Tannic acid
Encyclopedia
Tannin
A tannin is an astringent, bitter plant polyphenolic compound that binds to and precipitates proteins and various other organic compounds including amino acids and alkaloids.The term tannin refers to the use of...
, a type of polyphenol
Polyphenol
Polyphenols are a structural class of natural, synthetic, and semisynthetic organic chemicals characterized by the presence of large multiples of phenol structural units...
. Its weak acidity (pKa
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
around 6) is due to the numerous phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
groups in the structure. The chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
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. Commercial tannic acid is usually extracted from any of the following plant part: Tara pods (Caesalpinia spinosa
Caesalpinia spinosa
Caesalpinia spinosa Kuntze, commonly known as tara, is a small leguminous tree or thorny shrub native to Peru. C. spinosa is cultivated as a source of tannins based on a galloylated quinic acid structure...
), gallnuts from Rhus semialata or Quercus infectoria or Sicilian Sumac leaves (Rhus coriaria
Rhus coriaria
Rhus coriaria, commonly called Elm-Leaved Sumach or Tanner's Sumach is a deciduous shrub to small tree in the Anacardiaceae or Cashew family, native to southern Europe. The dried fruit are used as a spice, particularly in combination with other spices in the mixture called Za'atar.- Uses :Caution...
).
According to the definitions provided in external references such as international pharmacopoeia, Food Chemical 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
Chestnut
Chestnut , some species called chinkapin or chinquapin, is a genus of eight or nine species of deciduous trees and shrubs in the beech family Fagaceae, native to temperate regions of the Northern Hemisphere. The name also refers to the edible nuts they produce.-Species:The chestnut belongs to the...
or oak
Oak
An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus...
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 which is highly soluble in water; one gram
Gram
The gram is a metric system unit of mass....
dissolves in 0.35 mL of water.
While 'tannic acid' is a specific type of 'tannin
Tannin
A tannin is an astringent, bitter plant polyphenolic compound that binds to and precipitates proteins and various other organic compounds including amino acids and alkaloids.The term tannin refers to the use of...
' (plant polyphenol), the two terms are sometimes (incorrectly) used interchangeably. The long standing misuse of the terms, and its inclusion in scholarly articles have compounded the confusion. This is particularly widespread in relation to green tea and black tea. Although tea contains tannin and various types of polyphenols, "contrary to widespread belief, tea does not contain tannic acid."
Tannic acid is not an appropriate standard for any type of tannin analysis because of its poorly defined composition.
Quercitannic and gallotannic acids
Quercitannic acid is one of the two forms of tannic acid found in oak bark and leaves. The other form is called gallotannic acid and is found in oak galls.The quercitannic acid molecule is also present in quercitron
Quercitron
Quercitron is a yellow natural dye obtained from the bark of the Eastern Black Oak , a forest tree indigenous in North America....
, a yellow dye obtained from the bark of the Eastern black oak (Quercus velutina), a forest tree indigenous in North America. It is described as as a yellowish brown amorphous substance.
In 1838, Jöns Jacob Berzelius wrote that quercitannate is used to dissolve morphine.
In 1865 in the fifth volume of "A dictionary of chemistry", Henry Watts
Henry Watts (chemist)
-Life:He was born in London on 20 January 1815. He went to a private school, and was articled at the age of fifteen as an architect and surveyor; but went on to support himself by teaching, chiefly mathematical, privately and at a school. He then attended University College, London. In 1841 he...
wrote :
It exhibits with ferric salts the same reactions as gallotannic acid. It differs however from the latter in not being convertible into gallic acidGallic acidGallic 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 C6H23COOH. Gallic acid is found both free and as part of...
, and not yielding pyrogallic acid by dry distillation. It is precipitated by sulfuric acid in red flocks. (StenhouseJohn StenhouseJohn Stenhouse FRS FRSE FIC FCS was a Scottish chemist. In 1854, he invented one of the first practical respirators.He was a co-founder of the Chemical Society in 1841.-Life:...
, Ann. Ch. Pharm. xlv. 16.)
According to Rochleder (ibid lxiii. 202), the tannic acid of black tea is the same as that of oak-bark.
In 1880, Etti gave for it the molecular formula C17H16O9. He described it as an unstable substance, having a tendency to give off water to form anhydrides (called phlobaphene
Phlobaphene
Phlobaphenes can be defined either as the reddish colored phenolic substances extracted from plant that are alcohol soluble and water insoluble or the reddish colored, water insoluble products that result from treatment of tannin extracts with mineral acids .The name phlobaphen come from the Greek...
s), one of which is called oak-red (C34H30O17). For him, it was not a glycoside.
In Allen’s "Commercial Organic Analysis", published in 1912, the formula given was C19H16O10.
Other authors gave other molecular formulas like C28H26O15, while another formula found is C28H24O11.
According to Lowe, two forms of the principle exist - one soluble in water, of the formula C28H28O14, and the other scarcely soluble, C28H24O12. Both are changed by the loss of water into oak red, C28H22O11.
Quercitannic acid was for a time a standard used to assess the phenolic content in spice
Spice
A spice is a dried seed, fruit, root, bark, or vegetative substance used in nutritionally insignificant quantities as a food additive for flavor, color, or as a preservative that kills harmful bacteria or prevents their growth. It may be used to flavour a dish or to hide other flavours...
s, given as quercitannic acid equivalent.
