Tannic acid is a specific commercial form of tannin, a type of polyphenol. Its weak acidity (pKa around 10) is due to these phenol groups in the structure. Tannic acid (TAN-ik AS-id) 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.
Tannic acid is used in the conservation of ferrous (iron based) metal objects to passivate and inhibit corrosion. Tannic acid is also used to produce tannate salts of certain anti-histamins and anti-tussives to impart increased stability or slow release properties to the API (active pharmaceutical ingredient).