Tannic acid

Tannic acid is a specific form of tannin, a type of polyphenol. Its weak acidity (pKa around 10) is due to the numerous phenol groups in the structure. The chemical formula for commercial tannic acid is often given as C76H52O46, which corresponds with decagalloyl glucose, but in fact it is a mixture of polygalloyl glucoses or polygalloyl quinic acid esters with the number of galloyl moieties per molecule ranging from 2 up to 12 depending on the plant source used to extract the tannic acid. Commercial tannic acid is usually extracted from any of the following plant parts: Tara pods (Caesalpinia spinosa), gallnuts from Rhus semialata or Quercus infectoria or Sicilian Sumac leaves (Rhus coriaria).

Chemical structure

Tannins are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term tannin (from Anglo-Norman tanner, from Medieval Latin tannāre, from tannum, oak bark) refers to the use of oak and other bark in tanning animal hides into leather; hence the words "tan" and "tanning" for the treatment of leather. However, the term "tannin" by extension is widely applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) to form strong complexes with proteins and other macromolecules.

Uses

Tannic acid is used in the conservation of ferrous (iron based) metal objects to passivate and inhibit corrosion. Tannic acid reacts with the corrosion products to form a more stable compound, thus preventing further corrosion from occurring. Tannic acid is also used in food applications such as in the clarification of beer and wine.

Safety

Tannic acid is not a hazardous substance. However, it can cause irritation to the eyes, skin, and respiratory tract. It can also cause nausea, vomiting, and diarrhea if ingested in large amounts.

See also

• Tannin
• Polyphenol
• Phenol
• Chemical formula
• Corrosion

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