Oxalates are compounds containing oxalate ions, found in nature and synthesized in the laboratory for various industrial uses. Oxalic acid, a strong reducing agent, is commonly used as a whitening agent and wood restorer. Solubility varies among oxalate compounds, with calcium oxalate being insoluble and causing kidney stones. Oxalates can also be harmful in excess, and may be linked to autism.
An oxalate is any chemical compound that contains an oxalate ion, which is the anionic or negatively charged form of oxalic acid. All compounds of this family are therefore salts or esters of oxalic acid. Oxalic acid is the simplest dicarboxylic acid and has the formula HO2CCO2H; there are many different variations, though, and many potential combinations. They are found in abundance in nature, but can also be synthesized in the laboratory for large-scale use. The compounds have a range of industrial uses, from cleaning agents to food preservatives. Depending on their primary chemical composition, however, different varieties can have strikingly different properties. Some are water soluble while others are not, for example, and others can be very strong reducing agents, even going so far as to leach nutrients such as iron from the blood. Others cause buildups, particularly of minerals like calcium. People with certain health problems are often advised to monitor their daily oxalate intake to reduce symptoms or flare-ups.
Sources and availability
Oxalic acid and other oxalate compounds occur widely in nature, usually as salts of calcium, sodium or potassium. They are common in fruits and vegetables, to which they often contribute an astringent or bitter taste. Foods high in oxalates include spinach, rhubarb, strawberries and parsley, although almost all plant foods contain some amount. They are not normally particularly harmful, although the leaves and roots of the rhubarb plant are particularly high in oxalates and eating them can be toxic.
The compounds can also be artificially synthesized in the laboratory. This route is often preferred for large-scale production of household cleaners or chemical agents, as it is often the most reliable way to ensure consistency of both strength and composition.
Oxalic acid
Oxalic acid itself is unusually strong for an organic acid and is a good reducing agent. Commercially it is widely used as a whitening agent, anti-rust and wood restorer. Like acid, it is toxic, extremely irritating, and must be used with caution to avoid ingestion or skin contact.
Solubility questions
Solubility refers to how quickly compounds dissolve or dissociate in water. Sodium and potassium oxalates tend to be fairly soluble in water, while calcium and magnesium salts are considerably less so. The solubility of calcium oxalate is usually documented at only 0.005 grams/liter (g/l) at physiological pH. This can impact everything from absorption to the stability of the larger substance it is a part of.
In fact, it is the insolubility of calcium oxalate that gives these compounds their medical significance. Formed when soluble oxalate meets naturally occurring calcium ions in the body, calcium oxalate precipitates as a solid and can cause considerable damage, especially in the kidneys. Calcium oxalate is the most common component of kidney stones, and urine oxalate content is the most significant factor in causing kidney stone formation. Patients prone to kidney stones can be placed on low oxalate diets.
As a chelating agent
Oxalate is also a good chelating agent, which basically means that it is a substance that can bind to various metal ions through electrostatic attraction. As such, it is able to prevent ions from being used by the body, for better or for worse. Chelated iron oxalate appears to be a major factor in gout, for example. Due to its low solubility, it precipitates from the blood into the joints where the crystals can cause excruciating pain.
Average intake and health concerns
The average person’s daily intake of oxalate is usually between 80 and 120 milligrams per day (mg/day), although it can range from 40 to 350 mg/day depending on what you eat and how it is prepared. Consumption levels may be even higher in individuals following a typical Western diet high in preservatives and chemical additives.
Oxalates can also be produced through the metabolism of vitamin C and for this reason high doses of this vitamin, exceeding 2 grams per day, are not recommended. Most of the soluble oxalate is excreted in the urine, while the insoluble salts are excreted in the faeces. An excess of oxalate in the urine is known as hyperoxaluria and an excess in the body is called hyperoxalosis. Both conditions can be harmful, particularly if they persist for long periods of time.
Possible link with autism
Some scholars have linked oxalate metabolism disorders to diagnoses of autism in young children, and several theories have been advanced to explain it. There are reports that a controlled oxalate diet may be beneficial in helping autistic children control their condition, but so far this information is only anecdotal.
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