Tannins-Classification, Testing And Uses

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General properties

  • Tannins form colloidal solutions with water

  • They show acidic reaction, due to the presence of phenols

  • They are  soluble in acetone, alcohol and glycerin

  • Insoluble in other organic solvents

  • They have molecular weight ranging from 500 to over 3000

  • They impart a dark blue or green color with ferric salts

  • They cause the precipitation of protein

  • They cause the contraction of smooth muscles

Source

  • They are present exclusively in cell wall of certain plants

  • They are more accumulated in dead plant tissue

  • They are present in root and unripe fruits

Classification of tannins

Tannins are classified into two groups

  • Hydrolysable tannins

  • Non-hydrolysable tannins

Hydrolysable tannins

They are hydrolyzed by acids or other enzymes, to form Gallic acid or ellagic acid. Examples plants containing hydrolysable tannins are

  • Terminalia chebula- contains 20%-40% Gallic acid. it is used a s an astringent and stomachic

  • Teminalia arjuna- contains 20%-40% ellagic acid. Used as a cardio tonic and hypotensive drug

  • Emblica officinalis- contains22%-28% Gallic acid. Used as diuretic and laxative

  • Punica granatum- contains 22% gallotannic acid. It is used as astringent

  • Quercus infectoria- contains 50% to 70% gallotannic acid. It is used as anti dote

Non-hydrolysable tannins

         These are related to flavanoids. They are formed by condensing units of catechols.

  • Saraca indica- contains catechols and used as uterine tonic

  • Acacia catechu- contains quercetin and catechin and used as astringents

General uses

  • Used as an antiseptic on skin and mucus membrane

  • Used in healing of inflammation, burns

  • Used as uterine tonics

  • They are anti-diarrheal, styptic, and anti-infective agents

  • They also act beneficially in gastritis, and bowel disorders

Prolonged use of tannins may act as potential carcinogenic agent. Habitual chewing of betel nut and having black tea may affect the oral mucosa as an irritant and can cause cancer.

Test for tannins

  • Shows color reaction with ferrous chloride. Turns blue or green.

  • With  ammonia and potassium ferri cyanide it gives deep red color

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