Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula formula C2O2−4. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (Na2C2O4), and several esters such as dimethyl oxalate ((CH3)2C2O4). It is a conjugate base of oxalic acid. At neutral pH in aqueous solution, oxalic acid converts completely to oxalate.

two capital Cs connected to each other by a solid line and each connected to two separate Os by a solid line and a dashed line next to the solid line, the whole thing in brackets with a −2 to the top-right
The structure of the oxalate anion
IUPAC name
Systematic IUPAC name
  • 338-70-5 checkY
3D model (JSmol)
  • Interactive image
  • CHEBI:30623
  • 64235
  • C00209
  • 71081
  • PQ7QG47K6T checkY
  • InChI=1S/C2H2O4/c3-1(4)2(5)6/h(H,3,4)(H,5,6)/p-2
  • InChI=1S/C2H2O4/c3-1(4)2(5)6/h(H,3,4)(H,5,6)/p-2
  • C(=O)(C(=O)[O-])[O-]
Molar mass 88.018 g·mol−1
Conjugate acid Hydrogenoxalate[2]
Related compounds
Dinitrogen tetroxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Relationship to oxalic acid


The dissociation of protons from oxalic acid proceeds in a stepwise manner; as for other polyprotic acids, loss of a single proton results in the monovalent hydrogenoxalate anion HC2O4. A salt with this anion is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate. The equilibrium constant (Ka) for loss of the first proton is 5.37×10−2 (pKa = 1.27). The loss of the second proton, which yields the oxalate ion, has an equilibrium constant of 5.25×10−5 (pKa = 4.28). These values imply, in solutions with neutral pH, no oxalic acid and only trace amounts of hydrogen oxalate exist.[3] The literature is often unclear on the distinction between H2C2O4, HC2O4, and C2O2−4, and the collection of species is referred to as oxalic acid.[citation needed]



The oxalate anion exists in a nonplanar conformation where the O–C–C–O dihedrals approach 90° with approximate D2d symmetry.[4] When chelated to cations, oxalate adopts the planar, D2h conformation.[5][6] However, in the structure of caesium oxalate Cs2C2O4 the O–C–C–O dihedral angle is 81(1)°.[7][8] Therefore, Cs2C2O4 is more closely approximated by a D2d symmetry structure because the two CO2 planes are staggered. Two structural forms of rubidium oxalate Rb2C2O4 have been identified by single-crystal X-ray diffraction: one contains a planar and the other a staggered oxalate.

Nonplanar conformation found in caesium oxalate Cs2C2O4[8][9]
Planar conformation found in potassium oxalate K2C2O4[8][10]

The barrier to rotation about this bond is calculated to be roughly 2–6 kcal/mol for the free dianion, C2O2−4.[11][12][13] Such results are consistent with the interpretation that the central C−C bond is regarded as a single bond with minimal π interactions between the two CO2 units.[4] This barrier to rotation about the C−C bond (which formally corresponds to the difference in energy between the planar and staggered forms) may be attributed to electrostatic interactions as unfavorable O−O repulsion is maximized in the planar form.

Occurrence in nature


Oxalate occurs in many plants, where it is synthesized by the incomplete oxidation of saccharides.

Several plant foods such as the root and/or leaves of spinach, rhubarb, and buckwheat are high in oxalic acid and can contribute to the formation of kidney stones in some individuals. Other oxalate-rich plants include fat hen ("lamb's quarters"), sorrel, and several Oxalis species (also sometimes called sorrels). The root and/or leaves of rhubarb and buckwheat are high in oxalic acid.[14] Other edible plants with significant concentrations of oxalate include, in decreasing order, star fruit (carambola), black pepper, parsley, poppy seed, amaranth, chard, beets, cocoa, chocolate, most nuts, most berries, fishtail palms, New Zealand spinach (Tetragonia tetragonioides), and beans.[citation needed] Leaves of the tea plant (Camellia sinensis) contain among the greatest measured concentrations of oxalic acid relative to other plants. However, the drink derived by infusion in hot water typically contains only low to moderate amounts of oxalic acid due to the small mass of leaves used for brewing.[citation needed]

Physiological effects

Scanning electron micrograph of the surface of a kidney stone showing tetragonal crystals of weddellite (calcium oxalate dihydrate) emerging from the amorphous central part of the stone; the horizontal length of the picture represents 0.5 mm of the figured original.

