Calcium oxalate

Calcium oxalate anhydrous is a colourless cubic system powder.   It starts degrading into calcium carbonate releasing carbon monoxide from about 370 C on heating.   It is hygroscopic and easily absorbs moisture from the air to be monohydrate.   Anhydrate, monohydrate, trihydrate, etc. are found.  Monohydrate is dehydrated at 200 C to become anhydrate.   Calcium oxalate is slightly soluble in water, its solubility is the lowest among alkaline earth metal salts and the solubility in water at 13 C is 0.67mg/100mL.

 CAS No.  5794-28-5 (monohydrate), 25454-23-3 (anhydrate)
 Law*  deleterious substance
 Chemical formula  Ca(COO)2
 Molecular weight  128.097
 Density  2.12 g/cm3
* Poisonous and Deleterious Substance Control Law

The plants belonging to the family Araceae, especially the plants belonging to the genus Dieffenbachia, Philodendron or Arisaema contain much calcium oxalate.   These plants do not cause only poisoning of digestive organs following accidental ingestion but also poisoning when the body comes into contact with it.   Therefore, careful attention is required.

Many plants produce oxalic acid, which is an end product of metabolism and is harmful.   Consequently, the plant changes oxalic acid into calcium oxalate, so it becomes insoluble and therefore unharmful.   The number of hydration is varied among plants that calcium oxalate is contained in and the crystal form is varied according to the number of hydration, coexistent substances and physical environments.   There are a wide range of crystal forms, such as a hexagonal plate, octahedron, needle, quadrangular pyramid and rhombus.

Many plants belonging to the family Araceae contain long needle crystal bundles.   At accidental ingestion, the plant cells are destroyed during mastication and consequently the crystals physically injure mucous membranes to cause toxic symptoms, such as pain and edema.   Protease or phlogogenic substance often adhere to the crystal and that makes toxic symptoms complicated and serious.

Oxalic acid is an end product of metabolism also for humans.   When there is an obstruction of the urinary tract or deformity of urinary tract, it causes stagnant urinary flow and the urinary stone is often formed.   Calcium Stones (calcium oxalate stones, calcium phosphate stones) account for 80% of urinary stones while it is classified by its component.   There are also uric acid stones due to hyperuricemia (gout), magnesium ammonium phosphate stones due to urinary tract infection and cystine stones due to hereditary characteristics.   Oxalic acid as a product of metabolism is a factor of the urinary stone but in reality, 70 % of calcium oxalate lithiasis is due to an exogenous factor; oxalic acid from dietary sources.   Green vegetables, tea or coffee, banana, chocolate, peanut, almond, etc. all contain high amounts of oxalic acid compared to other foods, therefore, it can be a high contributing factor for urinary stones.   Heavy ingestion of green vegetables, such as spinach, should be avoided but on the other hand, since many of the green vegetables are nutritious, it is better to master how to ingest them.   For example, originally spinach contains 800mg of oxalic acid per 100g, whereas after boiling it for 3 minutes, this reduced by 50 to 60%.   Gastrointestinal absorption of oxalic acid can be decreased by ingestion together with calcium.