Chemical Tests - For Tannins

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Chapter: Pharmacognosy and Phytochemistry : Drugs Containing Tannins

Goldbeater’s skin test: Goldbeater’s skin is a membrane produced from the intestine of Ox. It behaves just like untanned animal hide.


CHEMICAL TESTS

 

 

1.       Goldbeater’s skin test: Goldbeater’s skin is a membrane produced from the intestine of Ox. It behaves just like untanned animal hide. A piece of goldbeaters skin previously soaked in 2% hydrochloric acid and washed with distilled water is placed in a solution of tannin for 5 minutes. It is then washed with distilled water and transferred to 1% ferrous sulphate solution. A change of the colour of the goldbeater’s skin to brown or black indicates the presence of tannin.

 

2.       Hydrolysable and condensed tannins both give the positive goldbeater’s test, whereas pseudotannins show very little colour or negative test.

 

3.       Phenazone Test: To 5 ml of aqueous solution of tannin containing drug, add 0.5 g of sodium acid phosphate. Warm the solution, cool, and filter. Add 2% phenazone solution to the filtrate. All tannins are precipitated as bulky, coloured precipitate.

 

4.       Gelatin Test: To a 1% gelatine solution, add little 10% sodium chloride. If a 1% solution of tannin is added to the gelatine solution, tannins cause precipitation of gelatine from solution.

 

5.       Test for Catechin (Matchstick Test): Catechin test is the modification of the well-known phloroglucinol test for lignin. Matchstick contains lignin. Dip a matchstick in the dilute extract of the drug, dry, moisten it with concentrated hydrochloric acid, and warm it near a flame. Catechin in the presence of acid produces phloroglucinol which stains the lignified wood pink or red.

 

6.       Test for chlorogenic acid: A dilute solution of chlorogenic acid containing extract, if treated with aqueous ammonia and exposed to air, slowly turns green indicating the presence of chlorogenic acid.

 

7.       Vanillin-hydrochloric acid test: Drug shows pink or red colour with a mixture of vanillin: alcohol : dilute HCl in the ratio 1:10:10. The reaction produces phloroglucinol which along with vanillin gives pink or red colour.

 

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