Tannic acid is not a single constituent but a type of hydro-lysable 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.
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.
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