Oxalic acid is an

organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are ...

with the systematic name ethanedioic acid and

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

. It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early investigators isolated oxalic acid from

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants t ...

s 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 has much greater acid strength than acetic acid. It is a

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth met ...

and its

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...

, known as oxalate (), is a

chelating agent Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands a ...

for metal cations. Typically, oxalic acid occurs as the dihydrate with the formula .

History

The preparation of salts of oxalic acid (crab acid) from plants had been known, at least since 1745, when the Dutch botanist and physician Herman Boerhaave isolated a salt from wood sorrel. By 1773, François Pierre Savary of Fribourg, Switzerland had isolated oxalic acid from its salt in sorrel. In 1776, Swedish chemists

Carl Wilhelm Scheele Carl Wilhelm Scheele (, ; 9 December 1742 – 21 May 1786) was a Swedish German pharmaceutical chemist. Scheele discovered oxygen (although Joseph Priestley published his findings first), and identified molybdenum, tungsten, barium, hydr ...

and

Torbern Olof Bergman Torbern Olaf (Olof) Bergman (''KVO'') (20 March 17358 July 1784) was a Swedish chemist and mineralogist noted for his 1775 ''Dissertation on Elective Attractions'', containing the largest chemical affinity tables ever published. Bergman was the ...

produced oxalic acid by reacting sugar with concentrated

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

; 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. In 1824, the German chemist

Friedrich Wöhler Friedrich Wöhler () FRS(For) Hon FRSE (31 July 180023 September 1882) was a German chemist known for his work in inorganic chemistry, being the first to isolate the chemical elements beryllium and yttrium in pure metallic form. He was the fi ...

obtained oxalic acid by reacting

cyanogen Cyanogen is the chemical compound with the formula ( C N)2. It is a colorless and highly toxic gas with a pungent odor. The molecule is a pseudohalogen. Cyanogen molecules consist of two CN groups – analogous to diatomic halogen molec ...

with ammonia in aqueous solution. This experiment may represent the first synthesis of a natural product.

Preparation

Oxalic acid is mainly manufactured by the oxidation of

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...

s or

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

using

or air in the presence of vanadium pentoxide. A variety of precursors can be used including glycolic acid and

ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an o ...

. A newer method entails oxidative

carbonylation Carbonylation refers to reactions that introduce carbon monoxide into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry. The term carbon ...

of alcohols to give the diesters of oxalic acid: These diesters are subsequently hydrolyzed to oxalic acid. Approximately 120,000

tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...

s are produced annually.Wilhelm Riemenschneider, Minoru Tanifuji "Oxalic acid" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2002, Wiley-VCH, Weinheim. . Historically oxalic acid was obtained exclusively by using caustics, such as sodium or

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...

, on sawdust, followed by acidification of the oxalate by mineral acids, such as

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...

. Oxalic acid can also be formed by the heating of sodium formate in the presence of an alkaline catalyst.

Laboratory methods

Although it can be readily purchased, oxalic acid can be prepared in the laboratory by oxidizing sucrose using

in the presence of a small amount of vanadium pentoxide as a

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

.''Practical Organic Chemistry'' by Julius B. Cohen, 1930 ed. preparation #42 The hydrated solid can be dehydrated with heat or by

azeotropic distillation In chemistry, azeotropic distillation is any of a range of techniques used to break an azeotrope in distillation. In chemical engineering, ''azeotropic distillation'' usually refers to the specific technique of adding another component to gene ...

. Developed in the Netherlands, an electrocatalysis by a

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...

complex helps reduce carbon dioxide to oxalic acid; this conversion uses carbon dioxide as a feedstock to generate oxalic acid.

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. Because the anhydrous material is both acidic and

hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are n ...

(water seeking), it is used in esterifications.

