Flow‐Injection Turbidimetric Determination of Tannins in Tea Samples Using Copper(II)/Acetate as Precipitant Reagent

Abstract

A flow‐injection (FI) turbidimetric method for determination of tannins content in tea samples based on the precipitation reaction with copper(II) in acetate medium is proposed. In this method, 350 µL of 0.05 mol L−1 copper(II) and 350 µL of sample were injected simultaneously in two water carrier streams of a merging zones system. After mixing at a 50 cm coiled reactor, the mixture converged with a flow stream of 0.10 mol L−1 ammonium acetate solution producing a precipitate in a 100 cm coiled reactor that was monitored at 500 nm. Under the optimized experimental conditions, the calibration graph for tannic acid was linear in the concentration range from 18.0 to 725 mg L−1 with a detection limit of 6.49 mg L−1. The relative standard deviations (RSD) were smaller than 1.2% (n = 10) for solutions containing 110 and 365 mg L−1 of tannic acid, and the analytical frequency was 90 determinations per hour. The results obtained in the analysis of tea samples (green, black, and brown) were compared with those provided by the Folin–Ciocalteu method. #Financial support from FAPESP, PADCT/CNPq, and also the scholarships granted by CAPES to E. Piccin and H.J. Vieira are gratefully acknowledged.