Tannic Acid

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Tannic Acid

KEY FACTS

OTHER NAMES:

Penta-(m-digalloyl)-glucose; tannin; gallotannin

FORMULA:

C₇₆H₅₂O₄₆

ELEMENTS:

Carbon, hydrogen, oxygen

COMPOUND TYPE:

Organic

STATE:

Solid

MOLECULAR WEIGHT:

1701.20 g/mol

MELTING POINT:

Begins to decompose at 210°C (410°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Soluble in water, ethyl alcohol, and acetone

OVERVIEW

Tannic acid (TAN-ik AS-id) is a pale yellow amorphous powder, shiny scales, or spongy material that gradually darkens when exposed to air. It is odorless but has a strong, bitter taste. It is a derivative of the simple sugar glucose (C₆H₁₂O₆) in which five hydroxyl groups (-OH) have been replaced by large complex side chains known as digalloyl groups. Tannic acid occurs naturally in the bark of hemlock, chestnut, mangrove, and oak trees; in sumac plants; and in plant galls, the hard swellings that develop on trees infested with insect parasites. The name tannic acid comes from the fact that the compound is used in the process known as tanning. It transforms the proteins in raw animal hides into forms that resist the natural process of decomposition, converting the hides into leather. Tannic acid belongs to a large class of chemical compounds known as tannins that have the common chemical property of being able to precipitate proteins. Precipitation is the process by which the soluble form of a substance is converted to an insoluble form, which then settles out of solution. In addition to its applications in tanning, tannic acid is also used for staining wood, dyeing cloth, and treating minor cuts and wounds.

HOW IT IS MADE

Tannic acid is extracted from natural source by using the principle that the compound is soluble in water and alcohol, while many components with which it occurs are not. The most common sources of tannic acid have traditionally been the Chinese nutgall, which consists of about 70 percent tannic acid, and Turkish nutgall, which contains about 50 to 60 percent tannic acid. The nutgall is ground into a powder and then mixed with a combination of water and/or alcohol and ether. The tannic acid in the powder dissolves in the water/alcohol portion of the mixture, and other components of the gall dissolve in the ether portion. The water and alcohol are then allowed to evaporate, leaving behind relatively pure tannic acid. In recent years, tannic acid has been obtained more economically from the bark of the quebracho tree, which grows in South America and is related to poison ivy.

Interesting Facts

A kind of ink called iron gall ink is made from tannic acid, iron(II) sulfate, gum arabic, and water. The recipe for this ink has been known since the Middle Ages. It is used for important documents because it does not wear away easily.
Tannic acid is mentioned in some very old medical books as a treatment for mushroom poisoning.
Studies suggest that people allergic to cats may be able to control their allergies by spraying surfaces in the home with a solution of tannic acid. The compound chemically alters the proteins in feline saliva that trigger allergic reactions.
Tannic acid is not a true acid because it does not contain the carboxyl group (−COOH) present in all organic acids. Instead, it is a polyphenol, a compound with many ("poly") phenol (C₆H₅OH) groups.

COMMON USES AND POTENTIAL HAZARDS

Tannic acid continues to be used for many of the applications for which it has been known for centuries, including

Words to Know

AMORPHOUS: Without crystalline structure.
ELECTROPLATING: A process by which a thin layer of one metal is deposited on top of a second metal by passing an electric current through a solution of the first metal.
MORDANT: A substance used in dyeing and printing that reacts chemically with both a dye and the material being dyed to help hold the dye permanently to the material.
MUTAGEN: A substance that causes a mutation in plants or animals. Mutations are changes in an organism's genetic composition.
PRECIPITATE: A solid material that settles out of a solution, often as the result of a chemical reaction.

the tanning of animal hides, the manufacture of specialized inks, and the treatment of minor cuts and bruises. Its medical applications are based on its astringent properties. An astringent is a compound that triggers a loss of water from tissue, thereby causing the tissues to shrink and contract. The compound also has a number of other commercial and industrial uses, including:

As a mordant in the coloring of fabrics and the printing of colored papers;
In the manufacture of many kinds of chemicals, including tannates (compounds of tannic acid and a metal), gallic acid (C₆H₂(OH)₃COOH; with many of the same uses as tannic acid), and pyrogallic acid (C₆H₃(OH)₃; also with similar uses);
As a clarifying (purifying) agent in the manufacture of beers and wines;
In electroplating;
In the manufacture of artificial horn and tortoise shell products;
As a coagulant in the manufacture of rubber to precipitate out impurities;
In the deodorizing of crude oil; and
In photographic processes.

Tannic acid is a mild irritant of the skin, eyes, and respiratory tract. It is toxic by ingestion. In large doses, it may cause liver damage. No evidence is available on its carcinogenic or mutagenic effects in humans.