Appendix XI M. Tannins in Herbal Drugs

(Ph. Eur. method 2.8.14)

Carry out all the extraction and dilution operations protected from light.

In the case of a herbal drug or a dry extract, to the stated amount of the powdered drug (180) (2.9.12) or the extract in a 250 mL round-bottomed flask add 150 mL of water R. Heat on a water-bath for 30 min. Cool under running water and transfer quantitatively to a 250 mL volumetric flask. Rinse the round-bottomed flask and collect the washings in the volumetric flask, then dilute to 250.0 mL with water R. Allow the solids to settle and filter the liquid through a filter paper 125 mm in diameter. Discard the first 50 mL of the filtrate.

In the case of a liquid extract or a tincture, dilute the stated amount of the liquid extract or tincture to 250.0 mL with water R. Filter the mixture through a filter paper 125 mm in diameter. Discard the first 50 mL of the filtrate.

Total polyphenols Dilute 5.0 mL of the filtrate to 25.0 mL with water R. Mix 2.0 mL of this solution with 1.0 mL of phosphomolybdotungstic reagent R and 10.0 mL of water R and dilute to 25.0 mL with a 290 g/L solution of sodium carbonate R. After 30 min measure the absorbance (2.2.25) at 760 nm (A1), using water R as the compensation liquid.

Polyphenols not adsorbed by hide powder To 10.0 mL of the filtrate, add 0.10 g of hide powder CRS and shake vigorously for 60 min. Filter and dilute 5.0 mL of the filtrate to 25.0 mL with water R. Mix 2.0 mL of this solution with 1.0 mL of phosphomolybdotungstic reagent R and 10.0 mL of water R and dilute to 25.0 mL with a 290 g/L solution of sodium carbonate R. After 30 min measure the absorbance (2.2.25) at 760 nm (A2), using water R as the compensation liquid.

Standard Dissolve immediately before use 50.0 mg of pyrogallol R in water R and dilute to 100.0 mL with the same solvent. Dilute 5.0 mL of the solution to 100.0 mL with water R. Mix 2.0 mL of this solution with 1.0 mL of phosphomolybdotungstic reagent R and 10.0 mL of water R and dilute to 25.0 mL with a 290 g/L solution of sodium carbonate R. After 30 min measure the absorbance (2.2.25) at 760 nm (A3), using water R as the compensation liquid.

Calculate the percentage content of tannins expressed as pyrogallol from the expression:

m1
 = 
mass of the sample to be examined, in grams;
m2
 = 
mass of pyrogallol, in grams.