Oxalic acid is anorganic acid with the systematic nameethanedioic acid andchemical formulaHO−C(=O)−C(=O)−OH, also written as(COOH)2 or(CO2H)2 orH2C2O4. It is the simplestdicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name is derived from early investigators who isolated oxalic acid fromflowering plants of the genusOxalis, commonly known as wood-sorrels. It occurs naturally in many foods. It can be toxic when eaten in significant quantities, and contact with concentrated forms can cause chemical burns.
The preparation of salts of oxalic acid from plants had been known since at least 1745, when the Dutch botanist and physicianHerman Boerhaave isolated a salt fromwood sorrel, akin tokraft process.[9][10]By 1773, François Pierre Savary of Fribourg,Switzerland had isolated oxalic acid from its salt in sorrel.[11][12]
In 1776, Swedish chemistsCarl Wilhelm Scheele andTorbern Olof Bergman[13][14] produced oxalic acid by reacting sugar with concentratednitric acid; Scheele called the acid that resultedsocker-syra orsåcker-syra (sugar acid). By 1784, Scheele had shown that "sugar acid" and oxalic acid from natural sources were identical.[15] The modern name was introduced (along withmany other acid names) in 1787, byde Morveau,Lavoisier and co-authors.[16]
In 1824, the German chemistFriedrich Wöhler obtained oxalic acid by reactingcyanogen with ammonia in aqueous solution.[17] This experiment may represent the first synthesis of anatural product.[18]
Anhydrous oxalic acid exists as twopolymorphs; in one thehydrogen-bonding results in a chain-like structure, whereas the hydrogen bonding pattern in the other form defines a sheet-like structure.[25] Because the anhydrous material is both acidic andhydrophilic (water seeking), it is used inesterifications.
Oxalic acid's pKa values vary in the literature from 1.25 to 1.46 and from 3.81 to 4.40.[28][29] The 100th ed of the CRC, released in 2019, has values of 1.25 and 3.81.[30]Oxalic acid is relatively strong compared to othercarboxylic acids:
H2C2O4 ⇌ HC2O−4 + H+
pKa1 = 1.27
HC2O−4 ⇌ C2O2−4 + H+
pKa2 = 4.27
Oxalic acid undergoes many of the reactions characteristic for other carboxylic acids. It forms esters such asdimethyl oxalate (m.p. 52.5 to 53.5 °C, 126.5 to 128.3 °F).[31] It forms an acid chloride calledoxalyl chloride.
Oxalic acid is an important reagent inlanthanide chemistry. Hydrated lanthanide oxalates form readily in very strongly acidic solutions as a denselycrystalline, easily filtered form, largely free of contamination by nonlanthanide elements:
2 Ln3+ + 3 H2C2O4 → Ln2(C2O4)3 + 6 H+
Thermal decomposition of these oxalates gives theoxides, which is the most commonly marketed form of these elements.[33]
Oxalic acid vapor decomposes at 125–175 °C (257–347 °F) toCO2 andformic acid (HCOOH).Photolysis with 237–313nmUV light also produces carbon monoxide (CO) and water.[35]
Evaporation of a solution ofurea and oxalic acid in 2:1 molar ratio yields a solid crystalline compoundH2C2O4·2CO(NH2)2, consisting of stacked two-dimensional networks of the neutral molecules held together byhydrogen bonds with the oxygen atoms.[36]
At least two pathways exist for the enzyme-mediated formation of oxalate. In one pathway,oxaloacetate, a component of theKrebs citric acid cycle, is hydrolyzed to oxalate and acetic acid by the enzymeoxaloacetase:[37]
Plants of the genusFenestraria produce optical fibers made from crystalline oxalic acid to transmit light to subterranean photosynthetic sites.[40]
Carambola, also known as starfruit, also contains oxalic acid along withcaramboxin. Citrus juice contains small amounts of oxalic acid.
