Calcium oxalate is an organic compound with the chemical formula CaC2O4(H2O)x. It belongs to the class of dicarboxylic acids and derivatives, which have two carboxylic acid groups. It forms needle-shaped crystals and is the main reason for kidney stones. It also is a constituent of beerstone, a scale that forms on containers used in breweries. It is insoluble in water and acetic acid, but soluble in nitric acid and hydrochloric acid. It is slightly hygroscopic. It is produced in the manufacture of oxalic acid and also in the production of ceramic glazes. It is found naturally as the minerals Whewellite and Weddellite. It is also present in certain plants, including dumb cane (Dieffenbachia), sorrel, rhubarb, and spinach.
The oxalate from these foods binds with the calcium in the gastrointestinal tract and then travels to the kidneys, where it combines with other substances to form the stones. A person can prevent these crystals from forming by eating high oxalate foods with foods rich in calcium, such as milk. They can also boil high-oxalate vegetables to remove some of the oxalate.
The Raman spectra shown in Figure 6 show that the oxalate monohydrate and dihydrate crystals have a similar shape, but that their crystal water structure is different. This explains the very different ratios of the vibrations at 1490 and 1464 cm-1, which depend on the supersaturation concentration. The stoichiometry of the free oxalate ions in solution also varies significantly as a function of pH, due to the deprotonation of the citric acid species.