oxalic acid (ŏksăl´Ĭk) or ethanedioic acid (ĕth´āndīōĬk), HO₂CCO₂H, a colorless, crystalline organic carboxylic acid that melts at 189°C with sublimation. Oxalic acid and oxalate salts are poisonous. Oxalic acid is found in many plants, e.g., sorrel and rhubarb, usually as its calcium or potassium salts. Oxalic acid is the only possible compound in which two carboxyl groups are joined directly; for this reason oxalic acid is one of the strongest organic acids. Unlike other carboxylic acids (except formic acid), it is readily oxidized; this makes it useful as a reducing agent for photography, bleaching, and ink removal. Oxalic acid is usually prepared by heating sodium formate with sodium hydroxide to form sodium oxalate, which is converted to calcium oxalate and treated with sulfuric acid to obtain free oxalic acid.

oxalic acid A dicarboxylic acid, chemically COOH—COOH. Poisonous in large amounts; present especially in spinach, chocolate, rhubarb, and nuts. The toxicity of rhubarb leaves is due to their high content of oxalic acid.

High concentrations of oxalates in the urine can form kidney stones; while most of these oxalates are of endogenous metabolic origin, patients with hyperoxaluria are advised to avoid dietary sources of oxalates.

oxalic acid (oks-al-ik) n. an extremely poisonous acid found in many plants, including sorrel and the leaves of rhubarb, and in some bleaching powders. When swallowed it produces burning sensations in the mouth and throat, vomiting of blood, breathing difficulties, and circulatory collapse. Treatment is with calcium lactate or other calcium salts, lime water, or milk. Formula: C₂H₂O₄.

oxalic acid (C₂H₂O₄) Poisonous, colourless crystalline organic acid whose salts occur naturally in some plants, such as sorrel and rhubarb. It is used for metal and textile cleaning and in tanning. Properties: m.p. 101.5°C (214.7°F).

oxalic acid (ethanedioic acid) A crystalline solid, (COOH)₂, that is slightly soluble in water. Oxalic acid is strongly acidic and very poisonous. It occurs in certain plants, e.g. sorrel and the leaf blades of rhubarb.

Oxalic Acid

Oxalic acid is the more common name of ethanedioic acid. The name ethanedioic acid communicates that the molecule has two carbon atoms (as in ethane) and two acid groups (COOH).

It is a white solid used in removal of certain kinds of stains, in removing calcium ions from solutions, and in tanning leather. It occurs naturally and is toxic. The potassium and calcium salts of oxalic acid are found naturally in cabbage, spinach, and rhubarb leaves, and are also found in the bark of some species of eucalyptus trees. The metabolism of sugar by many species of mold results in the production of oxalic acid. Ingestion of large amounts can cause kidney damage, convulsions, and death.

The most common uses of oxalic acid are in tanning leather and removing rust and ink stains. In stain removal, it acts as a reducing agent (a substance that donates electrons to other substances) and is relied on by most dry cleaners for this purpose. Iron rust stains contain iron in its oxidized form (Fe III); the oxalic acid reduces it to its colorless reduced form (Fe II). Oxalic acid is also used to clean metals in many industries and is also used in the purification of glycerol (glycerin).

Few people ingest toxic amounts of oxalic acid directly. However, if a child or pet swallows antifreeze (which typically contains ethylene glycol and has a sweet taste), enzymes in the body will metabolize the ethylene glycol to oxalic acid, which is the reason antifreeze is toxic.

In many industrial processes oxalic acid is used to remove calcium ions from solutions. The reaction of calcium ions with oxalic acid produces an insoluble solid, calcium oxalate.

Oxalic acid

Oxalic acid is the more common name of ethanedioic acid. The name ethanedioic acid communicates that the molecule has two carbon atoms (as in ethane) and two acid groups (COOH).

It is a white solid used in removal of certain kinds of stains, in removing calcium ions from solutions, and in tanning leather. It occurs naturally and is toxic. The potassium and calcium salts of oxalic acid are found naturally in cabbage, spinach , and rhubarb leaves, and are also found in the bark of some species of eucalyptus trees. The metabolism of sugar by many species of mold results in the production of oxalic acid. Ingestion of large amounts can cause kidney damage, convulsions, and death.

The most common uses of oxalic acid are in tanning leather and removing rust and ink stains. In stain removal, it acts as a reducing agent (a substance that donates electrons to other substances) and is relied on by most dry cleaners for this purpose. Iron rust stains contain iron in its oxidized form (Fe III); the oxalic acid reduces it to its colorless reduced form (Fe II). Oxalic acid is also used to clean metals in many industries and is also used in the purification of glycerol (glycerin).

Few people ingest toxic amounts of oxalic acid directly. However, if a child or pet swallows antifreeze (which typically contains ethylene glycol and has a sweet taste), enzymes in the body will metabolize the ethylene glycol to oxalic acid, which is the reason antifreeze is toxic.

In many industrial processes oxalic acid is used to remove calcium ions from solutions. The reaction of calcium ions with oxalic acid produces an insoluble solid, calcium oxalate.

Oxalic Acid

OVERVIEW

Oxalic acid (ok-SAL-ik AS-id) is a transparent, colorless, crystalline solid that often occurs as the dihydrate (HOOCCOOH·2H₂O). The dihydrate melts and begins to decompose at 101.5°C (214.7°F), forming the anhydrous acid. The compound is odorless, but has a characteristic tart, acidic taste. The acid should never be tasted, however, as it is very toxic.

