Tannins ppt by Dr.U.Srinivasa, Professor andHead,Srinivas college of P

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TANNINS Dr.U.Srinivasa , D.Pharm , M. Pharm., M.Phil., Ph.D. Professor and Head, (Dept. of Pharmacognosy ) Srinivas college of pharmacy, Mangalore. Email. usreenivas1966@rediffmail.com


CONTENTS 1. Definition 2. General properties and 3. Methods of estimation of tannins


DEFINATION Tannins are defined as complex organic, non – nitrogenous, phenolic compounds of high molecular weight widely distributed in plant kingdom. They found in large quantities in dead and drying tissue.


General properties : 1. Solubility : Soluble in water, alcohol, glycerin, dilute alkalies but insoluble in organic solvents except acetone. 2. Taste : All tannins are astringent in taste 3.All tannins are acid in reaction due to polyphenolic/carboxylic acid group.

Chemical nature:

Chemical nature Chemically , they contain the mixture of complex organic compounds in which polyphenols are present, generally with ortho- dihydroxy/ ortho- trihydroxy groups on a phenyl ring.

Estimation of tannins:

Estimation of tannins 1. Spectroscopic method 2. Ferric chloride method 3. Hide powder method 4. Agglutination method

Spectroscopic method:

Spectroscopic method The first step is the extraction of tannins from crude drug. PROCEDURE : The powdered crude drug is extracted with three proportions of aqueous methanol (1:1) in the ratio of 0.1ml/1mgof powder drug. This extract is used for the estimation of various types of tannins

Methods of estimation:

Methods of estimation Tannic acid equivalent (TAE). Proantho- cyanidin content/ condensed tannin content. Ellagi-tannin content Gallo-tannin content

Tannic acid equivalent (TAE):

Tannic acid equivalent (TAE) A suitable volume of the extract is diluted to 1 ml and 1ml of fresh sample of diluted human blood is added. Then the mixture is centrifuged to remove the tannin protein complex.


The residual haemoglobin is estimated by its absorbance at 578 nm TAE can be calculated by using a known Tannic acid sample. It gives the relative astringency of plant extract and is a direct measure of total tannin equivalent.

Proantho- cyanidin content:

Proantho - cyanidin content 1ml of the plant extract in methanol + 6ml of n-butanol-Hcl (19:1) + 0.2ml of 2% solution of Ferric ammonium sulphate and 2ml Hcl, the mixture is heated at 95˚c for 40 minutes . After cooling the absorbance is measured at 560 nm.(Red)

Ellagi tannin content:

Ellagi tannin content 0.5 ml of the plant extract under nitrogen environment + 2ml of 0.1 m Nitrous acid . A blue colour is produced which is measured after 15 minutes at 600nm Using Ellagic acid as standard.

Gallo-tannin estimation:

Gallo-tannin estimation 0.5 ml of plant extract + 1.5 ml of 12% potassium iodate in 33% methanol. A red brown coloration is developed . It is measured immediately at 556 nm. By using Gallic acid as reference standard .

Ferric chloride method:

Ferric chloride method This method consists of determining the limiting dilution of the drug which still produces a coloration with ferric chloride. However , this test is not specific because may other plant constituents give a blue coloration with ferric chloride. Tannic acid is used as a standard.

Hide powder method:

Hide powder method The principle of this method is based on fact that tannins react quantitatively with proteins of hide. The dry weight of the powder is determined before and after treatment with a specially prepared hide powder.


The difference in the dry weight indicates the amount that has been absorbed by the hide, thus eliminated from the solution by this treatment.

Agglutination method:

Agglutination method A suspension of red blood cells (RBC) is agglutinated by tannins. (However haemolysis does not occur). A sticky membrane is formed on the surface of erythrocytes causing them to adhere to each other.


The quantity of RBC precipitated is directly proportional to the quantity of physiological or astringent tannin present. END POINT: It is the stage when RBC are completely precipitated , when the solution becomes coloration on shaking. The filtrate must give no coloration with ferric chloride.


The astringent value of the test substance is then compared with pure tannic acid. Eg White oak bark – 8-10%, Gambiar –10- 15%, Gall – 60-70%

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