Dihydrate

The dihydrate .2 has space group ''C''⁵_2''h''–''P''2₁/''n'', with lattice parameters ''a'' = 611.9 pm, ''b'' = 360.7 pm, ''c'' = 1205.7 pm, ''β'' = 106°19', ''Z'' = 2.T. M. Sabine, G. W. Cox and B. M. Craven (1969): "A neutron diffraction study of lphaoxalic acid dihydrate" ''Acta Crystallographica Section B'', volume B25, pages 2437-2441. The main inter-atomic distances are: C−C 153 pm, C−O₁ 129 pm, C−O₂ 119 pm.F. R. Ahmed and D. W. J. Cruickshank (1953): "A refinement of the crystal structure analyses of oxalic acid dihydrate". ''Acta Crystallographica'' volume 6, pages 385-392. Theoretical studies indicate that oxalic acid dihydrate is one of very few crystalline substances that exhibit

negative area compressibility Negative may refer to: Science and mathematics * Negative number * Negative mass * Negative energy * Negative charge, one of the two types of electric charge * Negative (electrical polarity), in electric circuits * Negative result (disambigua ...

. Namely, when subjected to isotropic tension stress (negative

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

), the ''a'' and ''c'' lattice parameters increase as the stress decreases from −1.17  GPa to −0.12 GPa and from −1.17 GPa to −0.51 GPa, respectively.Francisco Colmenero (2019): "Negative area compressibility in oxalic acid dihydrate". ''Materials Letters'', volume 245, pages 25-28.

Reactions

Acid-base properties

Oxalic acid's p''K''_a values vary in the literature from 1.25–1.46 and 3.81–4.40. The 100th ed of the CRC, released in 2019, has values of 1.25 and 3.81. Oxalic acid is relatively strong compared to other

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyli ...

s: Oxalic acid undergoes many of the reactions characteristic of other carboxylic acids. It forms esters such as dimethyl oxalate (

m.p. A member of parliament (MP) is the representative in parliament of the people who live in their electoral district. In many countries with bicameral parliaments, this term refers only to members of the lower house since upper house members ofte ...

). 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 shown to reduce

manganese dioxide Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for is for dry-ce ...

manganese Manganese is a chemical element with the Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of ...

ores to allow the leaching of the metal by

. 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: :2Ln³⁺ + 3C₂O₄H₂ → Ln₂(C₂O₄)₃ + 6H⁺ Thermal decomposition of these oxalates gives the oxides, which is the most commonly marketed form of these elements.

Other

Oxalic acid and oxalates can be oxidized by

permanganate A permanganate () is a chemical compound containing the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom is in the +7 oxidation state, the permanganate(VII) ion is a strong oxidizing agent. The ion is a ...

in an autocatalytic reaction. Oxalic acid vapor decomposes at 125–175

°C The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The d ...

into

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

and formic acid HCOOH. Photolysis with 237–313 nm UV light also produces

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

CO and water.James Higgins, Xuefeng Zhou, Ruifeng Liu, and Thomas T.-S. Huang (1997): "Theoretical Study of Thermal Decomposition Mechanism of Oxalic Acid" ''Journal of Physical Chemistry A'', volume 101, issue 14, pages 2702–2708. Evaporation of a solution of

urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...

and oxalic acid in 2:1 molar ratio yields a solid crystalline compound ., consisting of stacked two-dimensional networks of the neutral molecules held together by hydrogen bonds with the oxygen atoms.S. Harkema, J. W. Bats, A. M. Weyenberg and D. Feil (1972) "The crystal structure of urea oxalic acid (2:1)". ''Acta Crystallographica Section B'', volume B28, pages 1646-1648.

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 In enzymology, an oxaloacetase () is an enzyme that catalyzes the chemical reaction: :oxaloacetate + H2O \rightleftharpoons oxalate + acetate Thus, the two substrates of this enzyme are oxaloacetate and H2O, whereas its two products are oxalat ...

: It also arises from the dehydrogenation of glycolic acid, which is produced by the metabolism of

Occurrence in foods and plants

Early investigators isolated oxalic acid from

wood-sorrel ''Oxalis'' ( (American English) or (British English)) is a large genus of flowering plants in the wood-sorrel family Oxalidaceae, comprising over 550 species. The genus occurs throughout most of the world, except for the polar areas; species ...

