What is "tannic acid"

Tannic \Tan"nic\, a. Of or pertaining to tan; derived from, or resembling, tan; as, tannic acid. Tannic acid. (Chem.)

1. An acid obtained from nutgalls as a yellow amorphous substance, C14H10O9, having an astringent taste, and forming with ferric salts a bluish-black compound, which is the basis of common ink. Called also tannin, and gallotannic acid.

2. By extension, any one of a series of astringent substances resembling tannin proper, widely diffused through the vegetable kingdom, as in oak bark, willow, catechu, tea, coffee, etc.

n. (context chemistry English) any of various complex phenols extracted from the bark or galls of trees; used in tanning

n. any of various complex phenolic substances of plant origin; used in tanning and in medicine [syn: tannin]

Tannic acid is a specific commercial form of tannin, a type of polyphenol. Its weak acidity ( pK around 10) is due to the numerous phenol groups in the structure. The chemical formula for commercial tannic acid is often given as CHO, 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 Chemicals 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; 2850 grams dissolves in one litre of water (1.7 moles per liter).

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.