The formation of naturally occurring calcium oxalatepatinas on certainlimestone andmarble statues and monuments has been proposed to be caused by the chemical reaction of the carbonate stone with oxalic acid secreted bylichen or othermicroorganisms.[41][42]
Many soil fungus species secrete oxalic acid, which results in greater solubility of metal cations and increased availability of certain soil nutrients, and can lead to the formation of calcium oxalate crystals.[43][44] Some fungi such asAspergillus niger have been extensively studied for the industrial production of oxalic acid;[45] however, those processes are not yet economically competitive with production from oil and gas.[46]Cryphonectria parasitica may excrete oxalic acid containing solutions at the advancing edge of its chestnutcambium infection. The lower pH (<2.5) of more concentrated oxalic acid excretions may degrade cambium cell walls and have a toxic effect on chestnut cambium cells. Cambium cells that burst provide nutrients for a blight infection advance.[47][48]
Theconjugate base of oxalic acid is the hydrogenoxalate anion, and its conjugate base (oxalate) is acompetitive inhibitor of thelactate dehydrogenase (LDH) enzyme.[49] LDH catalyses the conversion ofpyruvate tolactic acid (end product of the fermentation (anaerobic) process) oxidising the coenzymeNADH toNAD+ andH+ concurrently. Restoring NAD+ levels is essential to the continuation of anaerobic energy metabolism throughglycolysis. As cancer cells preferentially use anaerobic metabolism (seeWarburg effect) inhibition of LDH has been shown to inhibit tumor formation and growth,[50] thus is an interesting potential course of cancer treatment.
Oxalic acid plays a key role in the interaction between pathogenic fungi and plants. Small amounts of oxalic acid enhances plant resistance to fungi, but higher amounts cause widespread programmed cell death of the plant and help with fungi infection. Plants normally produce it in small amounts, but some pathogenic fungi such asSclerotinia sclerotiorum cause a toxic accumulation.[51]
Oxalate, besides being biosynthesised, may also be biodegraded.Oxalobacter formigenes is an important gut bacterium that helps animals (including humans) degrade oxalate.[52]
Oxalic acid's main applications include cleaning or bleaching, especially for the removal of rust (iron complexing agent). It is the primary active ingredient inBar Keepers Friend brand cleaner. Its utility in rust removal agents is due to its forming a stable, water-soluble salt with ferric iron,ferrioxalate ion.
Oxalic acid is an ingredient in some tooth whitening products. About 25% of produced oxalic acid is used as amordant in dyeing processes. It is also used inbleaches, especially forpulpwood, cork, straw, cane, feathers, and for rust removal and other cleaning, in baking powder, and as a third reagent in silica analysis instruments.
Dilute solutions (0.05–0.15M) of oxalic acid can be used to remove iron from clays such askaolinite to produce light-coloredceramics.[54]
Oxalic acid can be used to clean minerals like many other acids. Two such examples are quartz crystals and pyrite.[55][56][57]
Oxalic acid is sometimes used in the aluminumanodizing process, with or without sulfuric acid.[58] Compared to sulfuric-acid anodizing, the coatings obtained are thinner and exhibit lower surface roughness.
Oxalic acid is also widely used as a wood bleach; most often it is supplied in its crystalline form, so as to be mixed with water to its proper dilution for use.[citation needed]
Oxalic acid is also used in electronic and semiconductor industries. In 2006 it was reported being used inelectrochemical–mechanical planarization of copper layers in the semiconductor devices fabrication process.[59]
Oxalic acid has an oralLDLo (lowest published lethal dose) of 600 mg/kg.[66] It has been reported that the lethal oral dose is 15 to 30 grams.[67] The toxicity of oxalic acid is due to kidney failure caused by precipitation of solidcalcium oxalate.[68]
^Apelblat, Alexander; Manzurola, Emanuel (1987). "Solubility of oxalic, malonic, succinic, adipic, maleic, malic, citric, and tartaric acids in water from 278.15 to 338.15 K".The Journal of Chemical Thermodynamics.19 (3):317–320.Bibcode:1987JChTh..19..317A.doi:10.1016/0021-9614(87)90139-X.
^CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. William M. Haynes, David R. Lide, Thomas J. Bruno (2016-2017, 97th ed.). Boca Raton, FL. 2016.ISBN978-1-4987-5428-6.OCLC930681942.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)
^Boerhaave, Herman (1745).Elementa Chemiae. Vol. 2. Basil, Switzerland: Johann Rudolph Im-hoff. pp. 35–38.(in Latin) From p. 35:"Processus VII. Sal nativum plantarum paratus de succo illarum recens presso. Hic Acetosae." (Procedure 7. A natural salt of plants prepared from their freshly pressed juice. This [salt obtained] from sorrel.)
^H. E., Roscoe; Schorlemmer, C. (1890).A Treatise on Chemistry. Vol. 3. New York, NY: D. Appleton and Co. p. 105.
