Spectrophotometric determination of tannins in tea and beer samples with iron(III) and 1,10-phenanthroline as reagents

Abstract

A simple and accurate spectrophotometric method is proposed for the determination of tannins in tea and beer samples based on the reduction of iron(III) to iron(II) by tannins at 80 degrees C for 20 min. The iron(II) was then reacted with 1,10-phenanthroline at pH 4.4 to form a coloured complex. Background correction could be effected by precipitating the tannins in the sample solution twice with gelatin and kaolin. Absorbance measurements were made at 540 nm and the calibration graph was linear from 0 to 5.5 micrograms ml-1 of tannic acid with a slope of 0.213 A p.p.m.-1. The precision for the determination of tannins in a tea sample containing 9.45% of tannins was 1.8%. Most of the ingredients commonly found in tea and beer samples do not interfere with the determination. Several tea and beer samples were analysed for their tannin content using the proposed method.

A new method for quantification of total polyphenol content in medicinal plants based on the reduction of Fe(III)/1,10-phenanthroline complexes

“…It was noted that the development of a color solution is very slow at room temperature, so the final solution was kept in a water bath (80˚C for 20 min.) as done in previous works [16].…”

“…Fe phen 3  complexes, it was pointed out that most of the compounds normally found in those samples showed no interference [16]. So in that study a background correction based on the precipitation of tannins with gelatin and kaolin was carried out to minimize interference due to other reducing substances (resulting in a "free tannin solution") before the absorbance measurements.…”

“…Addition of a suitable masking agent, like EDTA, is needed for the complexion of the unreacted residual Fe(III) in order to avoid the absorbance interference of [3,16].…”

“…Despite these interesting strategies, the indirect determination of reducing agents weaker than ascorbic acid and tannins (such as formaldehyde, glucose and citric acid) in complex samples seems to demand more elaborate analytical procedures or consume more chemical reagents. Lau et al developed an accurate spectrophotometric method for the determination of tannins in tea and beer samples [16]. In that study, the reduction of Fe(III) to Fe(II) by tannins was carried out at 80˚C for 20 min and then the Fe(II) formed was reacted with 1,10-phen (at pH = 4.4) to form the orange-red 3 complex.…”

“…However, in order to reduce the interference from other reducing compounds in these samples, it was necessary to obtain a "tannin free" solution. For this purpose, a background correction was carried out by precipitating tannins twice (with gelatine and kaolin) which rendered the method tedious and time consuming [16].…”