A NEW FLOW-INJECTION SPECTROPHOTOMETRIC METHOD FOR THE DETERMINATION OF TANNINS IN TEA AND BEER USING IRON(III) AND 1,10-PHENANTHROLINE

A new flow-injection spectrophotometric method for the determination o f tannins in tea and beer based on the reduction of Fe(III) to Fe(II) by tannins and the subsequent formation of the coloured complex Fe(II) -1,10-phenanthroline is proposed. The FIA set-up used is controlled by a personal computer running appropriate software for automatic sample injection and data acquisition and processing. The calibration curve obtained is linear between 0-30 mg litre-1 gallic acid and the relativ e standard deviation for a standard containing 10 mg litre-1 gallic ac id is 0-7% (n = 10). The sample throughput achieved is 60 sample h-1. Unlike the batch method, on which it is based, the proposed method req uires no sample pretreatment in order to avoid interferences; it only involves appropriate dilution of the samples, and so provides more rel iable results in a substantially shorter time.