Calcium oxalate (in archaic terminology, oxalate of lime) is a

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early invest ...

with the chemical formula . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate occurs naturally as the mineral whewellite, forming envelope-shaped crystals, known in plants as raphides. The two rarer hydrates are dihydrate , which occurs naturally as the mineral

weddellite Weddellite ( Ca C2 O4·2 H2O) is a mineral form of calcium oxalate named for occurrences of millimeter-sized crystals found in bottom sediments of the Weddell Sea, off Antarctica. Occasionally, weddellite partially dehydrates to whewellite, formin ...

, and trihydrate , which occurs naturally as the mineral caoxite, are also recognized. Some foods have high quantities of calcium oxalates and can produce sores and numbing on ingestion and may even be fatal. Tribes with diets that depend highly on fruits and vegetables high in calcium oxalate, such as in

Micronesia Micronesia (, ) is a subregion of Oceania, consisting of about 2,000 small islands in the western Pacific Ocean. It has a close shared cultural history with three other island regions: the Philippines to the west, Polynesia to the east, ...

, reduce the level of it by boiling and cooking them. They are a constituent in 76% of human kidney stones. Calcium oxalate is also found in beerstone, a scale that forms on containers used in

breweries A brewery or brewing company is a business that makes and sells beer. The place at which beer is commercially made is either called a brewery or a beerhouse, where distinct sets of brewing equipment are called plant. The commercial brewing of beer ...

Occurrence

Many plants accumulate calcium oxalate as it has been reported in more than 1000 different genera of plants. The calcium oxalate accumulation is linked to the detoxification of calcium () in the plant. Upon decomposition, the calcium oxalate is oxidised by bacteria, fungi, or wildfire to produce the soil nutrient Calcium carbonate. The

poison Poison is a chemical substance that has a detrimental effect to life. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figuratively, with a broa ...

ous plant dumb cane ('' Dieffenbachia'') contains the substance and on ingestion can prevent speech and be suffocating. It is also found in

sorrel Sorrel (''Rumex acetosa''), also called common sorrel or garden sorrel, is a perennial herbaceous plant in the family Polygonaceae. Other names for sorrel include spinach dock and narrow-leaved dock ('dock' being a common name for the genus '' ...

rhubarb Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of '' Rheum'' in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick rhi ...

(in large quantities in the leaves), cinnamon, turmeric and in species of '' Oxalis'', Araceae, '' Arum italicum'',

taro Taro () (''Colocasia esculenta)'' is a root vegetable. It is the most widely cultivated species of several plants in the family Araceae that are used as vegetables for their corms, leaves, and petioles. Taro corms are a food staple in Afri ...

kiwifruit Kiwifruit (often shortened to kiwi in North American, British and continental European English) or Chinese gooseberry is the edible berry of several species of woody vines in the genus ''Actinidia''. The most common cultivar group of kiwif ...

, tea leaves, agaves, Virginia creeper ('' Parthenocissus quinquefolia''), and ''

Alocasia ''Alocasia'' is a genus of rhizomatous or tuberous, broad-leaved, perennial, flowering plants from the family Araceae. There are about 90 accepted species native to tropical and subtropical Asia and eastern Australia. Around the world, many gr ...

'' and in

spinach Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to central and western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common edible vegetable consumed either f ...

in varying amounts. Plants of the genus '' Philodendron'' contain enough calcium oxalate that consumption of parts of the plant can result in uncomfortable symptoms. Insoluble calcium oxalate crystals are found in plant stems, roots, and leaves and produced in idioblasts.

Vanilla Vanilla is a spice derived from orchids of the genus '' Vanilla'', primarily obtained from pods of the Mexican species, flat-leaved vanilla ('' V. planifolia''). Pollination is required to make the plants produce the fruit from whic ...

plants exude calcium oxalates upon harvest of the orchid seed pods and may cause contact dermatitis. Calcium oxalate, as ‘beerstone’, is a brownish precipitate that tends to accumulate within vats, barrels, and other containers used in the brewing of beer. If not removed in a cleaning process, beerstone will leave an unsanitary surface that can harbour microorganisms. Beerstone is composed of calcium and magnesium salts and various organic compounds left over from the brewing process; it promotes the growth of unwanted microorganisms that can adversely affect or even ruin the flavour of a batch of beer. Calcium oxalate crystals in the urine are the most common constituent of human kidney stones, and calcium oxalate crystal formation is also one of the toxic effects of ethylene glycol poisoning.

Chemical properties

Calcium oxalate is a combination of calcium ions and the conjugate base of

, the oxalate anion. Its aqueous solutions are slightly basic because of the basicity of the oxalate ion. The basicity of calcium oxalate is weaker than that of sodium oxalate, due to its lower solubility in water. Solid calcium oxalate hydrate has been characterized by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

. It is a

coordination polymer A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2 ...

. featuring planar oxalate anions linked to calcium, which also has water

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...

Medical significance

Calcium oxalate can produce sores and numbing on ingestion and may even be fatal.

Morphology and diagnosis

The monohydrate and dihydrate can be distinguished by the shape of the respective crystals. * Calcium oxalate ''dihydrate'' crystals are octahedral. A large portion of the crystals in a urine sediment will have this type of morphology, as they can grow at any pH and naturally occur in normal urine. * Calcium oxalate ''monohydrate'' crystals vary in shape, and can be shaped like dumbbells, spindles, ovals, or picket fences, the last of which is most commonly seen due to ethylene glycol poisoning. File:Calcium oxalate crystals in urine.jpg, Urine microscopy showing calcium oxalate crystals in the urine. The octahedral crystal morphology is clearly visible. File:Calcium oxalate crystals (urine) - kalsiyum oksalat kristalleri (idrar) - 01.png, Urine microscopy showing a ''calcium oxalate monohydrate'' crystal (dumbbell shaped) and a ''calcium oxalate dihydrate'' crystal (envelope shaped) along with several erythrocytes. File:Calcium oxalate crystals (urine) - kalsiyum oksalat kristalleri (idrar) - 02.png, Urine microscopy showing several ''calcium oxalate monohydrate'' crystals (dumbbell shaped, some of them clumped) and a ''calcium oxalate dihydrate'' crystal (envelope shaped) along with several erythrocytes. File:Calcium Oxalate Detail.png, Urinary sediment showing several calcium oxalate crystals. 40X

Kidney stones

About 76% of kidney stones are partially or entirely of the calcium oxalate type. They form when urine is persistently saturated with calcium and oxalate. Between 1% and 15% of people globally are affected by kidney stones at some point. In 2015, they caused about 16,000 deaths worldwide. Some of the oxalate in urine is produced by the body. Calcium and oxalate in the diet play a part but are not the only factors that affect the formation of calcium oxalate stones. Dietary oxalate is an organic ion found in many vegetables, fruits, and nuts. Calcium from bone may also play a role in kidney stone formation. In one study of modulators of calcium oxalate crystallization in urine, magnesium-

alkali citrate Alkali citrate (also known as alkaline citrate) is an inhibitor of kidney stones. It is used to increase urine citrate levels - this prevents calcium oxalate stones by binding to calcium and inhibiting its binding to oxalate. It is also used to i ...

was shown to inhibit CaOx (calcium oxalate) crystallization, “probably via actions of the citrate, but not the Mg.” This was in comparison to magnesium, citrate, and magnesium citrate. Currently the preparation of magnesium-potassium citrate that was used in one positive study is not available in the United States.

Industrial applications

Calcium oxalate is used in the manufacture of ceramic glazes.

References

{{DEFAULTSORT:Calcium Oxalate Oxalates Calcium compounds Kidney