What Is Tannic Acid Definition Structure Reactions and Practical Uses
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 Properties and Chemical Behavior
1. What is tannic acid in chemistry?
Tannic acid is a water-soluble polyphenolic compound derived from plant tannins, commonly represented by the approximate formula C76H52O46. It belongs to the group of hydrolysable tannins, especially gallotannins, and is composed of multiple gallic acid units esterified to a glucose core. In chemistry, tannic acid is studied for its acidic phenolic –OH groups, ability to form complexes with proteins and metal ions, and its antioxidant properties.
2. What is the chemical formula of tannic acid?
The commonly cited chemical formula of tannic acid is C76H52O46, although its exact composition can vary slightly. Tannic acid is not a single simple molecule but a mixture of closely related polygalloyl glucose esters. Key points include:
- It contains many phenolic –OH groups.
- It has multiple ester linkages formed from gallic acid and glucose.
- Its high oxygen content explains its strong hydrogen bonding and water solubility.
3. Is tannic acid a strong or weak acid?
Tannic acid is a weak acid because it donates protons from phenolic –OH groups rather than from a strong mineral acid group. Its acidity arises from:
- Multiple phenolic hydroxyl (–OH) groups.
- Partial stabilization of the phenoxide ion by resonance.
- A relatively low degree of ionization in water.
4. How does tannic acid react with iron(III) ions?
Tannic acid reacts with iron(III) ions to form a dark blue or black iron–tannate complex. The reaction involves coordination between Fe3+ ions and phenolic oxygen atoms. A simplified representation is:
- Fe3+(aq) + tannic acid(aq) → Fe–tannate complex(s)
5. What type of compound is tannic acid?
Tannic acid is a polyphenolic organic compound classified as a hydrolysable tannin. Chemically, it:
- Contains multiple aromatic rings.
- Has many phenolic –OH functional groups.
- Includes ester linkages between gallic acid units and glucose.
6. How is tannic acid hydrolyzed?
Tannic acid is hydrolyzed by acids or enzymes to yield gallic acid and glucose. During hydrolysis:
- The ester bonds between gallic acid and glucose are broken.
- Water participates in the cleavage reaction.
- Tannic acid + H2O → gallic acid + glucose
7. Why does tannic acid precipitate proteins?
Tannic acid precipitates proteins because it forms strong hydrogen bonds and cross-links with protein molecules. The mechanism involves:
- Interaction between phenolic –OH groups and amide groups in proteins.
- Formation of large, insoluble tannin–protein complexes.
8. What is the difference between tannic acid and gallic acid?
The main difference is that tannic acid is a polyester of gallic acid and glucose, while gallic acid is a single small phenolic acid molecule. Key distinctions:
- Gallic acid: Molecular formula C7H6O5, contains one aromatic ring.
- Tannic acid: Approximate formula C76H52O46, contains multiple gallic acid units.
- Tannic acid hydrolyzes to produce gallic acid.
9. What are the functional groups present in tannic acid?
Tannic acid contains multiple phenolic hydroxyl (–OH) groups and ester (–COO–) linkages. Its main functional groups are:
- Phenol groups attached to aromatic rings.
- Ester groups connecting gallic acid units to glucose.
- Aromatic rings contributing to resonance stabilization.
10. What are the main uses of tannic acid in chemistry and industry?
Tannic acid is mainly used as a complexing agent, antioxidant, and protein-precipitating agent in chemistry and industry. Common applications include:
- Preparation of iron gall ink via Fe3+ complex formation.
- Leather tanning by stabilizing collagen proteins.
- Use as a laboratory reagent for detecting alkaloids and proteins.
- Application as an antioxidant due to its polyphenolic structure.
