Solution of tannic acid precipitates heavy metals, alkaloids, glycosides and gelatin.

1. To a solution of tannic acid in water add a few drops of ferric chloride solution. A bluish-black precipitate is formed confirming the presence of tannic acid.
2. Gelatin test: To 5 ml of 1% solution of tannic acid in water, add 1% solution of gelatin containing 10% sodium chloride. A bulky white precipitate is formed.
3. Phenazone test: To 5 ml of a 1% solution of tannic acid in water, add 0.5 g of sodium acid phosphate. Warm, cool and filter. To the filtrate add 2% solution of phenazone. A bulky dirty white precipitate is formed.
4. Goldbeater’s skin test: A small piece of prepared skin mimicking untanned animal hide is soaked in 2% hydrochloric acid for about 10 min. Rinse it off with distilled water and place it in solution of tannic acid for 5 min. Wash with distilled water and transfer to 1% ferrous sulphate solution. A brownish-black colour on the skin indicates the presence of tannins.
5. TLC Test: To 5 ml of a 1% solution of tannic acid in water add 0.5 ml of hydrochloric acid and heat on a water bath for 30 min. Filter. This is the test solution.

For the standard solution, weigh 100 mg gallic acid reference sample and dissolve it in 100 ml water.

Apply both the test and standard solutions to a precoated silica gel plate and develop in a solvent system of chloroform: ethyl formate: formic acid (5:4:1). Air dry and spray with a 1:1 solution of 1% ferric chloride in 10% ethanol and 1% potassium ferricyanide in 50% ethanol.

A black spot exactly comparable to that of standard is seen for the sample.