Oxalic acid
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Oxalic acid is an organic acid with the systematic name ethanedioic acid and chemical formula Template:Chem2, also written as Template:Chem2 or Template:Chem2 or Template:Chem2. It is the simplest dicarboxylic 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 from flowering plants of the genus Oxalis, commonly known as wood-sorrels. It occurs naturally in many foods. Excessive ingestion of oxalic acid or prolonged skin contact can be dangerous.
Oxalic acid is a much stronger acid than acetic acid. It is a reducing agent<ref name="Oxalic Acid: Uses">Template:Cite book</ref> and its conjugate bases hydrogenoxalate (Template:Chem2) and oxalate (Template:Chem2) are chelating agents for metal cations. It is used as a cleaning agent, especially for the removal of rust, because it forms a water-soluble ferric iron complex, the ferrioxalate ion. Oxalic acid typically occurs as the dihydrate with the formula Template:Chem2.
History
The preparation of salts of oxalic acid from plants had been known since at least 1745, when the Dutch botanist and physician Herman Boerhaave isolated a salt from wood sorrel, akin to kraft process.<ref>Template:Cite book</ref> <ref>Template:Cite book</ref> By 1773, François Pierre Savary of Fribourg, Switzerland had isolated oxalic acid from its salt in sorrel.<ref>Template:Cite book</ref><ref>Template:Cite book</ref>
In 1776, Swedish chemists Carl Wilhelm Scheele and Torbern Olof Bergman<ref>Template:Cite book</ref><ref>Template:Cite book</ref> produced oxalic acid by reacting sugar with concentrated nitric acid; Scheele called the acid that resulted socker-syra or såcker-syra (sugar acid). By 1784, Scheele had shown that "sugar acid" and oxalic acid from natural sources were identical.<ref>Template:Cite book</ref> The modern name was introduced (along with many other acid names) in 1787, by de Morveau, Lavoisier and co-authors.<ref>Template:Cite web</ref>
In 1824, the German chemist Friedrich Wöhler obtained oxalic acid by reacting cyanogen with ammonia in aqueous solution.<ref>Template:Cite book</ref> This experiment may represent the first synthesis of a natural product.<ref name=Ullmann/>
Production
Industrial
Oxalic acid is mainly manufactured by the oxidation of carbohydrates like glucose using nitric acid or air in the presence of vanadium pentoxide. Another process uses oxygen to regenerate the nitric acid, using a variety of precursors including glycolic acid and ethylene glycol.<ref>Eiichi, Yonemitsu; Tomiya, Isshiki; Tsuyoshi, Suzuki; Yukio, Yashima "Process for the production of oxalic acid", Template:US patent, priority date March 15, 1969</ref> As of 2011, this process was only used by Mitsubishi in Japan.<ref>Template:Cite book</ref> A newer method entails oxidative carbonylation of alcohols to give the diesters of oxalic acid:
These diesters are subsequently hydrolyzed to oxalic acid. Approximately 120,000 tonnes are produced annually.<ref name=Ullmann>Template:Cite book</ref>
Historically oxalic acid was obtained exclusively by using caustics, such as sodium or potassium hydroxide, on sawdust, followed by acidification of the oxalate by mineral acids, such as sulfuric acid.<ref>Template:Cite book</ref> Oxalic acid can also be formed by the heating of sodium formate in the presence of an alkaline catalyst.<ref>Template:Cite web</ref>Template:Better source needed
Laboratory
Although it can be readily purchased, oxalic acid can be prepared in the laboratory by oxidizing sucrose using nitric acid in the presence of a small amount of vanadium pentoxide as a catalyst.<ref name="cohen">Practical Organic Chemistry by Julius B. Cohen, 1930 ed. preparation #42</ref>
The dihydrate can be converted to the anhydrous form by heating or azeotropic distillation.<ref>Template:OrgSynth</ref>
Structure
Anhydrous
Anhydrous oxalic acid exists as two polymorphs; in one the hydrogen-bonding results in a chain-like structure, whereas the hydrogen bonding pattern in the other form defines a sheet-like structure.<ref>Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. Template:ISBN.</ref> Because the anhydrous material is both acidic and hydrophilic (water seeking), it is used in esterifications.
