A bottle of tannic acid (water solution).
Tannic acid, a specific commercial form of tannin, is a polyphenol. Its weak acidity (pKa around 6) is due to the numerous phenol groups in the structure. The chemical formula for commercial tannic acid is often given as C76H52O46, but in fact it contains a mixture of related compounds. Its structure is based mainly on glucose esters of gallic acid. It is a yellow to light brown amorphous powder which is highly soluble in water; one gram dissolves in 0.35 mL of water.
Tannic acid is a basic ingredient in the chemical staining of wood. The tannic acid or tannin is 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 tannic acid in the bark of redwood (Sequoia) is a strong natural defense against wildfire, decomposition and infestation by certain insects such as woodboring beetles. 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.
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 have compounded the confusion. This is particularly widespread in relation to green tea and black tea. Although tea contains various types of polyphenols, "contrary to widespread belief, tea does not contain tannic acid." [1]
Tannic acid was once used as a treatment for strychnine poisoning in the late 19th and early 20th centuries.[citation needed] During WWII, tannic acid dressings were prescribed to treat "burns, whether caused by incendiary bombs, mustard gas or lewisite... Where tannic acid is not available, strong, lukewarm tea is a good substitute."[2]
There are many unverified and/or dubious claims about tannic acid, such as that soaking feet in tannic acid (or strong tea) can help prevent blisters.[citation needed] 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."
References
General References
The Merck Index, 9th edition, Merck & Co., Rahway, New Jersey, 1976.
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