Tannic Acid

TANNIC ACID

Tannic acid is not a single constituent but a type of hydrolysable tannin that contains several units of gallic or ellagic acids esterified with the glucosyl OH to produce complex tannin compounds. Its exact composition varies according to its source. Turkish galls have a maximum complexity of hexa or heptagalloyl glucose, whereas Chinese galls are octa or nonagalloyl glucose, which affords methylgallate and pentagalloyl glucose on hydrolysis.

Tannic acid is extracted with a mixture of water, alcohol, and ether. The extracted liquid separates into two layers. The aqueous lower layer contains gallotannins, whereas the ethereal layer contains free gallic acid and other similar compounds. Aqueous and etheral extracts are treated separately for further purification.

Tannic acid occurs as amorphous powder containing brownish spongy masses. It has a faint odour and strong astringent taste. It is soluble in water, alcohol, and acetone but insoluble in organic solvents.

Tannic acid has strong astringent properties. It is used as an antidote in cases of alkaloidal poisoning as it precipitates alkaloids as tannate salts. It finds its uses in tanning, dyeing industries and for ink manufacture. Its preparation can be used topically for the treatment of bedsores and minor ulcerations. It is utilized in the laboratory as a reagent for detection of gelatine and proteins.