STUDIES ON THE METHOD OF DETERMINATION OF VEGETABLE TANNINS, THE COMPLEXOMETRY USED FOR QUANTITATIVE DETERMINATION OF TANNIN

A method is described which is based upon the formation of a precipitate of poly- nucleus complex compound between the tannins and metallic ions, and estimation of the complexed quantity of metallic ions by complexometric titration for the determination of vegetable tannins. The tannin is precipitated completely by Zn⁺⁺ at pH 11±0.2, 35±2℃, and afterwards EDTA is used to titrate back the excess of Zn⁺⁺. By comparison made with the official method (the Hide powder method) as standard, we obtained the relation between the complexed quantity of Zn⁺⁺ and the percentage content of tannin: Tannin%=(0.1556×1M Zn⁺⁺ required (ml))/(W (sample, g))×100. This method is more accurate than L?wenthal's. It is more simple, precise and widely applicable than the Hide powder method and L?wenthal's method. The average deviation is not over 2%. It is very rapid and the whole determination can be finished within 40 minutes.