Chemical Nature of crude Drugs: Tannins.


  • Chemically tannins are derivatives of polyhydroxy benzoic acid, capable of combining with proteins. They are complex, high molecular Wt. Organic nitrogenous substances.

  • They are non- crystallizable, soluble in alcohol & water

  • 1. They have tanning property.

  • 2. They precipitate proteins and alkaloids.

  • 3. They are acidic in nature.

  • 4. They have an astringent test.

  • 5. They cause the contraction of smooth muscle.

  • 6. They give dark blue, green black color with ferric salt or iron.

  • 7. They give the Goldbeater skin Test positive.

  • 8. They prevent putrification of leather.

Classification: -

  1. Hydrolysable. 2. Condensed.
Hydrolysable tannins
Condensed tannins
They are hydrolysable by acids or enzymes
They are more resistant to hydrolysis
Chemically they are esters of Phenolic acids
They are related to flavonoid pigments.
On treatment with acids or enzymes they produce Gallic acid or Ellagic acids.
On treatment with acids or enzymes, they decompose into insoluble red compound known as phlobaphenes
On dry distillation they are converted into pyrogallols
On dry distillation they produce catechol
With iron salts they produce blue colors.
With iron salts they produce green colors.
Ex. Clove, Myrobalon
Black catechu, Acacia bark.

Chemical Test of Tannins: -

1) Gold beater’s skin Test: - Goldbeater skin is a membrane prepared from intestine of ox.

Gold beaters skin is treated with 2 % HCl. Wash the skin with water & keep the skin in the test solution for 5 minute. Again wash the skin with water. Treat the skin with 1 % FesO4 soln

If true tannins are present brown or black spot is produced on the skin.

2) Gelatin Test: - If solution of tannin is added to 1 % solution of gelatin containing 10 % NaCl, ppt. is produced.

3) Aqueous solution of tannins + Aq. solution of ferric chloride.

Hydrolysable tannin gives blue black colors & condensed tannin gives brownish green colors

4) Tannins give red color with vanillin hydrochloride reagent.

Isolation of Tannins (extraction): -

  • Raw material------ >Extract with ether + alcohol------> Filter ---->Shake Filtrate with water----------->Aqueous solution Takes most of tannin------->Evaporate aqueous extract to dryness under vacuum.

  • Crude Tannin dissolves in alcohol----- > Alcoholic solution id shaken with ether-------> Ppt of pure product is obtained from ether solution.

Topics u may like to read