Excess consumption of oxalate-rich foods has been linked to kidney stone formation of metal ions, such as calcium oxalate, a risk factor for kidney stones.[16]

Some fungi of the genus Aspergillus produce oxalic acid.[17]

As a ligand for metal ions


Oxalate also forms coordination compounds where it is sometimes abbreviated as ox. It is commonly encountered as a bidentate ligand. When the oxalate chelates to a single metal center, it always adopts the planar conformation. As a bidentate ligand, it forms a 5-membered MC2O2 ring. An illustrative complex is potassium ferrioxalate, K3[Fe(C2O4)3]. The drug oxaliplatin exhibits improved water solubility relative to older platinum-based drugs, avoiding the dose-limiting side-effect of nephrotoxicity. Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction. One of the main applications of oxalic acid is rust-removal, which arises because oxalate forms water-soluble derivatives with the ferric ion.



An excess oxalate level in the blood is termed hyperoxalemia, and high levels of oxalate in the urine is termed hyperoxaluria.



Although unusual, consumption of oxalates (for example, the grazing of animals on oxalate-containing plants such as Bassia hyssopifolia, or human consumption of wood sorrel or, specifically in excessive quantities, black tea) may result in kidney disease or even death due to oxalate poisoning. The New England Journal of Medicine reported acute oxalate nephropathy "almost certainly due to excessive consumption of iced tea" in a 56-year-old man, who drank "sixteen 8-ounce glasses of iced tea daily" (roughly 3.8 liters). The authors of the paper hypothesized that acute oxalate nephropathy is an underdiagnosed cause of kidney failure and suggested thorough examination of patient dietary history in cases of unexplained kidney failure without proteinuria (an excess of protein in the urine) and with large amounts of calcium oxalate in urine sediment.[18] Oxalobacter formigenes in the gut flora may help alleviate this.[19]



Primary hyperoxaluria is a rare, inherited condition, resulting in increased excretion of oxalate, with oxalate stones being common.


