Oxalic Acid (also called Ethanedioic Acid) is a colourless, crystalline, toxic organic compound belonging to the family of dicarboxylic acids; melting at 187 C; soluble in water, alcohol, and ether. Oxalic acid occurs naturally in quite a large number of plants. The human body also synthesizes oxalic acid from ascorbic acid (Vitamin C). Oxalic acid may combine with calcium, iron, sodium, magnesium, or potassium to form less soluble salts known as oxalates. Oxalates also occur naturally in plants. Plant foods with high concentrations of oxalic acid (over 200 ppm) include (but are not limited to): lamb's-quarter, buckwheat, star fruit, black pepper, purslane, poppy seeds, rhubarb, tea, spinach, plantains, cocoa and chocolate, ginger, almonds, cashews, garden sorrel, mustard greens, bell peppers, sweet potatoes, soybeans, tomatillos, beets and beet greens, oats, pumpkin, cabbage, green beans, mango, eggplant, tomatoes, lentils, and parsnips.
Oxalic acid is commercially manufactured by heating sodium formate in the presence of an alkali catalyst to form sodium oxalate, which should be converted to free oxalic acid when treated with sulfuric acid. It is also prepared by oxidizing carbohydrates with nitric acid, by heating saw dust with caustic alkalies or by fermentation of sugar solutions in the presence of certain molds. 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 acids in organic compounds. Unlike other carboxylic acids, oxalic acid (and formic acid) is readily oxidized and combine with calcium, iron, sodium, magnesium, or potassium to form less soluble salts called oxalates.
Oxalic acid is a strong acid that irritates the lining of the gut when consumed, and can prove fatal in large doses. Oxalic acid also combines with metals such as calcium in the body to form salts known as oxalates, which further irritate the gut and kidneys. The most common kind of kidney stone is made of calcium oxalate. Oxalic acid and oxalates are useful as reducing agents for photography, bleaching, and rust removal. They are widely used as an purifying agent in pharmaceutical industry, precipitating agent in rare-earth metal processing, bleaching agent in textile and wood industry, rust-remover for metal treatment, grinding agent, waste water treatment. acid rinse in laundries and removing scale from automobile radiators.
Since oxalic acid binds with important nutrients, making them inaccessible to the body, regular consumption of large amounts of foods high in oxalic acid over a period of weeks to months may result in nutrient deficiencies, most notably of calcium. Because it binds vital nutrients such as calcium, long-term consumption of foods high in oxalic acid can lead to nutrient deficiencies. Healthy individuals can safely consume such foods in moderation, but those with kidney disorders, gout, or rheumatoid arthritis are typically advised to avoid foods high in oxalic acid or oxalates. Foods that contain significant quantities of oxalic acid include cocoa, chocolate, most nuts, most berries, beans, and beets, among many others. In addition to its natural occurance in plants, oxalic acid may also be found in household chemical products such as some bleaches and rustproofing treatments.
Oxalic acid is a strong acid, and is irritating to tissue all by itself. Extremely high doses are fatal. Oxalates, on the other hand, form tiny little insoluble crystals with sharp edges, which are also irritating to tissue. So, high levels of oxalic acid/oxalates in the diet lead to irritation of the digestive system, and particularly of the stomach and kidneys. They may also contribute to the formation of kidney stones (the most common form of kidney stone is composed of calcium oxalate).