CaC2O4

Calcium oxalate, with the chemical formula CaC2O4, is an inorganic compound that occurs in several crystalline forms. The two common hydrated forms are whewellite, CaC2O4·H2O (monohydrate), and weddellite, CaC2O4·2H2O (dihydrate). In nature, calcium oxalate crystals are widespread in plants as part of biomineralization, where they can form raphides, druses, or other crystal types within specialized cells and are thought to play roles in calcium regulation and defense against herbivory. In animals, calcium oxalate is a major component of kidney stones and is commonly found in urine as crystals under certain physiological conditions.

Chemically, calcium oxalate forms by the reaction of calcium ions with oxalate ions. It is poorly soluble in water, which leads to precipitation when calcium and oxalate ions are present together in sufficient concentrations. In the laboratory, CaC2O4 can be prepared by mixing calcium salts, such as calcium chloride, with oxalate sources, such as ammonium oxalate, to yield a white precipitate that can be collected and weighed after drying or ignition, a method historically used in gravimetric analyses to determine calcium content.

Industrial and biomedical relevance includes its role in plant physiology, its contribution to urine stone formation, and its use as a standard precipitate in analytical chemistry. Calcium oxalate is relatively inert and requires acidic conditions or strong chelating agents to dissolve.

Safety and handling considerations reflect its low solubility and potential health effects associated with kidney stones; precautions are advised when handling oxalate-containing materials.