_ {2} O + HCOOCH_ {3}}}$
Formic acid reacts with methanol to form water and methyl formate.

### Safety-related parameters

Formic acid is considered a flammable liquid. Flammable vapor-air mixtures can form above the flash point . The compound has a flash point of 45 ° C. The explosion range is between 10  % by volume (190 g / m 3 ) as the lower explosion limit (LEL) and 45.5% by volume (865 g / m 3 ) as the upper explosion limit (UEL). With a standard gap width of 1.76 mm, it is assigned to explosion group IIA. The ignition temperature is 520 ° C. The substance therefore falls into temperature class T1. The electrical conductivity is low at 6.08 · 10 −3 S · m −1 .

## use

Until 1998, formic acid was used as a preservative in fish, fruit and vegetable products under the E number E236 , but has since then - in contrast to Switzerland - no longer been approved as a food additive in the EU . The related substances sodium and calcium formate are also no longer permitted as food additives (E237 and E238). In medicine it is used as an anti-inflammatory drug, as well as for the treatment of vulgar warts . A ready-to-use solution containing formic acid is applied to the wart. In the textile and leather industry they are used for pickling and impregnating. Sometimes it is also used as a disinfectant (also in acidic cleaning agents). According to the import regulations of the EU it is z. B. used for certain goods from other EU countries to prevent the spread of animal diseases . It also kills bacteria well. In the chemical industry it is used to neutralize alkaline reaction mixtures. In electronics production, formic acid is used as a reducing agent in the soldering process . It is used industrially for the decalcification of cooling water systems, since the resulting wastewater only contains the harmless calcium formate with a low COD value.

Beekeepers usually use them in a 60% concentration, in strictly indicated exceptional cases up to a maximum of 85% concentration in an aqueous solution to treat the bees against the Varroa mite . In genetics , formic acid can be used in conjunction with the enzyme AP endonuclease to create insertion mutants at random , known as in vitro mutagenesis. In the plastics industry it is used to glue polyamide plastics .

Concentrated formic acid is used to clean raw gemstones because it attacks limestone and other impurities heavily, exposing the gemstone without damaging it. This cleaning process should only be used with acid-resistant gemstones.

Scientists at the Leibniz Institute for Catalysis have experimentally achieved the catalytic release of hydrogen from formic acid even at room temperature. This hydrogen could e.g. B. converted to electricity in fuel cells . This possibility is to be used for small-scale storage of energy. The direct formation reaction of formic acid from hydrogen and carbon dioxide is thermodynamically very limited and the efficiency of the corresponding processes is therefore rather low.

## Health hazards

Formic acid can be broken down by the body. Direct contact with formic acid or concentrated vapors is irritating to the respiratory tract and eyes. At concentrations above ten percent, skin contact leads to severe burns and blisters. With long-term exposure, it can cause skin allergies, in extremely high doses as well as chemical burns and necrosis of the oral and pharyngeal mucosa, the esophagus and the gastrointestinal tract, acidosis , unconsciousness, drop in blood pressure, damage to the blood, liver and kidneys as well as pneumonia and damage to the heart cause.

The decomposition of formic acid can produce the breath toxin carbon monoxide . The acid must be stored in a well-ventilated, cool place. Containers in which formic acid is stored must also be closed with a pressure equalization screw connection, since the gases formed during decomposition can generate excess pressure.

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## literature

• Formic acid, formates, diglycol bis-chloroformate. VCH, Weinheim 1992, ISBN 3-527-28529-6 .
• Selected CHO radicals. Formic acid. Acetic acid. Oxalic acid (Gmelin Handbook of Inorganic and Organometallic Chemistry - 8th edition ELEM C TL C LFG 4). Springer, Berlin 1975, ISBN 3-540-93283-6 .
• Gundula Jänsch-Kaiser and Dieter Behrens: Formic acid and alkaline earth hydroxides. DECHEMA Society for Chemical Technology and Biotechnology e. V., ISBN 3-926959-00-2 .