Determining the levels of tannin in tea by amperometry of ferricyanide pre-reaction with a sample in a flow-injection system

An amperometric flow-injection assay was proposed for measuring the levels of tannin in tea. Electrode fouling owing to electro-oxidation of tannin could be efficiently avoided by utilizing ferricyanide as an oxidant to pre-oxidize tannin for indirect quantification without electrode modification. Appropriate concentrations of ferricyanide were pre-mixed with tannic acid and then measured by a platinum electrode in an electrochemical flow cell. The effects of working potential and flow rate on amperometric response were investigated, and the optimal condition was obtained when the working potential was −0.1 V (versus Ag/AgCl) and the flow rate was 2.28 mL min−1 in this work. The linear detection range for tannic acid could be effectively adjusted according with application demand, and the results of this study indicated two ranges of 10–50 μg mL−1 and 100–500 μg mL−1 (CV < 5%, n = 3). Beverage samples of tea with various fermentation degrees were measured (15 samples h−1) and verified with Folin–Ciocalteau method (r = 0.9794) as well as the ferrous tartrate method (r = 0.9918).