Quantitative estimation of tannins in plant tissue is generally not accurate as other phenolic substances may interfere with the non-specific chemical methods. Also it is not possible to extract completely condensed tannins. Repeated measurements are needed to make an accurate estimation. Chemical assay methods based on functional groups such as Folin-Denis procedure for total phenols and Vanillin-HCl procedure for catechins were used. Currently these are replaced by procedures based on tannin-protein interactions.

A. Method I

1. 20 g of accurately weighed sample of the powdered dried plant material is extracted with 3 × 30 ml portions of aqueous methanol (1:1).
2. Filter and wash the residue with methanol such that the final volume of the filtrate is 100 ml as collected in a 100 ml standard flask.
3. Take 0.5 ml in a 10 ml standard flask and add 1.5 ml of 12% potassium iodate in 33% methanol. Mix well and cool to 15°C. Make up the volume with aqueous methanol.
4. Measure the absorbance of the solution at 550 nm. Estimate the total gallotannin content based on measurements made on standard sample of tannic acid.

B. Method II

This spectrophotometric method is based on the development of a coloured complex between tannic acid and sodium tungstate. The colour of the complex is stable and is not affected by the presence of phenols, proteins or sugars.

1. Weigh accurately 20 g of powdered material that is passed through #20 mesh. Add 300 ml of petroleum ether, shake well and set aside overnight.
2. Filter and wash with 5 × 20 ml of petroleum ether. Discard the petroleum ether fractions and air dry the residue.
3. Carefully transfer the residue to a 500 ml beaker, add 200 ml 95% alcohol, shake occasionally and set aside overnight. Filter through sintered funnel.
4. Take 10 ml of the filtrate in a 25 ml centrifuge tube, add 2 ml of a 10% solution of lead acetate and heat on a water bath till the precipitate coagulates.
5. Centrifuge for 3 min, pour off the supernatant and drain as completely as possible. Add 5 to 10 drops of sulphuric acid and mix thoroughly. Add water up to the 20-ml mark in the centrifuge tube and centrifuge again for 3 min.
6. Perform steps 1 to 5 on 2 mg standard sample of tannic acid.
7. To both the centrifuge tubes containing the tannic acid solutions add, 2 ml of sodium tungstate reagent (prepared by heating under reflux a mix of 100 g of pure sodium tungstate, 30 g of arsenic acid, 300 ml of water and 50 ml hydrochloric acid and making up to 1000 ml with water), 10 ml of a 20% solution of sodium carbonate and make up to volume.
8. Measure the absorbance of the colour developed in both the tubes at 760 nm and determine the tannic acid content of the plant material based on the absorbance of the known weight of standard sample.