Calcium oxalate

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Structural formula
Structural formula calcium oxalate
Surname Calcium oxalate
other names
  • Calcium oxalate
  • Calcium ethanedioate
Molecular formula CaC 2 O 4
Brief description

colorless crystals (monohydrate)

External identifiers / databases
CAS number
  • 563-72-4
  • 5794-28-5 (monohydrate)
EC number 209-260-1
ECHA InfoCard 100.008.419
PubChem 33005
ChemSpider 30549
Wikidata Q412399
Molar mass 128.10 g · mol -1
146.12 g · mol -1 (monohydrate)
Physical state



2.12 g cm −3 (monohydrate)

Melting point

200 ° C (release of crystal water)


practically insoluble in water (0.0061 g l −1 at 20 ° C)

safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
07 - Warning


H and P phrases H: 302-312
P: 280
Toxicological data

375 mg kg −1 ( LD 50ratoral )

Thermodynamic properties
ΔH f 0

−1360.6 kJ / mol

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Calcium oxalate is a calcium salt of oxalic acid . It has the molecular formula CaC 2 O 4 and belongs to the group of oxalates .


The scanning electron microscope image of the surface of a kidney stone shows tetragonal crystals of calcium oxalate dihydrate ( Weddellite ) that have grown out of the amorphous center. Image width: 0.45 mm.
Common rhubarb ( Rheum rhabarbarum )

Calcium oxalate occurs naturally as the main component of kidney stones and beer stone . In addition, it occurs in many plants as a defense against eating (e.g. in the dock , wormwood , parsnips , taro and in Dieffenbachia and other arum plants ). Calcium oxalate also collects in the tissue of cacti . For example, 85% of the dry matter in Cephalocereus senilis consists of calcium oxalate. The crystal form of the calcium oxalate in the plant cells is mostly prismatic with acute angles or star-shaped ( drusen ). A needle-like crystal habit is less common . In addition to these biological sources, calcium oxalate occurs naturally in hydrate form as the rare minerals whewellite and weddellite . The mineral of the trihydrate is called caoxite .

Extraction and presentation

Calcium oxalate is an intermediate in the production of oxalic acid. Here, sodium oxalate with calcium hydroxide transferred to the hardly soluble calcium oxalate, from which the oxalic acid with sulfuric acid being released.

Crystal structures

Anhydrous calcium oxalate does not exist as a single crystal , but is obtained from the hydrates by heating . Its crystal structure was therefore determined by X-ray diffraction on the crystalline powder and shows monoclinic symmetry in space group P 2 / m . Calcium oxalate monohydrate also crystallizes in the monoclinic crystal system. At room temperature the space group is P 2 1 / n and above 328 K the space group is I 2 / m . Calcium oxalate dihydrate crystallizes in the tetragonal crystal system with space group I 4 / m . Calcium oxalate trihydrate crystallizes in the triclinic crystal system with space group P 1 .

Chemical properties

TGA measurement on calcium oxalate monohydrate

Calcium oxalate is a colorless salt of oxalic acid . The compound forms a stable monohydrate. When heated, the water of crystallization is released from 100 ° C. Above 300 ° C the compound decomposes with the release of carbon monoxide and the formation of calcium carbonate . This decomposes on further heating above 500 ° C into calcium oxide and carbon dioxide .

All three reaction steps are endothermic. In the presence of atmospheric oxygen , the second reaction stage takes place as an exothermic oxidation reaction with the formation of carbon dioxide.

With a dihydrate and a trihydrate, two further metastable hydrates are known.


Calcium oxalate serves as evidence for calcium ions . If an oxalate-containing solution is added to the calcium-containing solution to be determined, the poorly soluble calcium oxalate precipitates. It also serves as a bleaching agent and is used as a metal cleaner. It is also created as a glossy layer on the surface of natural stones (see Shine of natural stones ).

Calcium oxalate monohydrate is used in thermogravimetry to calibrate thermal balances.

