Gallic acid is a trihydroxybenzoic acid, a type of phenolic acid, a
type of organic acid, also known as 3,4,5-trihydroxybenzoic acid,
found in gallnuts, sumac, witch hazel, tea leaves, oak bark, and
other plants. The chemical formula is C6H2(OH)3COOH. Gallic acid is
found both free and as part of hydrolyzable tannins.
Salts and esters of gallic acid are termed 'gallates'. Despite its
name, it does not contain gallium.
Gallic acid is commonly used in the pharmaceutical industry. It is
used as a standard for determining the phenol content of various
analytes by the Folin-Ciocalteau assay; results are reported in
gallic acid equivalents.Gallic acid can also be used as a starting
material in the synthesis of the psychedelic alkaloid.
Historical context and uses
Gallic acid is an important component of iron gall ink, the
standard European writing and drawing ink from the 12th to 19th
century with a history extending to the Roman empire and the Dead
Sea Scrolls. Pliny the Elder (23-79 AD) describes his experiments
with it and writes that it was used to produce dyes. Galls (also
known as oak apples) from oak trees were crushed and mixed with
water, producing tannic acid. It could then be mixed with green
vitriol (ferrous sulfate) - obtained by allowing sulfate-saturated
water from a spring or mine drainage to evaporate - and gum arabic
from acacia trees; this combination of ingredients produced the
ink.
Gallic acid was one of the substances used by Angelo Mai
(17821854), among other early investigators of palimpsests, to
clear the top layer of text off and reveal hidden manuscripts
underneath. Mai was the first to employ it, but did so "with a
heavy hand", often rendering manuscripts too damaged for subsequent
study by other researchers.
George Washington used gallic acid to communicate with
spies[clarification needed] during the American Revolutionary War,
according to the miniseries America: The Story of Us.
Gallic acid is a component of some pyrotechnic whistle mixtures.
