Calcium oxalate

Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalate with the chemical formula CaC2O4·(H2O)x, where x varies from 0 to 3. All forms are colorless or white. The monohydrate occurs naturally as the mineral whewellite, forming envelope-shaped crystals, known in plants as raphides. The rarer dihydrate (mineral: weddellite) and trihydrate (mineral: caoxite) are also recognized. Calcium oxalates are a major constituent of human kidney stones. Calcium oxalate is also found in beerstone, a scale that forms on containers used in breweries.

Calcium oxalate
Calcium oxalate
The ball-and-stick model of calcium oxalate
IUPAC name
Calcium oxalate
  • 5794-28-5 (monohydrate) checkY
  • 25454-23-3 (dihydrate) ☒N
  • 192389-49-4 (trihydrate)
3D model (JSmol)
ECHA InfoCard 100.008.419 Edit this at Wikidata
EC Number
  • 209-260-1
  • InChI=1S/C2H2O4.Ca/c3-1(4)2(5)6;/h(H,3,4)(H,5,6);/q;+2/p-2 checkY
  • InChI=1/C2H2O4.Ca/c3-1(4)2(5)6;/h(H,3,4)(H,5,6);/q;+2/p-2
  • C(=O)(C(=O)[O-])[O-].[Ca+2]
CaC2H2O5 (monohydrate)
CaC2O4 (anhydrous)
Molar mass 128.097 g/mol
Appearance white solid
Density 2.20 g/cm3, monohydrate[1]
Melting point 200 °C (392 °F; 473 K) decomposes (monohydrate)
0.67 mg/L (20 °C)
2.7 × 10-9 for CaC
Main hazards Harmful, Irritant
GHS pictograms GHS07: Harmful
GHS Signal word Warning
H302, H312
NFPA 704 (fire diamond)
Related compounds
Other anions
Calcium carbonate
Calcium acetate
Calcium formate
Other cations
Sodium oxalate
Beryllium oxalate
Magnesium oxalate
Strontium oxalate
Barium oxalate
Radium oxalate
Iron(II) oxalate
Iron(III) oxalate
Related compounds
Oxalic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)
Infobox references
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)
Portion of CaC2O4·2H2O lattice, highlighting the connectivity of the oxalate ligand.[1](Carbon: black; Oxygen: red; Calcium: green)


Many plants accumulate calcium oxalate as it has been reported in more than 1000 different genera of plants.[3] The calcium oxalate accumulation is linked to the detoxification of calcium (Ca2+) in the plant.[4]

The poisonous plant dumb cane (Dieffenbachia) contains the substance and on ingestion can prevent speech and be suffocating. It is also found in sorrel, rhubarb (in large quantities in the leaves), cinnamon, turmeric and in species of Oxalis, Araceae, Arum italicum, taro, kiwifruit, tea leaves, agaves, Virginia creeper (Parthenocissus quinquefolia), and Alocasia and in spinach in varying amounts. Plants of the genus Philodendron contain enough calcium oxalate that consumption of parts of the plant can result in uncomfortable symptoms. Insoluble calcium oxalate crystals are found in plant stems, roots, and leaves and produced in idioblasts. Vanilla plants exude calcium oxalates upon harvest of the orchid seed pods and may cause contact contact dermatitis.

Calcium oxalate, as ‘beerstone’, is a brownish precipitate that tends to accumulate within vats, barrels, and other containers used in the brewing of beer. If not removed in a cleaning process, beerstone will leave an unsanitary surface that can harbour microorganisms.[5] Beerstone is composed of calcium and magnesium salts and various organic compounds left over from the brewing process; it promotes the growth of unwanted microorganisms that can adversely affect or even ruin the flavour of a batch of beer.

Calcium oxalate crystals in the urine are the most common constituent of human kidney stones, and calcium oxalate crystal formation is also one of the toxic effects of ethylene glycol poisoning.

Chemical propertiesEdit

Calcium oxalate is a combination of calcium ions and the conjugate base of oxalic acid, the oxalate anion. Its aqueous solutions are slightly basic because of the basicity of the oxalate ion. The basicity of calcium oxalate is weaker than that of sodium oxalate, due to its lower solubility in water.

Medical significanceEdit

Calcium oxalate can produce sores and numbing on ingestion and may even be fatal.

Morphology and diagnosisEdit

The monohydrate and dihydrate can be distinguished by the shape of the respective crystals.

  • Calcium oxalate dihydrate crystals are octahedral. A large portion of the crystals in a urine sediment will have this type of morphology, as they can grow at any pH and naturally occur in normal urine.
  • Calcium oxalate monohydrate crystals vary in shape, and can be shaped like dumbbells, spindles, ovals, or picket fences, the last of which is most commonly seen due to ethylene glycol poisoning.[6]

Kidney stonesEdit

Calcium oxalate monohydrate stones can be spiculated, resembling the head of a morning star.

About 80% of kidney stones are partially or entirely of the calcium oxalate type. They form when urine is persistently saturated with calcium and oxalate. Some of the oxalate in urine is produced by the body. Calcium and oxalate in the diet play a part, but are not the only factors that affect the formation of calcium oxalate stones. Dietary oxalate is an organic ion found in many vegetables, fruits, and nuts. Calcium from bone may also play a role in kidney stone formation.

Industrial applicationsEdit

Calcium oxalate is used in the manufacture of ceramic glazes.[7]

See alsoEdit


  1. ^ a b S. Deganello (1981). "The Structure of Whewellite, CaC2O4.H2O, at 328 K". Acta Crystallogr. B. 37 (4): 826–829. doi:10.1107/S056774088100441X.
  2. ^ Euler. "Ksp Table: Solubility product constants near 25 °C". Retrieved 10 June 2021.
  3. ^ Francesci, V.R.; Nakata (2005). "Calcium oxalate in plants: formation and function". Annu Rev Plant Biol. 56 (56): 41–71. doi:10.1146/annurev.arplant.56.032604.144106. PMID 15862089.
  4. ^ Martin, G; Matteo Guggiari; Daniel Bravo; Jakob Zopfi; Guillaume Cailleau; Michel Aragno; Daniel Job; Eric Verrecchia; Pilar Junier (2012). "Fungi, bacteria and soil pH: the oxalate–carbonate pathway as a model for metabolic interaction". Environmental Microbiology. 14 (11): 2960–2970. doi:10.1111/j.1462-2920.2012.02862.x. PMID 22928486.
  5. ^ Ryan, James (27 May 2018). "What is beerstone (and how to remove it)". Retrieved 28 May 2018.
  6. ^ "Urine Crystals". Cornell University. Retrieved 12 July 2014.
  7. ^ "Calcium Oxalate Data Sheet". Hummel Croton Inc. Retrieved 23 April 2017.