Tannic acid (Acidum tannicum), a commercial form of tannin, is a polyphenol. Its weak acidity (pKa around 10) is due to these phenol groups in the structure. 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.

Tannic acid is the most common mordant for cellulose fibers such as cotton. Tannin is often combined with alum and/or iron. The tannin mordant should be done first as metal mordants combine well with the fiber-tannin complex.

The presence of tannic acid in the bark of redwood (Sequoia) is a strong natural defense against wildfire, decomposition and infestation. It is found in the seeds, bark, cones, and heartwood.

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.

A popular home remedy to stop the bleeding after wisdom tooth extraction is applying tea bags (Lipton's or green tea) in the back of the jaws and biting down, given that the tannic acid in tea helps to clot blood.

It is said[weasel words] that soaking feet in tannic acid (or strong tea) can help prevent blisters.[citation needed]

But the use of tea for toughening skin appears to be apocryphal, in as much as tea is said to be incapable of tanning leather.

Tannic acid was once used as a treatment for strychnine poisoning in the late 19th and early 20th centuries.

Historical Properties & Uses

Because tannins are naturally astringent, almost all plants containing them are used as astringents. Tannic acids help eliminate diarrhea, reduce swelling of hemorrhoids, loosen catarrh in the respiratory system, and control various kinds of internal bleeding. Externally, tannins are beneficial in rubs for aching muscles and joints, in salves for open, slow-healing sores, and as antiseptics. Plants high in tannic acid have been used to treat cancers, but tannic acid itself has been found to be carcinogenic under certain conditions.

In very small amounts, tannic acid is approved for food use and is added to many commercial foods. Since the astringecy of tannic acid involves the precipitation of protein, the addition of protein (such as milk or cream) to a tannin-rich tea would "bind" the tannin, rendering it biologically inert.

Method of Action

Tannic Acid Is Astringent
The astringency of tannins comes from their ability to precipitate protein. This astringency varies from plant to plant depending upon the kinds of tannins present, the total concentration of the tannins, and ratio of the different types of tannins to each other. The two main tannins are gallic acid and catechinic acid. Another common tannin is ellagic acid.

Tannic Acid Has Antibiotic Properties
Several studies have shown tannic acid has antiviral, antimicrobial, and antibacterial properties. In many cases it acts directly on the organism to inactivate it. Tannins have also been implicated in hyaluronidase system. That is, they destroy hyaluronidase in much the same manner as does echinacea, thereby defending the cells of the body against viral invasion.
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