Oxalic acid is one of the first organic acids to have been discovered and studied. It was first isolated by the German chemist Johann Christian Wiegleb (1732–1800) in 1769 and first synthesized by the Swedish chemist Karl Wilhelm Scheele (1742–1786) in 1776. Friedrich Wöhler's (1800–1882) synthesis of oxalic acid entirely from inorganic materials was a critical step in disproving the Vitalistic Theory of chemistry. The theory claimed that compounds found in living organisms could be produced only by the act of some supernatural being, and not by human actions.

KEY FACTS

OTHER NAMES:

Ethanedioic acid

FORMULA:

HOOCCOOH

ELEMENTS:

Carbon, hydrogen, oxygen

COMPOUND TYPE:

Dicarboxylic acid; organic acid (organic)

STATE:

Solid

MOLECULAR WEIGHT:

90.04 g/mol

MELTING POINT:

189.5°C (373.1°F); begins to sublime at 157°C (315°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Soluble in water and ethyl alcohol; slightly soluble in ether; insoluble in most other organic solvents

Oxalic acid occurs naturally in a number of vegetable products, including spinach, rhubarb, tea, chocolate, oats, pumpkin, lentils, beets, parsnips, and many kinds of nuts and berries. The amount present in foods is generally so low that it does not present risk to people who eat such products. Oxalic acid also occurs as a product of carbohydrate metabolism in animals.

HOW IT IS MADE

Traditionally, oxalic acid has been extracted from natural products by treating them with an alkaline solution, followed by crystallization of the acid. Sodium hydroxide is the alkaline material most commonly used for this procedure. Today, a number of methods are available for the commercial preparation of oxalic acid. In one procedure, carbon monoxide gas is bubbled through a concentrated solution of sodium hydroxide to produce oxalic acid. Alternatively, sodium formate (COONa) is heated in the presence of sodium hydroxide or sodium carbonate to obtain the acid. Another popular method of preparing oxalic acid involves the oxidation of sucrose (common table sugar) or more complex carbohydrates using nitric acid as a catalyst. The reaction results in the formation of oxalic acid and water as the primary products.

COMMON USES AND POTENTIAL HAZARDS

Over a half million kilograms (about 1 million pounds) of oxalic acid are produced in the United States each year. The greatest portion of the product is used in a variety of cleaning products, including substances for the bleaching and cleaning of wood, cork, cane, feathers, and natural and synthetic fibers. Many metal polishes, auto radiator cleaners, and laundry rinses also contain oxalic acid. Some rust-proofing materials also contain the compound.

The list of other household and industrial applications of oxalic acid is extensive and includes:

• In the printing and dyeing of fabrics, especially calico;
• For the treatment of leather products, especially the leather used to make book covers;
• As a paint and varnish remover as well as a stain remover for ink and rust marks;
• In the manufacture of blue ink, celluloid, rubber, and other synthetic products;
• For the decolorization of glycerol and the stabilization of hydrocyanic acid (HCN);
• As a purification agent for various chemicals, especially methanol (methyl alcohol); and
• In the preparation of certain medicines and pharmaceuticals.

Interesting Facts

• One family of plants, the Oxalidaceae, get their name from the high concentrations of oxalic acid they contain. Perhaps the best known member of the family is the wood sorrel.
• A number of molds produce oxalic acid as a major metabolic product. Some species of Penicil lium and Aspergillus, for example, convert glucose into oxalic acid.

Oxalic acid is a strong skin, eye, and respiratory irritant in pure form. It is also toxic by ingestion, causing nausea, vomiting, diarrhea, kidney damage, convulsions, coma, and death. Since people are exposed to pure oxalic acid only in the workplace, these hazards are usually not of concern to most individuals. Of more concern to the general public is the possibility of ingesting unusually large amounts of foods containing oxalic acid. In the body, the acid tends to react with calcium ions (Ca₂₊) forming calcium oxalate, which is insoluble. This reaction has two harmful effects. First, it removes calcium needed by the body for other biological functions, resulting in a calcium deficiency problem. Second, it may result in the precipitation of calcium oxalate crystals on the inner lining of blood vessels and the small tubes in the kidneys, reducing the flow of blood and resulting in the development of kidney disorders.

Words to Know

ANHYDROUS COMPOUND

A compound that lacks any water of hydration.

HYDRATE

A chemical compound formed when one or more molecules of water is added physically to the molecule of some other substance.

CATALYST

A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.

METABOLISM

Process that includes all of the chemical reactions that occur in cells by which fats, carbohydrates, and other compounds are broken down to produce energy and the compounds needed to build new cells and tissues.

SUBLIME

To go from solid to gaseous form without passing through a liquid phase.

SYNTHESIS

A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.

FOR FURTHER INFORMATION

"Material Safety Data Sheet." Hill Brothers Chemical Company. http://hillbrothers.com/msds/pdf/oxalic-acid.pdf (accessed on October 23, 2005).

"Oxalic Acid." Al's Home Improvement Center. http://alsnetbiz.com/homeimprovement/oxalic_acid.html (accessed on October 23, 2005).

"Oxalic Acid Dihydrate." Chemical Land 21. http://www.chemicalland21.com/arokorhi/industrialchem/organic/OXALIC%20ACID.htm (accessed on October 23, 2005).

"The Rhubarb Compendium." Rhubarb Poison Information Center. http://www.rhubarbinfo.com/rhubarb-poison.html (accessed on October 23, 2005).