Background correction method for the determination of ascorbic acid in soft drinks, fruit juices and cordials using direct ultraviolet spectrophotometry

Abstract

A background correction method has been developed for the determination of ascorbic acid in soft drinks, fruit juices and cordials using UV spectrophotometry. The sample blank is produced by the catalytic oxidation of the acid by copper(II). To correct for the absorption due to copper(II), EDTA was added after the oxidation, and standard solutions were prepared to contain the same concentration of the copper(II)-EDTA complex, which does not catalyse the oxidation of ascorbic acid at room temperature. The decomposition of ascorbic acid in real samples should be carried out at 50 °C because citric acid, which is usually present in the samples under study, retards the copper(II)-catalysed oxidation of ascorbic acid.

Absorbance measurements were made at 267 nm and pH 6. The calibration graph was linear for 0–20 µg ml^–1 of ascorbic acid. The precision was 0.1–0.5% for ascorbic acid concentrations in the range 5–13 µg ml^–1. The proposed method is selective, and many ingredients commonly found in soft drinks, fruit juices and cordials do not interfere. The method was utilised to determine ascorbic acid in a wide range of samples.