Formic acid, which has a pungent, penetrating odor, is found naturally in insects, weeds, fruits and vegetables, and forest emissions. It appears in mostants and instingless bees of the genusOxytrigona.[6][7]Wood ants from the genusFormica can spray formic acid on their prey or to defend the nest. Thepuss moth caterpillar (Cerura vinula) sprays it as well when threatened by predators. It is also found in thetrichomes ofstinging nettle (Urtica dioica). Apart from that, this acid is incorporated in many fruits such as pineapple (0.21 mg per 100 g), apple (2 mg per 100 g) and kiwi (1 mg per 100 g), and in many vegetables, namely onion (45 mg per 100 g), eggplant (1.34 mg per 100 g), and in extremely low concentrations, cucumber (0.11 mg per 100 g).[8] Formic acid is a naturally occurring component of theatmosphere primarily due to forest emissions.[9]
As early as the 15th century, somealchemists andnaturalists were aware that ant hills give off an acidic vapor. The first person to describe the isolation of this substance (by the distillation of large numbers of ants) was English naturalistJohn Ray, in 1671.[10][11] Ants secrete the formic acid for attack and defense purposes. Formic acid was first synthesized fromhydrocyanic acid by French chemistJoseph Gay-Lussac. In 1855, another French chemist,Marcellin Berthelot, developed a synthesis fromcarbon monoxide similar to the process used today.[12]
Formic acid was long considered achemical compound of only minor interest in the chemical industry. In the late 1960s, significant quantities became available as a byproduct ofacetic acid production. It now finds increasing use as a preservative and antibacterial inlivestock feed.[12]
Cyclic dimer of formic acid; dashedgreen lines represent hydrogen bonds
Formic acid is a colorless liquid having a pungent, penetrating odor[13] at room temperature, comparable to the relatedacetic acid. Formic acid is about 10 times stronger of an acid than acetic acid; its (logarithmic) dissociation constant (pKa) is 3.745, compared to the pKa of 4.756 for acetic acid.[3]
It ismiscible with water and most polarorganicsolvents, and is somewhat soluble inhydrocarbons. In hydrocarbons and in the vapor phase, it consists ofhydrogen-bondeddimers rather than individual molecules.[14][15] Owing to its tendency to hydrogen-bond, gaseous formic acid does not obey theideal gas law.[15] Solid formic acid, which can exist in either of twopolymorphs, consists of an effectively endless network of hydrogen-bonded formic acid molecules. Formic acid forms a high-boilingazeotrope with water (107.3 °C; 77.5% formic acid). Liquid formic acid tends tosupercool.
Formic acid shares most of the chemical properties of othercarboxylic acids. Because of its high acidity, solutions in alcohols form esters spontaneously; inFischer esterifications of formic acid, it self-catalyzes the reaction and no additional acid catalyst is needed.[19] Formic acid shares some of thereducing properties ofaldehydes, reducing solutions of metal oxides to their respective metal.[20]
Formic acid is unique in its ability to participate in addition reactions withalkenes. Formic acids and alkenes readily react to form formateesters. In the presence of certain acids, includingsulfuric andhydrofluoric acids, however, a variant of theKoch reaction occurs instead, and formic acid adds to the alkene to produce a larger carboxylic acid.[23]
In 2009, the worldwide capacity for producing formic acid was 720 thousand tonnes (1.6 billion pounds) per year, roughly equally divided between Europe (350 thousand tonnes or 770 million pounds, mainly in Germany) and Asia (370 thousand tonnes or 820 million pounds, mainly in China) while production was below 1 thousand tonnes or 2.2 million pounds per year in all other continents.[25] It is commercially available in solutions of various concentrations between 85 and 99 w/w %.[14] As of 2009[update], the largest producers areBASF,Eastman Chemical Company,LC Industrial, andFeicheng Acid Chemicals, with the largest production facilities inLudwigshafen (200 thousand tonnes or 440 million pounds per year, BASF, Germany),Oulu (105 thousand tonnes or 230 million pounds, Eastman, Finland),Nakhon Pathom (n/a, LC Industrial), andFeicheng (100 thousand tonnes or 220 million pounds, Feicheng, China). 2010 prices ranged from around €650/tonne (equivalent to around $800/tonne) in Western Europe to $1250/tonne in the United States.[25]
In industry, 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 issodium methoxide.Hydrolysis of the methyl formate produces formic acid:
HCO2CH3 + H2O → HCOOH + CH3OH
Efficient hydrolysis of methyl formate requires a large excess of water. Some routes proceed indirectly by first treating the methyl formate withammonia to giveformamide, which is then hydrolyzed withsulfuric acid:
HCO2CH3 + NH3 → HC(O)NH2 + CH3OH
2 HC(O)NH2 + 2H2O + H2SO4 → 2HCO2H + (NH4)2SO4
A disadvantage of this approach is the need to dispose of theammonium sulfate byproduct. This problem has led some manufacturers to develop energy-efficient methods of separating formic acid from the excess water used in direct hydrolysis. In one of these processes, used byBASF, the formic acid is removed from the water byliquid-liquid extraction with an organic base.[citation needed]
A significant amount of formic acid is produced as a byproduct in the manufacture of other chemicals. At one time, acetic acid was produced on a large scale by oxidation ofalkanes, by a process that cogenerates significant formic acid.[14] This oxidative route to acetic acid has declined in importance, so the aforementioned dedicated routes to formic acid have become more important.[citation needed]
The catalytichydrogenation of CO2 to formic acid has long been studied. This reaction can be conducted homogeneously.[26][27][28] Electrolytic reduction of CO2 to formate has been investigated but not commercialized.
