Determination of Hydrolyzable Tannins (Gallotannins and Ellagitannins) after Reaction with Potassium Iodate

TLDR: In the new method, hydrolyzable tannins are reacted at 85 degrees C for 20 h in methanol/sulfuric acid to quantitatively release methyl gallate, and with plant samples, relative standard deviations of less than 3% were obtained.

Abstract: A widely used method for analyzing hydrolyzable tannins afer reaction with KIO3 has been modified to include a methanolysis step followed by oxidation with KIO3. In the new method, hydrolyzable tannins (gallotannins and ellagitannins) are reacted at 85 °C for 20 h in methanol/sulfuric acid to quantitatively release methyl gallate. Dried plant samples can be methanolyzed under the same conditions to convert hydrolyzable tannins to methyl gallate. Oxidation of the methyl gallate by KIO3 at pH 5.5, 30 °C, forms a chromophore with λmax 525 nm, which is determined spectrophotometrically. The detection limit of the method is 1.5 μg of methyl gallate, and with plant samples, relative standard deviations of less than 3% were obtained. Keywords: Tannin; polyphenolic compound; hydrolyzable tannin; methyl gallate; galloyl glucose; gallotannin; potassium iodate