Oxalic acid

Oxalic acid is the ligand in coordination chemistry. Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate, which is the primary constituent of the most common kind of kidney stone.

Preparation

Although it can be readily purchased, oxalic acid can be prepared in the laboratory by sodium hydroxide.^[2]

Typically oxalic acid is obtained as the hydrogen-bonding results in a chain-like structure whereas the hydrogen bonding pattern in the other form defines a sheet-like structure.^[4]

Reactions

Oxalic acid is a relatively strong "weak acid" with pKa₁=1.27 and pKa₂=4.28. Oxalic acid exhibits many of the reactions characteristic of other carboxylic acids. It forms esters such as dimethyloxalate (m.p. 52.5–53.5 °C).^[5]. It forms an acid chloride called oxalyl chloride.

Oxalate, the conjugate base of oxalic acid, is an excellent platinum-based drugs, avoiding the dose-limiting side-effect of nephrotoxicity.

Occurrence in nature

Oxalic acid and oxalates are abundantly present in many plants, most notably fat hen (lamb's quarters), sour grass, and rhubarb and buckwheat are listed being high in oxalic acid.^[6]

Foods that are edible but that still contain significant concentrations of oxalic acid include—in decreasing order—star fruit (carambola), casein found in various dairy products.

Leaves of the Camellia sinensis) contain among the greatest measured concentrations of oxalic acid relative to other plants. However the infusion beverage typically contains only low to moderate amounts of oxalic acid per serving, due to the small mass of leaves used for brewing.

Physiological effects

  The affinity of divalent metal ions is sometimes reflected in their tendency to form insoluble precipitates. Thus in the body, oxalic acid also combines with metals ions such as nutrients such as calcium, long-term consumption of foods high in oxalic acid can be problematic. Healthy individuals can safely consume such foods in moderation, but those with kidney disorders, gout, rheumatoid arthritis, or certain forms of chronic vulvar pain (vulvodynia) are typically advised to avoid foods high in oxalic acid or oxalates. Conversely, calcium supplements taken along with foods high in oxalic acid can cause calcium oxalate to precipitate out in the gut and drastically reduce the levels of oxalate absorbed by the body (by 97% in some cases.)^[7][8] The calcium oxalate precipitate (better known as kidney stones) obstruct the kidney tubules.

Oxalic acid can also be produced by the metabolism of vitamin C). Under certain conditions of concentration and pH, oxalic acid can precipitate in the kidneys as calcium oxalate crystals, forming an estimated 80% of kidney stones.^[9]

Some Aspergillus species produce oxalic acid, which reacts with blood or tissue calcium to precipitate calcium oxalate.^[10] There is some preliminary evidence that the administration of probiotics can affect oxalic acid excretion rates^[11] (and presumably oxalic acid levels as well.)

Methods to reduce the oxalate content in food are of current interest.^[12]

Other uses

• In household chemical products such as rustproofing treatments.
• In wood restorers where the acid dissolves away a layer of dry surface wood to expose fresh material underneath.
• As an additive to automotive wheel cleaners.
• As a mordant in dyeing processes.
• Vaporized oxalic acid, or a 6% solution of oxalic acid in sugar syrup, is used by some beekeepers as an insecticide against the parasitic Varroa mite.
• As a rust remover in such applications as automotive shops and for the restoration of antiques.
• As a recommended surface pretreatment for stainless steels (surface etch) before application of solid metal or polymer self-lubricating coatings.
• For polishing stones and marble.
• Use to destroy warts.^{[citation needed]}

Tests for oxalic acid

iodine can be done.^{[citation needed]}

References