United
States Pharmacopeia (USP) Reference Standard
Synonym: Gallotannin, Tannin
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CAS Number 1401-55-4
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Empirical Formula (Hill Notation) C76H52O46
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Molecular Weight 1701.20
·
Beilstein Registry
Number 8186396
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MDL number MFCD00066397
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PubChem Substance ID 329751222
Properties
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Related Categories
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InChI Key
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LRBQNJMCXXYXIU-PPKXGCFTSA-N
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autoignition temp.
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980 °F
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mp
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218 °C(lit.)
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solubility
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H2O:
soluble
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acetone: very soluble
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alcohol: very soluble
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benzene: insoluble
(practically)
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Tannic acid is a specific form of tannin, a
type of polyphenol. Its weak acidity (pKa around 10) is due to the numerous phenol groups in the structure. The chemical formula for commercial tannic acid is often given as C76H52O46,
which corresponds with decagalloyl glucose, but in fact it is a mixture of
polygalloyl glucoses or polygalloyl quinic acid esters with the number of
galloyl moieties per molecule ranging from 2 up to 12 depending on the plant
source used to extract the tannic acid.
Detailed
description
Commercial tannic acid is usually extracted from any
of the following plant parts: Tara pods (Caesalpinia spinosa),
gallnuts from Rhus semialata or Quercus infectoria or Sicilian Sumac leaves (Rhus coriaria).
According to the definitions provided in external
references such as international pharmacopoeia, Food Chemicals Codex and FAO-WHO tannic
acid monograph only tannins sourced from the above-mentioned plants can be
considered as tannic acid. Sometimes extracts from chestnut or oak wood are also
described as tannic acid but this is an incorrect use of the term. It is a
yellow to light brown amorphous powder; 2850 grams dissolves in one litre of water (1.7 moles per
liter).
While tannic acid is a specific type of tannin (plant polyphenol),
the two terms are sometimes (incorrectly) used interchangeably. The
long-standing misuse of the terms, and its inclusion in scholarly articles has
compounded the confusion. This is particularly widespread in relation to green
tea and black tea, both of which contain tannin but not tannic acid.[1]
Tannic acid is not an appropriate standard for any
type of tannin analysis because of its poorly defined composition.
Uses
Tannins are a basic ingredient in the chemical
staining of wood,
and are already present in woods like oak, walnut,
and mahogany.
Tannic acid can be applied to woods low in tannin so chemical stains that
require tannin content will react. The presence of tannins in the bark of redwood (Sequoia) is a strong natural defense against wildfire, decomposition and infestation by certain insects such as termites.
It is found in the seeds, bark, cones,
and heartwood.
Tannic acid is a common mordant used in the dyeing
process for cellulose fibers such as cotton, often combined with alum and/or iron. The tannin mordant should be done first as metal mordants
combine well with the fiber-tannin complex. However this use has lost
considerable interest.
Similarly tannic acid can also be used as an
aftertreatment to improve wash fastness properties of acid dyed polyamide. It is also an
alternative for fluorcarbon aftertreatments to impart anti-staining properties
to polyamide yarn or carpets. However, due to economic considerations currently
the only widespread use as textile auxiliary is the use as an agent to improve
chlorine fastness, i.e. resistance against dye bleaching due to cleaning with
hypochlorite solutions in high-end polyamide 6,6-based carpets and swimwear. It
is, however, used in relatively small quantities for the activation of upholstery flock; this serves
as an anti-static treatment.
Tannic acid is used in the conservation of ferrous
(iron based) metal objects to passivate and inhibit corrosion. Tannic acid
reacts with the corrosion products to form a more stable compound, thus
preventing further corrosion from taking place. After treatment the tannic acid
residue is generally left on the object so that if moisture reaches the surface
the tannic acid will be rehydrated and prevent or slow any corrosion. Tannic
acid treatment for conservation is very effective and widely used but it does
have a significant visual effect on the object, turning the corrosion products
black and any exposed metal dark blue. It should also be used with care on
objects with copper alloy components as the tannic acid can have a slight
etching effect on these metals.
Tannic acid is also found in commercially available
iron/steel corrosion treatments, such as Hammerite
Kurust.
Use in food
Use of tannic acid in food applications is far more
widespread and significant amounts are used as process aids in beer
clarification, aroma compound in soft drinks and juices. Equally important are
applications in the wine industry, where it finds use as a natural clarifying
agent, colour stabilizer and taste enhancer.
In many parts of the world, such uses are permitted.
In the United States, tannic acid is generally recognized
as safe by the Food and Drug
Administration.
According to EU directive 89/107/EEC tannic
acid cannot be considered as a food additive and consequently
does not hold an E number. Under directive
89/107/EEC tannic acid can be referred to as a food ingredient. The E-number
E181 is sometimes incorrectly used to refer to tannic acid; this in fact refers
to the INS number assigned to tannic acid under the FAO-WHO Codex Alimentarius
system.
Tannins are a common impurity in the bevarage ayahuasca made
from mimosa hostilis root bark. Tannins contribute to the nausea and
gastrointestinal upset associated with ayahuasca consumption. Addition of
gelatin to ayahuasca binds to these tannins, causing them to sink.
Uses as a medication
In conjunction with magnesium and sometimes
activated charcoal, tannic acid was once used as a treatment for many toxic
substances, such as strychnine, mushroom, and
ptomaine poisonings in the late 19th and early 20th centuries.[13]
The introduction of tannic acid treatment of severe
burn injuries in the 1920s significantly reduced mortality rates.[14] During World War I, tannic acid
dressings were prescribed to treat "burns, whether caused by incendiary
bombs, mustard gas or lewisite.[15] After the war this
use was abandoned due to the development of more modern treatment regimens.
Tannic acid is still used in pharmaceutical
applications to produce albumin tannate which is used as an
antidiarrheal agent. Tannic acid is also used to produce tannate salts of certain antihistamine and antitussive products
to impart increased stability or slow release properties to the active
pharmaceutical ingredient. Further to this,
tannic acid is the principle but perhaps minimally effective ingredient in
antiallergen sprays.[16]
Tannins have also been reported to exert many
physiological effects, such as to accelerate blood clotting, reduce blood
pressure, decrease the serum lipid level, produce liver necrosis, and modulate
immunoresponses.[17] This would explain
common folklore such as that soaking feet in tannic acid (or strong tea) can
treat or prevent blisters, foot odor and rough, dry feet.[18]
Crocodilian coloration
Skin color in Crocodilia (crocodiles
and alligators) is very dependent on water quality. Algae-laden waters produce
greener skin, while tannic acid in the water from decay of leaves from
overhanging trees (which produces some types of blackwater rivers) often produce darker skin
in these animals.
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