  1. ^ "Oxalate".
  2. ^ "oxalate(2−) (CHEBI:30623)". www.ebi.ac.uk. Retrieved 2 January 2019. oxalate(2−) (CHEBI:30623) is conjugate base of oxalate(1−) (CHEBI:46904) … oxalate(1−) (CHEBI:46904) is conjugate acid of oxalate(2−) (CHEBI:30623)
  3. ^ Riemenschneider, Wilhelm; Tanifuji, Minoru (2000). "Oxalic Acid". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a18_247. ISBN 3-527-30673-0.
  4. ^ a b Dean, Philip A. W. (2012). "The Oxalate Dianion, C2O42-: Planar or Nonplanar?". Journal of Chemical Education. 89 (3): 417–418. Bibcode:2012JChEd..89..417D. doi:10.1021/ed200202r.
  5. ^ Reed, D. A.; Olmstead, M. M. (1981). "Sodium oxalate structure refinement" (PDF). Acta Crystallographica Section B. 37 (4): 938–939. Bibcode:1981AcCrB..37..938R. doi:10.1107/S0567740881004676.
  6. ^ Beagley, B.; Small, R. W. H. (1964). "The structure of lithium oxalate". Acta Crystallographica. 17 (6): 783–788. Bibcode:1964AcCry..17..783B. doi:10.1107/S0365110X64002079.
  7. ^ In the figure 81(1)°, the (1) indicates that 1° is the standard uncertainty of the measured angle of 81°
  8. ^ a b c Dinnebier, Robert E.; Vensky, Sascha; Panthöfer, Martin; Jansen, Martin (2003). "Crystal and Molecular Structures of Alkali Oxalates: First Proof of a Staggered Oxalate Anion in the Solid State". Inorganic Chemistry. 42 (5): 1499–1507. doi:10.1021/ic0205536. PMID 12611516.
  9. ^ Dinnebier, R.E.; Vensky, S.; Panthofer, M.; Jansen, M. (2003). "CSD Entry WUWTIR: Di-cesium oxalate". Cambridge Structural Database: Access Structures. Cambridge Crystallographic Data Centre. doi:10.5517/cc6fzf0.
  10. ^ Dinnebier, R.E.; Vensky, S.; Panthofer, M.; Jansen, M. (2003). "CSD Entry QQQAZJ03: Di-potassium oxalate". Cambridge Structural Database: Access Structures. Cambridge Crystallographic Data Centre. doi:10.5517/cc6fzcy.
  11. ^ Clark, Timothy; Schleyer, Paul von Ragué (1981). "Conformational preferences of 34 valence electron A2X4 molecules: An ab initio Study of B2F4, B2Cl4, N2O4, and C
    ". Journal of Computational Chemistry. 2: 20–29. doi:10.1002/jcc.540020106. S2CID 98744097.
  12. ^ Dewar, Michael J.S.; Zheng, Ya-Jun (1990). "Structure of the oxalate ion". Journal of Molecular Structure: THEOCHEM. 209 (1–2): 157–162. doi:10.1016/0166-1280(90)85053-P.
  13. ^ Herbert, John M.; Ortiz, J. V. (2000). "Ab Initio Investigation of Electron Detachment in Dicarboxylate Dianions". The Journal of Physical Chemistry A. 104 (50): 11786–11795. Bibcode:2000JPCA..10411786H. doi:10.1021/jp002657c.
  14. ^ Streitweiser, Andrew Jr.; Heathcock, Clayton H. (1976). Introduction to Organic Chemistry. Macmillan. p. 737. ISBN 9780024180100.
  15. ^ Resnick, Martin I.; Pak, Charles Y. C. (1990). Urolithiasis, A Medical and Surgical Reference. W.B. Saunders Company. p. 158. ISBN 0-7216-2439-1.
  16. ^ Mitchell T, Kumar P, Reddy T, Wood KD, Knight J, Assimos DG, Holmes RP (March 2019). "Dietary oxalate and kidney stone formation". American Journal of Physiology. Renal Physiology. 316 (3): F409–F413. doi:10.1152/ajprenal.00373.2018. PMC 6459305. PMID 30566003.
  17. ^ Pabuççuoğlu, Uğur (2005). "Aspects of oxalosis associated with aspergillosis in pathology specimens". Pathology – Research and Practice. 201 (5): 363–368. doi:10.1016/j.prp.2005.03.005. PMID 16047945.
  18. ^ Syed, Fahd; Mena Gutiérrez, Alejandra; Ghaffar, Umbar (2 April 2015). "A Case of Iced-Tea Nephropathy". New England Journal of Medicine. 372 (14): 1377–1378. doi:10.1056/NEJMc1414481. PMID 25830441.
  19. ^ Siener, R.; Bangen, U.; Sidhu, H.; Hönow, R.; von Unruh, G.; Hesse, A. (2013). "The role of Oxalobacter formigenes colonization in calcium oxalate stone disease". Kidney International. 83 (June): 1144–1149. doi:10.1038/ki.2013.104. PMID 23536130.

Further reading

  • Euler. "Ksp Table: Solubility product constants near 25 °C". chm.uri.edu. Retrieved 10 June 2021.
  • Ibis, Fatma; Dhand, Priya; Suleymanli, Sanan; van der Heijden, Antoine E. D. M.; Kramer, Herman J. M.; Eral, Huseyin Burak (2020). "A combined experimental and modelling study on solubility of calcium oxalate monohydrate at physiologically relevant pH and temperatures". Crystals. 10 (10): 924. doi:10.3390/cryst10100924. ISSN 2073-4352.
  • Ulmgren, Per; Rådeström, Rune (1999). "Solubility of calcium oxalate in the presence of magnesium ions, and solubility of magnesium oxalate in sodium chloride medium". Nordic Pulp & Paper Research Journal. 14 (4): 330–335. doi:10.3183/npprj-1999-14-04-p330-335. ISSN 2000-0669. S2CID 96834193.
  • Oxalate.org - Oxalate content of 750+ foods from university and government sources