Individual evidence

  1. a b Entry on calcium oxalate. In: Römpp Online . Georg Thieme Verlag, accessed on September 29, 2014.
  2. a b Data sheet calcium oxalate monohydrate from AlfaAesar, accessed on January 31, 2010 ( PDF )(JavaScript required) .
  3. David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 85th edition. (Internet version: 2005), CRC Press / Taylor and Francis, Boca Raton, FL, PHYSICAL CONSTANTS OF INORGANIC COMPOUNDS, pp. 4-49 - 4-49.
  4. a b Calcium oxalate data sheet from Sigma-Aldrich , accessed on March 15, 2011 ( PDF ).Template: Sigma-Aldrich / name not given
  5. Not explicitly listed in Regulation (EC) No. 1272/2008 (CLP) , but with the specified labeling it falls under the group entry salts of oxalic acid with the exception of those specified elsewhere in this Annex in the Classification and Labeling Inventory of the European Chemicals Agency ( ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
  6. Data sheet calcium oxalate (PDF) from Carl Roth , accessed on January 31, 2010.
  7. David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Standard Thermodynamic Properties of Chemical Substances, pp. 5-20.
  8. T. Terrazas Salgado, J. Mauseth: Cacti: Biology and uses . Ed .: PS Nobel. University of California Press, Berkeley 2002, ISBN 0-520-23157-0 , Shoot anatomy and morphology, pp. 23–40 ( full text in Google Book Search [accessed June 12, 2016]).
  9. ^ O. Hochrein, A. Thomas, R. Kniep: Revealing the Crystal Structure of Anhydrous Calcium Oxalate, Ca [C 2 O 4 ], by a Combination of Atomistic Simulation and Rietveld Refinement . In: Journal of Inorganic and General Chemistry . tape 634 , no. 11 , 2008, p. 1826-1829 , doi : 10.1002 / zaac.200800207 .
  10. ^ S. Deganello: The basic and derivative structures of calcium oxalate monohydrate . In: Journal of Crystallography . tape 152 , no. 1-4 , 1980, pp. 247-252 , doi : 10.1524 / zkri.1980.152.14.247 .
  11. S. Deganello: The structure of whewellite at 328K . In: Acta Crystallographica Section B . tape 37 , no. 4 , 1981, p. 826-829 , doi : 10.1107 / S056774088100441X .
  12. ^ C. Sterling: Crystal structure analysis of Weddellite . In: Acta Crystallographica . tape 18 , no. 5 , 1965, pp. 917-921 , doi : 10.1107 / S0365110X65002219 .
  13. ^ S. Deganello, AR Kampf, PB Moore: The crystal structure of calcium oxalate trihydrate . In: American Mineralogist . tape 66 , no. 7-8 , 1981, pp. 859-865 .
  14. N. Kutaish, P. Aggarwal, D. Dollimore: Thermal analysis of calcium oxalate samples obtained by various preparative routes. In: Thermochim. Acta . 297, 1997, pp. 131-137. doi: 10.1016 / S0040-6031 (97) 00002-6 .
  15. ^ A b G. Sadovska, G. Wolf: Enthalpy of dissolution and thermal dehydration of calcium oxalate hydrates. In: J. Therm. Anal. Calorim. 119, 2015, pp. 2063-2068. doi: 10.1007 / s10973-014-4350-x .
  16. ^ HL Anderson, A. Kemmler, GWH Höhne, K. Heldt, R. Strey: Round robin test on the kinetic evaluation of a complex solid state reaction from 13 European laboratories. Part 1. Kinetic TG analysis. In: Thermochim. Acta . 332, 1999, pp. 33-53. doi: 10.1016 / S0040-6031 (99) 00045-3 .
  17. a b c H. L. Anderson, R. Strey, GWH Höhne, A. Kemmler, K. Heldt: Round robin test on the kinetic evaluation of a complex solid state reaction from 13 European laboratories. Part 2. Kinetic DSC analysis. In: Thermochim. Acta. 332, 1999, pp. 55-70. doi: 10.1016 / S0040-6031 (99) 00046-5 .