Formic acid can also be obtained by aqueous catalytic partial oxidation of wet biomass by theOxFA process.[29][30] AKeggin-type polyoxometalate (H5PV2Mo10O40) is used as the homogeneous catalyst to convert sugars, wood, waste paper, or cyanobacteria to formic acid and CO2 as the sole byproduct. Yields of up to 53% formic acid can be achieved.[citation needed]
In the laboratory, formic acid can be obtained by heatingoxalic acid inglycerol followed by steam distillation.[31] Glycerol acts as a catalyst, as the reaction proceeds through a glyceryl oxalate intermediate. If the reaction mixture is heated to higher temperatures,allyl alcohol results. The net reaction is thus:
Formic acid is named after ants which have high concentrations of the compound in their venom, derived fromserine through a5,10-methenyltetrahydrofolate intermediate.[34] The conjugate base of formic acid, formate, also occurs widely in nature. Anassay for formic acid in body fluids, designed for determination of formate after methanol poisoning, is based on the reaction of formate with bacterialformate dehydrogenase.[35]
A major use of formic acid is as apreservative andantibacterial agent in livestock feed. It arrests certain decay processes and causes the feed to retain itsnutritive value longer.
In Europe, it is applied onsilage, including fresh hay, to promote the fermentation oflactic acid and to suppress the formation ofbutyric acid; it also allows fermentation to occur quickly, and at a lower temperature, reducing the loss of nutritional value.[14] It is widely used to preserve winter feed forcattle,[36] and is sometimes added topoultry feed to killE. coli bacteria.[37][38] Use as a preservative for silage and other animal feed constituted 30% of the global consumption in 2009.[25]
Electrolytic conversion of electrical energy to chemical fuel has been proposed as a large-scale source of formate by various groups.[42] The formate could be used as feed to modifiedE. coli bacteria for producingbiomass.[43][44] Naturalmethylotroph microbes can feed on formic acid or formate.
Formic acid has been considered as a means ofhydrogen storage.[45] The co-product of this decomposition, carbon dioxide, can be rehydrogenated back to formic acid in a second step. Formic acid contains 53 g/L hydrogen at room temperature and atmospheric pressure, which is three and a half times as much as compressed hydrogen gas can attain at 350 bar pressure (14.7 g/L). Pure formic acid is a liquid with aflash point of 69 °C, much higher than that of gasoline (−40 °C) or ethanol (13 °C).[citation needed]
It is possible to use formic acid as an intermediary to produceisobutanol from CO2 using microbes.[46][47]
Formic acid has a potential application insoldering. Due to its capacity to reduce oxide layers, formic acid gas can be blasted at an oxide surface to increase solderwettability.[48]
Formic acid is used as a volatile pH modifier inHPLC andcapillary electrophoresis. Formic acid is often used as a component of mobile phase inreversed-phasehigh-performance liquid chromatography (RP-HPLC) analysis and separation techniques for the separation of hydrophobic macromolecules, such as peptides, proteins and more complex structures including intact viruses. Especially when paired withmass spectrometry detection, formic acid offers several advantages over the more traditionally usedphosphoric acid.[49][50]
Formic acid is also significantly used in the production of leather, includingtanning (23% of the global consumption in 2009[25]), and in dyeing and finishing textiles (9% of the global consumption in 2009[25]) because of its acidic nature. Use as a coagulant in theproduction of rubber[14] consumed 6% of the global production in 2009.[25]
Formic acid is also used in place of mineral acids for various cleaning products,[14] such aslimescale remover andtoilet bowl cleaner. Some formateesters are artificial flavorings and perfumes.
Formic acid application has been reported to be an effective treatment forwarts.[51]
In the nuclear industry, formic acid is used as the main component for the decomposition of residual nitric acid in denitrification during spent nuclear fuel reprocessing[52]. The process can be carried out either chemically or using catalysts.[53]
Formic acid has low toxicity (hence its use as a food additive), with anLD50 of 1.8g/kg (tested orally on mice). The concentrated acid is corrosive to the skin.[14]
Formic acid is readily metabolized and eliminated by the body. Nonetheless, it has specifictoxic effects; the formic acid andformaldehyde produced as metabolites ofmethanol are responsible for theoptic nerve damage, causing blindness, seen inmethanol poisoning.[54] Some chronic effects of formic acid exposure have been documented. Some experiments on bacterial species have demonstrated it to be amutagen.[55] Chronic exposure in humans may cause kidney damage.[55] Another possible effect of chronic exposure is development of a skinallergy that manifests upon re-exposure to the chemical.
Concentrated formic acid slowly decomposes to carbon monoxide and water, leading to pressure buildup in the containing vessel. For this reason, formic acid is stored with degassing caps in a well-ventilated area.[56]
Formic acid in 85% concentration is flammable, and diluted formic acid is on the U.S. Food and Drug Administration list of food additives.[57] The principal danger from formic acid is from skin or eye contact with the concentrated liquid or vapors. The U.S.OSHA Permissible Exposure Level (PEL) of formic acid vapor in the work environment is 5parts per million (ppm) of air.[58]
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