Kinetic-spectrophotometric method for the determination of ascorbic acid in orange juice, parsley and potatoes

Abstract

A kinetic-spectrophotometric method using stopped-flow techniques is described for the determination of ascorbic acid based on its reaction with 2,6-dichlorophenolindophenol (DCPl). The initial rate was measured with a specially designed system, which consists of an operational amplifier, a window comparator and a scaler which counts the pulses from a 1 MHz square-signal generator in order to measure the time interval, Δt, between two pre-set levels, E₁ and E₂, of the electrical signal. The regression curve for the results is (Δt)^–1=(0.4 ± 0.1)±(162240 ± 541)[ascorbic acid] with a correlation coefficient r=0.99988. It was found that 4 × 10^–5M DCPl gives the best stability, reproducibility and linearity of the calibration graph. The optimum oxalic acid concentration is 0.2 M. Ascorbic acid can be quickly determined in samples of orange juice, parsley and potato in the range 2 × 10^–5–5 × 10^–4M. Recoveries of 98.0, 95.0 and 90.0% were obtained for orange juice, parsley and potato, respectively. The standard additions method gave recoveries of 101.7, 100.1 and 100.2% for orange juices, parsley and potato, respectively. The results obtained have been compared with the official method. The accuracy and reproducibility of the method are better than 2%. The concentrations of ascorbic acid in orange juices from four different areas of Greece are also presented.