Tannic acid of pharmaceutical grade is commercially obtained from nut galls, which are grown as pathological outgrowths on the stems and branches of the oak tree due to the deposition of eggs of the gall wasp. Galls are obtained in three grades namely blue, green and white. Tannic acid is extracted from these powdered galls by fermenting them and subjecting them to extraction with water. Fermentation facilitates loosening of tannic acid from other constituents. There are several procedures adopted for the extraction of tannic acid from plant material. Common solvents employed for its extraction are ethanol, methanol, ethyl acetate, isopropanol, butyl acetate, absolute alcohol and hot water.

A. Method I

1. Extract about 500 g of the powdered plant material (#40 mesh) by agitation with 2 × 750 ml of methanol for 8–12 h each. Filter.
2. Extract the residue on the filter with further 750 ml of methanol.
3. Combine the methanol extracts and concentrate on a rotary vacuum evaporator to about one-fourth the original volume.
4. Evaporate further on water bath to a sticky mass. Add 1litre of water to this mass and set aside overnight in a refrigerator.
5. Discard the precipitate and filter the supernatant liquid through a fine muslin cloth.
6. Extract the filtrate with 4 × 25 ml portions of petroleum ether. Discard the petroleum ether layers.
7. Concentrate the aqueous solution to one-third volume and add sodium chloride till the solution is saturated.
8. Transfer carefully into a separating funnel and extract with 4 × 100 ml portions of ethyl acetate.
9. Combine the ethyl acetate layers and concentrate under vacuum when a brownish-yellow powder of tannic acid is separated.
10. Continue the evaporation on a steam-heated water bath for the complete removal of the solvent and dry the product to constant weight.

B. Method II

1. To about 100 g of the powdered plant material add 400 ml hot water at 80°C and keep on a steam-heated water bath for 2 h.
2. Filter, concentrate the filtrate to about one-third its original volume by evaporation on a hot plate.
3. Extract it with equal volume of petroleum ether (40–60°C) and repeat successive extractions with the same solvent till the petroleum ether layer leaves no residue on evaporation.
4. Separate the aqueous layer and saturate it with sodium chloride, i.e. till there is some deposition of the salt at the bottom of the solution.
5. Extract the mixture with 4 × 60 ml portions of ethyl acetate. Collect the ethyl acetate layers and concentrate under vacuum when yellowish-brown product of tannic acid is obtained.
6. Evaporate the excess solvent and dry the product to constant weight in a vacuum desiccator.