Dihydrate
The dihydrate Template:Chem2 has space group C52h–P21/n, with lattice parameters Template:Nowrap, Template:Nowrap, Template:Nowrap, Template:Nowrap, Template:Nowrap.<ref name=sabi1969>Template:Cite journal</ref> The main inter-atomic distances are: C−C 153 pm, C−O1 129 pm, C−O2 119 pm.<ref name=ahmed1953>Template:Cite journal</ref>
Reactions
Acid–base properties
Oxalic acid's pKa values vary in the literature from 1.25 to 1.46 and from 3.81 to 4.40.<ref>Template:Cite book</ref><ref>Template:Cite book</ref> The 100th ed of the CRC, released in 2019, has values of 1.25 and 3.81.<ref>Template:Cite book</ref> Oxalic acid is relatively strong compared to other carboxylic acids: Template:Block indent Oxalic acid undergoes many of the reactions characteristic for other carboxylic acids. It forms esters such as dimethyl oxalate (m.p. Template:Convert).<ref>Template:OrgSynth</ref> It forms an acid chloride called oxalyl chloride.
Metal-binding properties
Transition metal oxalate complexes are numerous, e.g. the drug oxaliplatin. Oxalic acid has been shown to reduce manganese dioxide (Template:Chem2) in manganese ores to allow the leaching of the metal by sulfuric acid.<ref>Template:Cite journal</ref>
Oxalic acid is an important reagent in lanthanide chemistry. Hydrated lanthanide oxalates form readily in very strongly acidic solutions as a densely crystalline, easily filtered form, largely free of contamination by nonlanthanide elements:
Thermal decomposition of these oxalates gives the oxides, which is the most commonly marketed form of these elements.<ref>Template:Cite book</ref>
Other
Oxalic acid and oxalates can be oxidized by permanganate ion in an autocatalytic reaction.<ref>Template:Cite journal</ref>
Oxalic acid vapor decomposes at Template:Convert to Template:Chem2 and formic acid (Template:Chem2). Photolysis with 237–313 nm UV light also produces carbon monoxide (Template:Chem2) and water.<ref name=zhou1997>Template:Cite journal</ref>
Evaporation of a solution of urea and oxalic acid in 2:1 molar ratio yields a solid crystalline compound Template:Chem2, consisting of stacked two-dimensional networks of the neutral molecules held together by hydrogen bonds with the oxygen atoms.<ref name=hark1972>Template:Cite journal</ref>
Occurrence
Biosynthesis
At least two pathways exist for the enzyme-mediated formation of oxalate. In one pathway, oxaloacetate, a component of the Krebs citric acid cycle, is hydrolyzed to oxalate and acetic acid by the enzyme oxaloacetase:<ref>Template:Cite journal.</ref>
It also arises from the dehydrogenation of glycolic acid, which is produced by the metabolism of ethylene glycol.
Occurrence in foods and plantsTemplate:Anchor
Early investigators isolated oxalic acid from wood-sorrel (Oxalis). Members of the spinach family and the brassicas (cabbage, broccoli, brussels sprouts) are high in oxalates, as are sorrel and umbellifers like parsley.<ref>Rombauer, Rombauer Becker, and Becker (1931/1997). Joy of Cooking, p.415. Template:ISBN.</ref> The leaves and stems of all species of the genus Chenopodium and related genera of the family Amaranthaceae, which includes quinoa, contain high levels of oxalic acid.<ref name="oxalic acid quinoa leaves">Template:Cite journal</ref> Rhubarb leaves contain about 0.5% oxalic acid, and jack-in-the-pulpit (Arisaema triphyllum) contains calcium oxalate crystals. Similarly, the Virginia creeper, a common decorative vine, produces oxalic acid in its berries as well as oxalate crystals in the sap, in the form of raphides. Bacteria produce oxalates from oxidation of carbohydrates.<ref name=Ullmann/>
Plants of the genus Fenestraria produce optical fibers made from crystalline oxalic acid to transmit light to subterranean photosynthetic sites.<ref>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.</ref>
Carambola, also known as starfruit, also contains oxalic acid along with caramboxin. Citrus juice contains small amounts of oxalic acid.
