A Colorimetric Determination of Tea Tannin with Ferrous Tartrate

As a substitute for the Löwenthal method for the determination of tea tannin, a colorimetric method using Mitchell's ferrous tartrate reagent was investigated.The reagent consisted of FeSO4 . 7H2O (100 mg.) and Rochelle salt (500 mg.) in 100 ml. of water. Tea tannin and ferrous tartrate produced a reddish violet color, having a maximum absorption at 540 mμ, which was most intensive in the buffer solution of pH 7.5. The absorbance of the colored solution was kept constant for 4 hours, and a slight decrease in absorbance was observed with the temperature rise of 20◊. Since the individual catechins gave different coloration and intensity with ferrous tartrate, the amount of tea tannin adopted was calculated by the Löwenthal method.The recommended procedure is as follows: The sample solution containing tea tannin is prepared by extracting powdered tea (100 mg.) with water (100 ml.) in a boiling water bath for 30 minutes and filtering. Five ml. of the sample solution and the same quantity of the reagent are transferred into a 25-ml. volumetric flask and filled to the mark with Sörensen's phosphate buffer (pH 7.5). Then the absorbance is measured at 540 mμ against a blank solution substituting water for the reagent, and the amount of tea tannin is determined from the calibration curve.The variation coefficient of the determination was about 2 %.By the continuous variation method, it seemed that an Fe reacted with one molecule of gallic acid, two of (-)-epigallocatechin, and three of (-)-epicatechin respectively.