How Is Tannic Acid Used in Chemistry and Everyday Life?
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.
FAQs on Tannic Acid: Structure, Types, Properties and Uses
1. What is tannic acid and what is its chemical formula?
Tannic acid is a specific type of tannin, which belongs to a class of astringent, polyphenolic biomolecules. It is not a single compound but a mixture of polymers. While an idealized chemical formula is often cited as C₇₆H₅₂O₄₆ (corresponding to decagalloyl glucose), commercial tannic acid is a complex mixture of esters of gallic acid with a core of glucose or quinic acid.
2. What are the main physical properties of tannic acid?
Tannic acid typically appears as a yellowish to light-brown amorphous powder. Its key physical properties include:
- Solubility: It is highly soluble in water, acetone, and alcohol, but poorly soluble in non-polar solvents like ether.
- Taste: It has a characteristic sharp, astringent taste.
- Decomposition: Upon heating, it decomposes to form pyrogallol and carbon dioxide.
- Appearance: It is non-crystalline and exists as a powder or a spongy, solid mass.
3. Where is tannic acid naturally found?
Tannic acid is naturally found in various parts of plants, where it often serves as a defence mechanism against herbivores. The most common commercial sources are nutgalls, which are abnormal growths on trees (especially oaks) caused by insects. It is also present in the bark, leaves, and wood of trees like oak, sumac, and chestnut.
4. What are the major industrial and commercial uses of tannic acid?
Tannic acid has a wide range of uses due to its ability to precipitate proteins. Key applications include:
- Leather Tanning: Its primary historical use is to convert animal hides into leather by cross-linking collagen fibres.
- Ink Manufacturing: It is a key component in traditional iron gall ink.
- Food and Beverage Industry: It is used as a clarifying agent for wine and beer, a flavouring agent, and a natural food additive.
- Medicine: Employed as an astringent to treat burns, diarrhoea, and as an antidote for certain types of poisoning.
5. How is tannic acid classified among other types of tannins?
Tannins are broadly divided into two main categories: hydrolysable tannins and condensed tannins. Tannic acid belongs to the hydrolysable tannins group. This means it can be broken down (hydrolysed) by weak acids or enzymes into simpler molecules, specifically gallic acid and a carbohydrate core like glucose. Condensed tannins, in contrast, do not hydrolyse easily and instead polymerise into insoluble reddish compounds when treated with acid.
6. Why is it difficult to assign a single, exact chemical structure to tannic acid?
It is difficult to assign a single structure to tannic acid because it is not a pure, single molecule but rather a polydisperse mixture. It consists of polygalloyl glucose or polygalloyl quinic acid esters. The number of galloyl groups attached to the central glucose molecule can vary (typically from 2 to 12) depending on the plant source from which it was extracted. Therefore, any chemical formula, like C₇₆H₅₂O₄₆, represents an idealized average structure rather than a precise, uniform molecule.
7. What is the key chemical difference between tannic acid and gallic acid?
The key difference lies in their chemical structure and complexity. Gallic acid (C₇H₆O₅) is a simple, single phenolic acid molecule. In contrast, tannic acid is a much larger and more complex polymer. It is essentially formed from multiple gallic acid units (or its derivatives) that are bonded via ester links to a central core molecule, which is typically glucose. In simple terms, gallic acid is the monomer or building block from which the polymer tannic acid is constructed.
8. How do the chemical properties of tannic acid contribute to its use in medicine and leather tanning?
The effectiveness of tannic acid in these applications stems from its astringent properties, which arise from its numerous phenolic hydroxyl (-OH) groups. These groups can form strong hydrogen bonds with proteins.
- In leather tanning: Tannic acid binds to the collagen protein in animal hides, cross-linking the fibres. This process displaces water and makes the hide more durable, flexible, and resistant to microbial decay.
- In medicine: When applied to tissue, its astringency causes proteins in the skin or mucous membranes to precipitate and contract, forming a protective layer. This helps reduce inflammation, secretions, and bleeding, which is why it is used to treat minor burns and diarrhoea.
