A multi-pumping flow system for spectrophotometric determination of oxalate in tea

A high oxalate uptake from the diet is thought to play a role in the development of hyperoxaluria, a documented risk factor in the formation of calcium oxalate kidney stones. In this sense, a flow system designed with multicommutation is proposed for spectrophotometric determination of oxalate in tea. The procedure is based on the SPADNS method (a complex of zirconium and the trisodium salt of 1,8-dihydroxy-2-(4-sulfophenylazo)-naphthalene-3,6-disulphonic acid), in which the oxalate reacts with the zirconium to form a colorless complex, thus bleaching the red color in a proportional amount to the oxalate concentration. A manifold with three solenoid micro-pumps was used and the decrease in the absorbance of Zr-SPADNS reagent is linear with oxalate concentration in the range of 1.0 to 6.0 mg L-1. The coefficient of variation, sampling rate and detection limit were estimated as 2.6% (4.0 mg L-1, n=10), 120 samples h(-1), and 0.30 mg L-1 (99.7% confidence level), respectively, with reagent consumption estimated as 70 mu g of SPADNS per determination. Under optimized conditions, no interference was observed in presence of fluoride for the concentration expected in infusions (less than 100 mu g L-1). The procedure was applied in different sample infusions included black, green and white tea. The recovery values were between 83 and 108% showing that the proposed protocol is able to determination of oxalate without interference problems in a complex sample after a simple dilution.