(''Oxalis''). Members of the spinach family and the brassicas (

cabbage Cabbage, comprising several cultivars of ''Brassica oleracea'', is a leafy green, red (purple), or white (pale green) biennial plant grown as an annual vegetable crop for its dense-leaved heads. It is descended from the wild cabbage ( ''B.&n ...

, broccoli,

brussels sprouts The Brussels sprout is a member of the Gemmifera cultivar group of cabbages (''Brassica oleracea''), grown for its edible buds. The leaf vegetables are typically 1.5–4.0 cm (0.6–1.6 in) in diameter and resemble miniature cabbag ...

) are high in oxalates, as are

sorrel Sorrel (''Rumex acetosa''), also called common sorrel or garden sorrel, is a perennial herbaceous plant in the family Polygonaceae. Other names for sorrel include spinach dock and narrow-leaved dock ('dock' being a common name for the genus '' ...

and umbellifers like

parsley Parsley, or garden parsley (''Petroselinum crispum'') is a species of flowering plant in the family Apiaceae that is native to the central and eastern Mediterranean region (Sardinia, Lebanon, Israel, Cyprus, Turkey, southern Italy, Greece, ...

. The leaves and stems of all species of the genus '' Chenopodium'' and related genera of the family

Amaranthaceae Amaranthaceae is a family of flowering plants commonly known as the amaranth family, in reference to its type genus ''Amaranthus''. It includes the former goosefoot family Chenopodiaceae and contains about 165 genera and 2,040 species, making i ...

, which includes

quinoa Quinoa (''Chenopodium quinoa''; , from Quechua ' or ') is a flowering plant in the amaranth family. It is a herbaceous annual plant grown as a crop primarily for its edible seeds; the seeds are rich in protein, dietary fiber, B vitamins, ...

, contain high levels of oxalic acid,. Rhubarb leaves contain about 0.5% oxalic acid, and jack-in-the-pulpit (''

Arisaema triphyllum ''Arisaema triphyllum'', the jack-in-the-pulpit, bog onion, brown dragon or Indian turnip, is a herbaceous perennial plant growing from a corm. It is a highly variable species typically growing in height with three-part leaves and flowers cont ...

'') contains calcium oxalate crystals. Similarly, the

Virginia creeper ''Parthenocissus quinquefolia'', known as Virginia creeper, Victoria creeper, five-leaved ivy, or five-finger, is a species of flowering vine in the grape family, Vitaceae. It is native to eastern and central North America, from southeastern ...

, 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

s. Plants of the genus '' Fenestraria'' produce optical fibers made from crystalline oxalic acid to transmit light to subterranean photosynthetic sites. Carambola, also known as starfruit, also contains oxalic acid along with

caramboxin Caramboxin (CBX) is a toxin found in star fruit (''Averrhoa carambola''). Individuals with some types of kidney disease are susceptible to adverse neurological effects including intoxication, seizures and even death after eating star fruit. Car ...

. Citrus juice contains small amounts of oxalic acid. Citrus fruits produced in organic agriculture contain less oxalic acid than those produced in conventional agriculture. The formation of naturally occurring calcium oxalate patinas on certain

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

and

marble Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite. Marble is typically not foliated (layered), although there are exceptions. In geology, the term ''marble'' refers to metamorphose ...

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.

Production by fungi

Many soil fungus species secrete oxalic acid, resulting in greater solubility of metal cations, increased availability of certain soil nutrients, and can lead to the formation of calcium oxalate crystals. Some fungi such as '' Aspergillus niger'' have been extensively studied for the industrial production of oxalic acid;Hermann Strasser, Wolfgang Burgstaller, Franz Schinner(1994): "High-yield production of oxalic acid for metal leaching processes by ''Aspergillus niger''". ''FEMS Microbiology Letters'', volume 119, issue 3, pages 365–370. however, those processes are not yet economically competitive with production from oil and gas.Jan S. Tkacz, Lene Lange (2012): ''Advances in Fungal Biotechnology for Industry, Agriculture, and Medicine''. 445 pages.

Biochemistry

The

of oxalic acid is the hydrogenoxalate anion, and its conjugate base ( oxalate) is a competitive inhibitor of the lactate dehydrogenase (LDH) enzyme. LDH catalyses the conversion of

pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic a ...

lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as nat ...

