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|Section2= |Section7= }} Tannic acid is a specific commercial 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 part: Tara pods (''Caesalpinia spinosa''), gallnuts from ''Rhus semialata'' or ''Quercus infectoria'' or Sicilian Sumac leaves (''Rhus coriaria''). According to the definitions provided in external references such as international pharmacopoeia, Food Chemical Codex and FAO-WHO tannic acid monograph only tannins sourced from the above mentioned plants can be considered as tannic acid. Sometimes extracts from chestnut or oak wood are also described as tannic acid but this is an incorrect use of the term. It is a yellow to light brown amorphous powder which is highly soluble in water; one gram dissolves in 0.35 mL of water. While tannic acid is a specific type of tannin (plant polyphenol), the two terms are sometimes (incorrectly) used interchangeably. The long-standing misuse of the terms, and its inclusion in scholarly articles has compounded the confusion. This is particularly widespread in relation to green tea and black tea, both of which contain tannin but not tannic acid. Tannic acid is not an appropriate standard for any type of tannin analysis because of its poorly defined composition.〔http://www.users.muohio.edu/hagermae/〕 == Quercitannic and gallotannic acids == Quercitannic acid is one of the two forms of tannic acid〔(L'Energie Homo-Hydrogne By Patricia Le Roux (French) )〕 found in oak bark and leaves.〔(''Quercus'' on www.henriettes-herb.com )〕 The other form is called gallotannic acid and is found in oak galls. The quercitannic acid molecule is also present in quercitron, a yellow dye obtained from the bark of the Eastern black oak (''Quercus velutina''), a forest tree indigenous in North America. It is described as a yellowish brown amorphous substance. In 1838, Jöns Jacob Berzelius wrote that quercitannate is used to dissolve morphine.〔Traité de chimie, Volume 2. Jöns Jakob Berzelius (friherre) and Olof Gustaf Öngren, A. Wahlen et Cie., 1838〕 In 1865 in the fifth volume of "A dictionary of chemistry", Henry Watts wrote : It exhibits with ferric salts the same reactions as gallotannic acid. It differs however from the latter in not being convertible into gallic acid, and not yielding pyrogallic acid by dry distillation. It is precipitated by sulfuric acid in red flocks. (Stenhouse, Ann. Ch. Pharm. xlv. 16.) In 1880, Etti gave for it the molecular formula C17H16O9. He described it as an unstable substance, having a tendency to give off water to form anhydrides (called phlobaphenes), one of which is called ''oak-red'' (C34H30O17). For him, it was not a glycoside. In Allen’s "Commercial Organic Analysis", published in 1912, the formula given was C19H16O10. Other authors gave other molecular formulas like C28H26O15, while another formula found is C28H24O11. According to Lowe, two forms of the principle exist - one soluble in water, of the formula C28H28O14, and the other scarcely soluble, C28H24O12. Both are changed by the loss of water into oak red, C28H22O11.〔(A manual of organic materia medica and pharmacognosy - An introduction to the study of the vegetable Kingdom and the vegetable and animal drugs (Sayre's Materia Medica) fourth edition, by Lucius E Sayre )〕 Quercitannic acid was for a time a standard used to assess the phenolic content in spices, given as ''quercitannic acid equivalent''. In an interesting historical note, the inventor Edward G. Acheson (Inventor of Carborundum) discovered that gallotannic acid greatly improved the plasticity of clay. In his report of this discovery in 1904 he noted that the only known historical reference to the use of organic material added to clay is the use of straw mixed with clay described in the Bible, Exodus 1:11 and that the Egyptians must have been aware of his (re-)discovery. He stated "This explains why the straw was used and why the children of Israel were successful in substituting stubble for straw, a course that would hardly be possible, were the fibre of the straw depended upon as a bond feasible for the clay, but quite reasonable where the extract of the plant was used." 〔 Edward G. Acheson, "Egyptianized Clay" in Transactions of the American Ceramic Society (1904), pp 31-65.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tannic acid」の詳細全文を読む スポンサード リンク
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