Tannic Acid - C₇₆H₅₂O₄₆

Tannic acid is defined as a type of polyphenol, and it is also known as either Gallotannic acid or Acidum tannicum.

Tannic acid contains numerous phenol groups, and thus it is a weak acid. This acid is naturally obtained from Quercus infectoria, tara pods, Sicilian Sumac leaves, and gallnuts from Rhus semialata. 

It has a chemical formula of C76H52O46.

Types of Acidum Tannicum

The two main types of Acidum tannicum are given as the Condensed tannins and Hydrolyzable tannins. The word tannin has been derived from the Celtic word for Oaktree which was used for leather processing once.

Properties of Tannic acid – C76H52O46

• The tannic acid chemical formula is C76H52O46

• Molecular Weight of Tannic acid is 1701.19 g/mol

• Its density is given by 2.12 g/cm3

• Its Melting Point is: decomposes above 200 °C

• pKa of Tannic acid is given as ca. 6

Structure of Tannic acid (C76H52O46)

Let us look at the tannic acid structure as represented below:

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Occurrence

Tannins are distributed in the species throughout the kingdom of plants. Commonly, they are found in both angiosperms and gymnosperms. Mole studied the tannin distribution in 44 families of monocotyledons (Cronquist), and 180 families of dicotyledons. Most families of dicot have contained tannin-free species (which are tested by their ability to precipitate proteins). The best-known families of which all the species tested having tannin are given as Actinidiaceae, Aceraceae, Bixaceae, Anacardiaceae, Myricaceae for dicot, Grossulariaceae, Najadaceae and Typhaceae in Monocot.

To the oak family, Fagaceae, 73 percent of the species tested (N = 22) have tannin. Mimosaceae, for those of acacias, only 39 percent of the species tested (N = 28) have tannin, among Solanaceae rate drops to 6 percent and 4 percent for the Asteraceae. A few families such as Cucurbitaceae, Boraginaceae, Papaveraceae have no tannin-rich species.

Most of the abundant polyphenols are said to be condensed tannins, which are found virtually in all families of plants, and comprising up to 50 percent of the leaf's dry weight. The tropical woods of tannins tend to be of a cathartic nature instead of the gallic type present in temperate woods.

There can be a loss in the bio-availability of still other tannins present in the plants because of pests, birds, and other pathogens.

Tests for Tannins

There exist three methods for the analysis of tannins: precipitation of alkaloids or proteins, reaction with phenolic rings, and depolymerization.

Alkaloid Precipitation

Alkaloids like cinchonine, caffeine, strychnine or quinine, precipitates tannins and polyphenols. This property is used in the quantitation method.

Goldbeater's Skin Test

When ox skin or goldbeater's skin is dipped in HCl acid, rinsed in water, and soaked in the tannin solution for 5 minutes, and washed in water, then treated with 1% of FeSO4 solution, it forms a blue-black colour if there exists tannin.

Ferric Chloride Test

In general, the use of ferric chloride (which is FeCl3) tests for phenolics. And, powdered plant leaves of the test plant (with 1.0 g) are weighed into a beaker, and then, 10 ml of distilled water is added. After that, the mixture is boiled for five minutes. Then, two drops of 5% FeCl3 are added. Formation of a greenish precipitate is an indication of the tannins’ presence. Alternatively, a portion of the extraction of water is diluted with distilled water in a ratio of 1:4, and a few drops of 10% of ferric chloride solution is added. A green or blue colour represents the presence of tannins.

Tannic vs Tannin Acid

Let us look at the primary points of difference between the tannic acid and tannins.

• Tannic acid is defined as a special kind of tannin, which is a kind of polyphenol. The low acidity (pKa, at nearly 6) is because of the various phenol groups of the structure. 

• While tannic acid is a differential form of tannin (which is plant polyphenol), the two words are often used interchangeably.

Tannic Acid Uses

Let us look at the tannic acid uses listed below:

• Tannic acid, in the production of albumin tannate which can be used as an antidiarrheal agent.

• It also has a wide application in the food industry because it can be used as a colour stabilizer, taste enhancer, and clarifying agent.

• It can be used in the process of dyeing of cellulose fibres.

• This acid can be used in the conservation of ferrous metal objects to inhibit corrosion.

• It is also used as a coagulant in rubber manufacturing.

• This acid is used to impart anti-staining properties to yarn or polyamide carpets.

• It can be used with a mixture of gelatin and albumin to manufacture tortoiseshell.

• In analytical chemistry, it can be used as a reagent.

Health Hazards

Tannic acid can damage the respiratory tract, eye, skin, and gastrointestinal tract. It also causes pain, irritation, blurred vision, and redness. When the intake of this acid happens through the skin, it can cause irritation and redness. When it is ingested, it might cause diarrhoea, nausea, and vomiting.