^Savary, F. P. (1773).Dissertatio Inauguralis De Sale Essentiali Acetosellæ. Jean François Le Roux. p. 17."Unum adhuc circa liquorem acidum, quem sal acetosellae tam sincerissimum a nobis paratum quam venale destillatione fundit phoenomenon erit notandum, nimirum quod aliquid ejus sub forma sicca crystallina lateribus excipuli accrescat, ..." (One more [thing] will be noted regarding the acid liquid, which furnished for us sorrel salt as pure as commercial distillations, [it] produces a phenomenon, that evidently something in dry, crystalline form grows on the sides of the receiver, ...)
^T., Bergman (1776)."VIII. De acido Sacchari".Opuscula Physica et Chemica. Vol. 1. Leipzig (Lipsia), Germany: I.G. Müller. pp. 238–263.
^Scheele, Carl W. (1784). "Om Rhabarber-jordens bestånds-delar, samt sått at tilreda Acetosell-syran".Kungliga Vetenskapsakademiens Nya Handlingar. 2. Vol. 5. pp. 183–187.(in Swedish) From p. 187:"Således finnes just samma syra som vi genom konst af socker med tilhjelp af salpeter-syra tilreda, redan förut af naturen tilredd uti o̊rten Acetosella." (Thus it is concluded [that] the very same acid as we prepare artificially by means of sugar with the help of nitric acid, [was] previously prepared naturally in the herbacetosella [i.e., sorrel].)
^Eiichi, Yonemitsu; Tomiya, Isshiki; Tsuyoshi, Suzuki; Yukio, Yashima "Process for the production of oxalic acid",U.S. patent 3,678,107, priority date March 15, 1969
^Wilhelm Riemenschneider and Minoru Tanifuji (October 15, 2011).Oxalic acid, in Ullmann's Encyclopedia of Industrial Chemistry.doi:10.1002/14356007.a18_247.pub2.
^Higgins, James; Zhou, Xuefeng; Liu, Ruifeng; Huang, Thomas T.-S. (1997). "Theoretical Study of Thermal Decomposition Mechanism of Oxalic Acid".The Journal of Physical Chemistry A.101 (14):2702–2708.Bibcode:1997JPCA..101.2702H.doi:10.1021/jp9638191.
^Dutton, M. V.; Evans, C. S. (1996). "Oxalate production by fungi: Its role in pathogenicity and ecology in the soil environment".Canadian Journal of Microbiology.42 (9):881–895.doi:10.1139/m96-114..
^Rombauer, Rombauer Becker, and Becker (1931/1997).Joy of Cooking, p.415.ISBN0-684-81870-1.
^Siener, Roswitha; Honow, Ruth; Seidler, Ana; Voss, Susanne; Hesse, Albrecht (2006). "Oxalate contents of species of the Polygonaceae, Amaranthaceae, and Chenopodiaceae families".Food Chemistry.98 (2):220–224.doi:10.1016/j.foodchem.2005.05.059.
^Attenborough, David. "Surviving."The Private Life of Plants: A Natural History of Plant Behaviour. Princeton, NJ: Princeton UP, 1995. 265+."OpenLibrary.org: The Private Life of Plants" Print.
^Sabbioni, Cristina; Zappia, Giuseppe (2016). "Oxalate patinas on ancient monuments: The biological hypothesis".Aerobiologia.7:31–37.doi:10.1007/BF02450015.S2CID85017563.
^Frank-Kamemetskaya, Olga; Rusakov, Alexey; Barinova, Ekaterina; Zelenskaya, Marina; Vlasov, Dmitrij (2012). "The Formation of Oxalate Patina on the Surface of Carbonate Rocks Under the Influence of Microorganisms".Proceedings of the 10th International Congress for Applied Mineralogy (ICAM). pp. 213–220.doi:10.1007/978-3-642-27682-8_27.ISBN978-3-642-27681-1.
^Dutton, Martin V.; Evans, Christine S. (1 September 1996). "Oxalate production by fungi: its role in pathogenicity and ecology in the soil environment".Canadian Journal of Microbiology.42 (9):881–895.doi:10.1139/m96-114.
^Havir, Evelyn; Anagnostakis, Sandra (November 1983). "Oxalate production by virulent but not by hypovirulent strains of Endothia parasitica".Physiological Plant Pathology.23 (3):369–376.doi:10.1016/0048-4059(83)90021-8.
^Durham, Sharon."Making Spinach with Low Oxalate Levels".AgResearch Magazine. No. January 2017. United States Department of Agriculture. Retrieved26 June 2017.The scientists analyzed oxalate concentrations in 310 spinach varieties—300 USDA germplasm accessions and 10 commercial cultivars. "These spinach varieties and cultivars displayed oxalate concentrations from 647.2 to 1286.9 mg/100 g on a fresh weight basis," says Mou.
