CALCIUM OXALATE (CAC2O4): PROPERTIES, STRUCTURE, USES, RISKS - CHEMISTRY - 2025

The calcium oxalate is an organic compound consisting of the elements carbon (C), oxygen (O) and calcium (Ca). Its chemical formula is CaC ₂ O ₄. It is generally found in its three hydrated forms: mono-, di- and trihydrate. That is, with one, two or three water molecules respectively in its crystalline structure.

Calcium oxalate is found in minerals, plants, fungi and other living beings such as mammals and even in humans as a consequence of the metabolism of some proteins. It can be found in the urine of humans and some animals.

Crystals of calcium oxalate CaC ₂ O ₄ in a urine sample observed under a microscope. J3D3 / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

Some foods like spinach, rhubarb, soy, and chocolate are high in oxalates, and when sensitive people eat them, calcium oxalate stones can form in their kidneys.

You can avoid the appearance of CaC ₂ O ₄ stones in the kidneys by ingesting a lot of fluids, especially water, avoiding foods high in oxalates and consuming those rich in calcium and magnesium.

Calcium oxalate forms undesirable scale in pipes and tanks in processes such as pulp and paper manufacturing, and also in breweries.

Structure

Calcium oxalate is formed by the calcium ion Ca ²⁺ and the oxalate ion C ₂ O ₄ ^2-. The oxalate anion is made up of two carbon atoms and four oxygen atoms. The negative charges of the oxalate anion are found on the oxygen atoms.

Chemical structure of calcium oxalate. VSimonian / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5). Source: Wikimedia Commons.

Nomenclature

• Calcium oxalate
• Oxalic acid calcium salt
• Ethanedioic acid calcium salt

Properties

Physical state

Colorless, white, yellow or brown crystalline solid that can be in three different hydrated forms.

Molecular weight

128.1 g / mol

Melting point

Calcium oxalate monohydrate decomposes at 200 ° C.

Specific weight

Monohydrate CaC ₂ O _{4 •} H ₂ O = 2.22 g / cm ³

Dihydrate CaC ₂ O _{4 •} 2H ₂ O = 1.94 g / cm ³

Trihydrate CaC ₂ O _{4 •} 3H ₂ O = 1.87 g / cm ³

Solubility

Almost insoluble in water: 0.00061 g / 100 g of water at 20 ° C. The monohydrate dissolves in dilute acid.

pH

Aqueous solutions of calcium oxalate are weakly basic.

Chemical properties

Calcium oxalate is the calcium salt of oxalic acid. This is a natural by-product of metabolism, so it is very abundant in the human body, and is part of many foods.

Oxalic acid and its conjugate base, oxalate, are highly oxidized organic compounds, with powerful chelating activity, that is, they can easily combine with positive ions with charges of +2 or +3.

Their aqueous solutions are weakly basic because the oxalate ion tends to take H ⁺ protons from the water, which releases OH ^- ions. After taking two H ⁺ protons the oxalate ion becomes oxalic acid H ₂ C ₂ O ₄:

C ₂ O ₄ ^2- + H ₂ O → HC ₂ O ₄ ^- + OH ^-

HC ₂ O ₄ ^- + H ₂ O → H ₂ C ₂ O ₄ + OH ^-

Presence in nature

In minerals

Calcium oxalate is the most common oxalate and comes in the form of the minerals whewellite, weddellite, and caoxite.

Whewellite is the monohydrate CaC ₂ O _{4 •} H ₂ O and is the most stable of the forms of this compound.

Whewellite mineral rock. Rob Lavinsky, iRocks.com - CC-BY-SA-3.0 / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0). Source: Wikimedia Commons.

Weddellite is the dihydrate CaC ₂ O _{4 •} 2H ₂ O and is less stable than the monohydrate.

Weddelite mineral crystals. Leon Hupperichs / CC BY (https://creativecommons.org/licenses/by/3.0). Source: Wikimedia Commons.

Caoxite is calcium oxalate trihydrate CaC ₂ O _{4 •} 3H ₂ O.

In plants and fungi

Calcium oxalate is found associated with dry soils and leaves, also with pathogenic fungi, free, in symbiosis or associated with plants. In the latter, crystals are formed by the precipitation of calcium in the form of its oxalate.