Uses
Tannins are a basic ingredient in the chemical staining of woodWood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
, and are already present in woods like oak
Oak
An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus...
, walnut
Walnut
Juglans is a plant genus of the family Juglandaceae, the seeds of which are known as walnuts. They are deciduous trees, 10–40 meters tall , with pinnate leaves 200–900 millimetres long , with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts , but not the hickories...
, and mahogany
Mahogany
The name mahogany is used when referring to numerous varieties of dark-colored hardwood. It is a native American word originally used for the wood of the species Swietenia mahagoni, known as West Indian or Cuban 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
Wildfire
A wildfire is any uncontrolled fire in combustible vegetation that occurs in the countryside or a wilderness area. Other names such as brush fire, bushfire, forest fire, desert fire, grass fire, hill fire, squirrel fire, vegetation fire, veldfire, and wilkjjofire may be used to describe the same...
, decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
and infestation by certain insects such as termite
Termite
Termites are a group of eusocial insects that, until recently, were classified at the taxonomic rank of order Isoptera , but are now accepted as the epifamily Termitoidae, of the cockroach order Blattodea...
s. It is found in the seeds
SEEDS
SEEDS is a voluntary organisation registered under the Societies Act of India....
, bark
Bark
Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines and shrubs. Bark refers to all the tissues outside of the vascular cambium and is a nontechnical term. It overlays the wood and consists of the inner bark and the outer bark. The inner...
, cones
Conifer cone
A cone is an organ on plants in the division Pinophyta that contains the reproductive structures. The familiar woody cone is the female cone, which produces seeds. The male cones, which produce pollen, are usually herbaceous and much less conspicuous even at full maturity...
, and heartwood.
Tannic acid is a common mordant
Mordant
A mordant is a substance used to set dyes on fabrics or tissue sections by forming a coordination complex with the dye which then attaches to the fabric or tissue. It may be used for dyeing fabrics, or for intensifying stains in cell or tissue preparations. The term mordant comes from the Latin...
used in the dyeing process for cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
fibers such as cotton
Cotton
Cotton is a soft, fluffy staple fiber that grows in a boll, or protective capsule, around the seeds of cotton plants of the genus Gossypium. The fiber is almost pure cellulose. The botanical purpose of cotton fiber is to aid in seed dispersal....
, often combined with alum
Alum
Alum is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate with the formula KAl2.12H2O. The wider class of compounds known as alums have the related empirical formula, AB2.12H2O.-Chemical properties:Alums are...
and/or iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
. 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
Polyamide
A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization or solid-phase synthesis, examples being nylons, aramids,...
. 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.
A particular and unique nich textile application of tannic acid is the activation of flock, which is basically an anti-static aftertreatment.
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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, tannic acid is generally recognized as safe
Generally recognized as safe
Generally recognized as safe is an American Food and Drug Administration designation that a chemical or substance added to food is considered safe by experts, and so is exempted from the usual Federal Food, Drug, and Cosmetic Act food additive tolerance requirements.-History:On January 1, 1958,...
by the Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...
.
According to EU directive 89/107/EEC tannic acid cannot be considered as a food additive
Food additive
Food additives are substances added to food to preserve flavor or enhance its taste and appearance.Some additives have been used for centuries; for example, preserving food by pickling , salting, as with bacon, preserving sweets or using sulfur dioxide as in some wines...
and consequently does not hold an E number
E number
E numbers are number codes for food additives that have been assessed for use within the European Union . They are commonly found on food labels throughout the European Union. Safety assessment and approval are the responsibility of the European Food Safety Authority...
. 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.
Uses as a medication
Tannic acid was once used as a treatment for many toxic substances, such as strychnineStrychnine
Strychnine is a highly toxic , colorless crystalline alkaloid used as a pesticide, particularly for killing small vertebrates such as birds and rodents. Strychnine causes muscular convulsions and eventually death through asphyxia or sheer exhaustion...
, mushroom, and ptomaine poisonings in the late 19th and early 20th centuries.
The introduction of tannic acid treatment of severe burn injuries in the 1920s significantly reduced mortality rates During World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
, tannic acid dressings were prescribed to treat "burns, whether caused by incendiary bombs, mustard gas or lewisite
Lewisite
Lewisite is an organoarsenic compound, specifically an arsine. It was once manufactured in the U.S. and Japan as a chemical weapon, acting as a vesicant and lung irritant...
... Where tannic acid is not available, strong, lukewarm tea is a good substitute." After the war this use was abandoned due to the development of more modern treatment regimens.
Today tannic acid is still used in pharmaceutical applications to produce albumine tannate which is used as an anti-diarrhea agent. 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).
There are many unverified and/or dubious claims about tannic acid, such as that soaking feet in tannic acid (or strong tea) can treat or prevent blisters; foot odor; and rough, dry feet. Other claims about tannic acid seem to either add to, or have been created by the confusion between tannic acid and tannin. One example is that "A popular home remedy to stop the bleeding after wisdom tooth extraction is applying tea bags in the back of the jaws and biting down, given that the tannic acid [sic] in tea helps to clot blood."
Tannic acid glycerite is used for local application.
Trivia
Skin color in CrocodiliaCrocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
(crocodiles and alligators) is very dependent on water quality. Algae-laden waters produce greener skin, while tannic acid from overhanging trees often produce darker skin.