(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 Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH ...

. As cancer cells preferentially use anaerobic metabolism (see Warburg effect) inhibition of LDH has been shown to inhibit tumor formation and growth, thus is an interesting potential course of cancer treatment. Oxalic acid plays an 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. Oxalate, besides being biosynthesised, may also be biodegraded. '' Oxalobacter formigenes'' is an important gut bacteria that helps animals (including humans) degrade oxalate.

Applications

Oxalic acid's main applications include cleaning or bleaching, especially for the removal of rust (iron complexing agent). Its utility in rust removal agents is due to its forming a stable, water-soluble salt with ferric iron,

ferrioxalate Ferrioxalate or trisoxalatoferrate(III) is a trivalent anion with formula . It is a transition metal complex consisting of an iron atom in the +3 oxidation state and three bidentate oxalate ions anions acting as ligands. The ferrioxalate anio ...

ion. The cleaning product Zud contains oxalic acid. Oxalic acid is an ingredient in some tooth whitening products. About 25% of produced oxalic acid will be used as a

mordant A mordant or dye fixative is a substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). It may be used for dyeing fabrics or for intensifying stains in ...

in dyeing processes. It is also used in

bleach Bleach is the generic name for any chemical product that is used industrially or domestically to remove color (whitening) from a fabric or fiber or to clean or to remove stains in a process called bleaching. It often refers specifically, to ...

es, especially for pulpwood, and for rust removal and other cleaning, in baking powder, and as a third reagent in silica analysis instruments.

Niche uses

Oxalic acid is used by some

beekeeper A beekeeper is a person who keeps honey bees. Beekeepers are also called honey farmers, apiarists, or less commonly, apiculturists (both from the Latin '' apis'', bee; cf. apiary). The term beekeeper refers to a person who keeps honey bees i ...

s as a miticide against the parasitic varroa mite. Thymovar combined with an oxalic acid treatment has proved effective against the varroa mite. Dilute solutions (0.05–0.15 M) of oxalic acid can be used to remove iron from clays such as

kaolinite Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahed ...

to produce light-colored

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, ...

s.Sung Oh Lee, Tam Tran, Byoung Hi Jung, Seong Jun Kim, and Myong Jun Kim (2007): "Dissolution of iron oxide using oxalic acid". ''Hydrometallurgy'', volume 87, issues 3–4. pages 91-99. Oxalic acid is used to clean minerals. Oxalic acid is sometimes used in the aluminum anodizing process, with or without sulfuric acid. Compared to sulfuric acid anodizing, the coatings obtained are thinner and exhibit lower surface roughness. Oxalic acid is also widely used as
wood bleach
most often in its crystalline form to be mixed with water to its proper dilution for use.

Semiconductor industry

Oxalic acid is also used in electronic and semiconductor industries. In 2006 it was reported being used in electro chemical–mechanical planarization of copper layers in the semiconductor devices fabrication process.

Content in food items

Toxicity

Oxalic acid has an oral LD_Lo (lowest published lethal dose) of 600 mg/kg. It has been reported that the lethal oral dose is 15 to 30 grams. The toxicity of oxalic acid is due to kidney failure caused by precipitation of solid calcium oxalate. Oxalate is known to cause mitochondrial dysfunction. Ingestion of

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.

Other effects

Oxalic acid can cause joint pain by formation of precipitates in the joints. Calcium hydroxide decreases urinary oxalate in both humans and rats.

Notes

Unless otherwise cited, all measurements are based on raw vegetable weights with original moisture content.

References

External links

Oxalic acid MS Spectrum
*

* ttps://web.archive.org/web/20081016043139/http://www.rhubarbinfo.com/rhubarb-poison.html About rhubarb poisoning (The Rhubarb Compendium)br>Oxalosis & Hyperoxaluria Foundation (OHF) The Oxalate Content of Food 2008 (PDF)Calculator: Water and solute activities in aqueous oxalic acid
{{Authority control Oxalates Household chemicals Dicarboxylic acids Chelating agents Beekeeping Western honey bee medications Nephrotoxins Conjugated ketones