Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . 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. They include all forbs (flowering plants without a woody stem), grasses and grass-like plants, a vast majority of ...

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 Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

. 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 m ...

and its conjugate base, known as oxalate (), is a chelating agent 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, hydro ...

and Torbern Olof Bergman 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) HonFRSE (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 first ...

obtained oxalic acid by reacting cyanogen 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 or ...

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, usi ...

using

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

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 expl ...

, on sawdust, followed by acidification of the oxalate by mineral acids, such as sulfuric acid. Oxalic acid can also be formed by the heating of sodium formate in the presence of an

alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of ...

ne catalyst.

Laboratory methods

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

oxidizing Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refi ...

using

in the presence of a small amount of vanadium pentoxide as a catalyst.''Practical Organic Chemistry'' by Julius B. Cohen, 1930 ed. preparation #42 The hydrated solid can be dehydrated with heat or by azeotropic distillation. Developed in the Netherlands, an

electrocatalysis An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst ...

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 pinkish ...

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

polymorph Polymorphism, polymorphic, polymorph, polymorphous, or polymorphy may refer to: Computing * Polymorphism (computer science), the ability in programming to present the same programming interface for differing underlying forms * Ad hoc polymorphi ...

s; 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 ...

(water seeking), it is used in

esterification In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glyceride ...

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 (disambig ...

. 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. Carboxyl ...

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 of ...

). 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-cel ...

manganese Manganese is a chemical element with the 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 industrial alloy u ...

ores to allow the leaching of the metal by sulfuric acid. 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 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 Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure . It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Est ...

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 si ...

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 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 bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing ...

s 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 Oxaloacetic acid (also known as oxalacetic acid or OAA) is a crystalline organic compound with the chemical formula HO2CC(O)CH2CO2H. Oxaloacetic acid, in the form of its conjugate base oxaloacetate, is a metabolic intermediate in many processes ...

, 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 ethylene glycol.

Occurrence in foods and plants

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 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, contain high levels of oxalic acid,.

Rhubarb Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of '' Rheum'' in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick r ...

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

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 wh ...

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 metamorpho ...

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 conjugate base 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 to

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 natur ...

(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, 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 ''Oxalobacter formigenes'' is a Gram negative oxalate-degrading anaerobic bacterium that was first isolated from the gastrointestinal tract of a sheep in 1985. To date, the bacterium has been found to colonizes the large intestines of numerous ...

'' 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 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 in dyeing processes. It is also used in bleaches, 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 THYMOVAR is a product to control the Varroa mite (''Varroa destructor'') on bees (''Apis mellifera'') and contains the essential oil thymol. THYMOVAR contains thymol, a volatile substance which sublimates in the air depending on temperature. On rel ...

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, porcelai ...

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 Mitochondrial disease is a group of disorders caused by mitochondrial dysfunction. Mitochondria are the organelles that generate energy for the cell and are found in every cell of the human body except red blood cells. They convert the energy of ...

. Ingestion of ethylene glycol results in oxalic acid as a metabolite which can also cause acute kidney failure.

Kidney stones

Most

kidney stone Kidney stone disease, also known as nephrolithiasis or urolithiasis, is a crystallopathy where a solid piece of material (kidney stone) develops in the urinary tract. Kidney stones typically form in the kidney and leave the body in the urine ...

s, 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