The formation of CaC ₂ O ₄ by fungi has an important influence on the biological and geochemical processes of soils, since it constitutes a reserve of calcium for the ecosystem.

Presence in the human body and mammals

Oxalate originates in the liver, in red blood cells or erythrocytes, and to a lesser extent in the kidney. It is formed from the metabolism of amino acids (such as phenylalanine and tryptophan) and by oxidation of glyoxal dialdehyde.

Also vitamin C can be converted into oxalate when performing its antioxidant function.

Calcium oxalate is found in stones that form in the kidneys of people or animals with kidney disease.

The so-called calcium oxalate stones or stones are formed by the crystallization or aggregation of CaC ₂ O ₄ in urine supersaturated with calcium and oxalate. This means that urine contains so much calcium and oxalate that it is not possible for this compound to remain dissolved, but rather to precipitate or become solid in the form of crystals.

In humans

The formation of grit or stones in the kidneys is a disease called nephrolithiasis; it attacks approximately 10% of the population and about 75% of these stones are composed of calcium oxalate CaC ₂ O ₄.

Calcium oxalate can form kidney stones. Author: VSRao. Source: Pixabay.

The formation and growth of calcium oxalate crystals in the kidney occur because in some people the urine is supersaturated with this salt. Calcium oxalate develops in acidic urine at pH less than 6.0.

Supersaturation occurs when the excretion or elimination of this salt (which is very poorly soluble in water) in the urine occurs in a small volume of water.

Factors that affect the appearance of stones in the kidneys

Among the factors that favor the formation of calcium oxalate grit are excess calcium in the urine or hypercalciuria, excess oxalate in the urine or hyperoxaluria, elements derived from diet and the absence of inhibitors.

Excess oxalate can occur when high amounts of spinach, rhubarb, soy, nuts, and chocolate are eaten, among other foods.

Chocolate can be a source of calcium oxalate and promote the appearance of stones in the kidneys. Author: Alexander Stein. Source: Pixabay.

However, there are substances that inhibit or prevent stone formation from occurring. Among the compounds that prevent stone formation are small molecules such as citrate and pyrophosphate, and large molecules such as glycoproteins and proteoglycans.

Ways to avoid the formation of calcium oxalate stones

A good strategy to prevent the recurrence of grit or calcium oxalate stones includes increasing your fluid intake, increasing your intake of calcium-rich foods (such as dairy), and restricting table salt (NaCl), animal protein and foods rich in oxalate.

In animals

Since the beginning of the year 2000, an increase in calcium oxalate stones has been observed in the urinary system of cats and dogs. It seems that this depends on the type of diet that these animals eat and has to do with the acidity of the urine and magnesium (Mg) deficiency.

Calcium oxalate stones found in a dog's urinary bladder. Joel Mills / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/). Source: Wikimedia Commons.

Body response to excess oxalate

There is evidence that both humans and animals respond to excess oxalate by increasing the number of bacteria that can degrade oxalate.

Some of these bacteria are Oxalobacter formigenes, Bifidobacterium sp., Porphyromonas gingivalis and Bacillus sp., among others, and are naturally present in the intestine.

Applications

According to sources consulted, calcium oxalate is used in the coating of ceramics.

It has been used to coat limestone sculptures and other artistic elements, and has been found to improve the hardness of the material, decrease its porosity, and increase its resistance to acids and alkalis.

Italy has experimented with coating museum limestone objects with calcium oxalate to preserve them. Author: Source: Moni Quayle. Pixabay.

Problems in some processes

In the pulp and paper industry, calcium oxalate can form scale which causes many problems in the process.

To prevent its formation in the ducts or pipes of industrial processes, the enzymatic degradation of oxalic acid has been proposed, by means of enzymes such as oxalate oxidase.

It also tends to accumulate like stone in the containers where beer is made, from where it must be eliminated to avoid the formation of microorganisms that can give an unpleasant taste to the drink.

Risks

In high concentrations oxalate can cause death in animals and occasionally in humans, mainly due to its corrosive effects.

The accumulation of oxalate and its conjugated acid, oxalic acid, can cause disorders such as heart malfunction, calcium oxalate stones, kidney failure, and even death from toxicity.

References

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