Tannic acid is a specific commercial form of tannin, a type of polyphenol. Tannic acid is a pale yellow amorphous powder, shiny scales, or spongy material that gradually darkens when exposed to air. It is odorless but has a strong, bitter taste. The chemical formula for commercial Tannic acid is often given as C76H52O46, Tannic acid occurs naturally in the bark of hemlock, chestnut, mangrove, and oak trees; in sumac plants; and in plant galls, the hard swellings that develop on trees infested with insect parasites.
The name tannic acid comes from the fact that the compound is used in the process known as tanning. It transforms the proteins in raw animal hides into forms that resist the natural process of decomposition, converting the hides into leather.
Tannic acid belongs to a large class of chemical compounds known as tannins that have the common chemical property of being able to precipitate proteins. Precipitation is the process by which the soluble form of a substance is converted to an insoluble form, which then settles out of solution. In addition to its applications in tanning, tannic acid is also used for staining wood, dyeing cloth, and treating minor cuts and wounds.
Quercitannic acid is one of the two forms of tannic acid found in oak bark and leaves. 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 as a yellowish brown amorphous substance.
Quercitannic acid was for a time a standard used to assess the phenolic content in spices, given as quercitannic acid equivalent.