The formation of naturally occurring calcium oxalate patinas on certain limestone and marble statues and monuments has been proposed to be caused by the chemical reaction of the carbonate stone with oxalic acid secreted by lichen or other microorganisms.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref>
Production by fungi
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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Some fungi such as Aspergillus niger have been extensively studied for the industrial production of oxalic acid;<ref name=stra1994>Template:Cite journal</ref> however, those processes are not yet economically competitive with production from oil and gas.<ref name=tkacz2012>Jan S. Tkacz, Lene Lange (2012): Advances in Fungal Biotechnology for Industry, Agriculture, and Medicine. 445 pages. Template:ISBN</ref> Cryphonectria parasitica may excrete oxalic acid containing solutions at the advancing edge of its chestnut cambium 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Biochemistry
The conjugate base of oxalic acid is the hydrogenoxalate anion, and its conjugate base (oxalate) is a competitive inhibitor of the lactate dehydrogenase (LDH) enzyme.<ref>Template:Cite journal</ref> LDH catalyses the conversion of pyruvate to lactic acid (end product of the fermentation (anaerobic) process) oxidising the coenzyme NADH to NAD+ and H+ concurrently. Restoring NAD+ levels is essential to the continuation of anaerobic energy metabolism through glycolysis. As cancer cells preferentially use anaerobic metabolism (see Warburg effect) inhibition of LDH has been shown to inhibit tumor formation and growth,<ref>Template:Cite journal</ref> 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 as Sclerotinia sclerotiorum cause a toxic accumulation.<ref>Template:Cite journal</ref>
Oxalate, besides being biosynthesised, may also be biodegraded. Oxalobacter formigenes is an important gut bacterium that helps animals (including humans) degrade oxalate.<ref>Template:Cite journal</ref>
Applications
Oxalic acid's main applications include cleaning or bleaching, especially for the removal of rust (iron complexing agent). It is the primary active ingredient in Bar 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 a mordant in dyeing processes. It is also used in bleaches, especially for pulpwood, cork, straw, cane, feathers, and for rust removal and other cleaning, in baking powder, and as a third reagent in silica analysis instruments.
Niche uses
Some beekeepers use oxalic acid as a miticide against the parasitic varroa mite.<ref>Template:Cite book</ref>
Dilute solutions (0.05–0.15 M) of oxalic acid can be used to remove iron from clays such as kaolinite to produce light-colored ceramics.<ref name=lee2007>Template:Cite journal</ref>
Oxalic acid can be used to clean minerals like many other acids. Two such examples are quartz crystals and pyrite.<ref>Jackson, Faith. "Quartz Crystal Cleaning". Template:Webarchive.</ref><ref>"Rock Currier – Cleaning Quartz". mindat.org</ref><ref>Georgia Mineral Society. "Cleaning Pyrites". Template:Webarchive. www.gamineral.org.</ref>
Oxalic acid is sometimes used in the aluminum anodizing process, with or without sulfuric acid.<ref>Template:Cite journal</ref> 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.Template:Citation needed
Semiconductor industry
Oxalic acid is also used in electronic and semiconductor industries. In 2006 it was reported being used in electrochemical–mechanical planarization of copper layers in the semiconductor devices fabrication process.<ref>Template:Cite journal</ref>
Proposed uses
Reduction of carbon dioxide to oxalic acid by various methods, such as electrocatalysis using a copper complex,<ref>Template:Cite journal</ref> is under study as a proposed chemical intermediate for carbon capture and utilization.<ref>Template:Cite journal</ref>
Content in food items
<ref>All data not specifically annotated is from Agriculture Handbook No. 8-11, Vegetables and Vegetable Products, 1984. ("Nutrient Data: Oxalic Acid Content of Selected Vegetables". ars.usda.gov [1])</ref>Template:Clarify
Toxicity
Oxalic acid has an oral LDLo (lowest published lethal dose) of 600 mg/kg.<ref>Template:Cite web</ref> It has been reported that the lethal oral dose is 15 to 30 grams.<ref>"CDC – Immediately Dangerous to Life or Health Concentrations (IDLH): Oxalic acid – NIOSH Publications and Products". cdc.gov</ref> The toxicity of oxalic acid is due to kidney failure caused by precipitation of solid calcium oxalate.<ref>EMEA Committee for veterinary medicinal products, oxalic acid summary report, December 2003Template:Dead link</ref>
Oxalate is known to cause mitochondrial dysfunction.<ref>Template:Cite journal</ref>
Ingestion of ethylene glycol results in oxalic acid as a metabolite which can also cause acute kidney failure.
Kidney stones
Most kidney stones, 76%, are composed of calcium oxalate.<ref name="Singh">Template:Cite journal</ref>
Notes
Template:NoteUnless otherwise cited, all measurements are based on raw vegetable weights with original moisture content.
See also
References
External links
Template:Commons Template:EB1911 poster
- Oxalic acid MS Spectrum
- Template:ICSC
- NIOSH Guide to Chemical Hazards (CDC)
- Table: Oxalic acid content of selected vegetables (USDA)
- Alternative link: Table: Oxalic Acid Content of Selected Vegetables (USDA)
- Template:Usurped
- Oxalosis & Hyperoxaluria Foundation (OHF) The Oxalate Content of Food 2008 (PDF)
- Oxalosis & Hyperoxaluria Foundation (OHF) Diet Information
- Calculator: Water and solute activities in aqueous oxalic acid Template:Webarchive
Template:Oxalates Template:Navbox linear saturated dicarboxylic acids