Calcium Oxalate, CaC₂O₄

Calcium Oxalate, CaC₂O₄, crystallises from very dilute solutions in the cold with 3 molecules of water, and at room temperature with 1 in monoclinic crystals, its density being 2.2. The water of crystallisation is only completely removed at 200° C., and at red heat the salt is decomposed into calcium carbonate and carbon monoxide. Calcium oxalate is only slightly soluble in water. By electrical conductivity methods, Kohlrausch found a solubility of 5.6 grm. per litre of the anhydrous salt at 18° C., but much higher values have been given. It is practically insoluble in a dilute solution of ammonium oxalate, but some salts, for example magnesium and zinc chlorides, increase the solubility.

Double salts are formed with calcium chloride, for example, CaC₂O₄.CaCl₂.7H₂O and 3CaC₂O₄.CaCl₂.8H₂O.

Calcium oxalate is found in many plants. In rhubarb, for instance, it occurs in aggregates of needle-like crystals. It is found in considerable quantity in certain lichens growing on limestone.