Indirect flow-injection determination of tannins in wines and tea by atomic absorption spectrometry

An automatic continuous precipitation system for the indirect determination of tannins in foods based on the standard gravimetric method was developed as an alternative to the Official Methods of Analysis based on the Folin-Ciocalteu reagent. Tannins were precipitated by injecting the copper-acetate complex into the sample, and the unprecipitated copper was determined by atomic absorption spectrometry. The calibration graph was linear over the range 1–25 μg ml−1 tannic acid, and the relative standard deviation for determination of 10 μg ml−1 tannic acid was 2.0%. The 3σ detection limit was 0.7 μg ml−1. No compounds normally found in wines or tea had any effect on the determination. The results obtained in the analysis of 11 wines and 5 teas are critically compared with those provided by the Folin-Ciocalteu method.