Determination of ascorbic acid in soft drinks, preserved fruit juices and pharmaceuticals by flow injection spectrophotometry: Matrix absorbance correction by treatment with sodium hydroxide.

Two flow injection systems for the spectrophotometric determination of ascorbic acid at 245 nm have been described. On treatment with sodium hydroxide a fraction of the ascorbic acid was decomposed into substances, which do not absorb in UV region, and the decrease in signal measured. This was directly related to the amount of ascorbic acid present. The calibration graph was linear over the range 1-25 and 1-50 microg/ml in the two methods with a correlation coefficient of 0.9981 and 0.9994, respectively. The detection limit (2sigma) was 0.5 and 0.2 microg/ml, respectively. The RSD for 1 microg/ml standard was 2.5 and 1.8% (n = 6) in the two methods, and the sampling throughput 30/hr. The methods permitted the use of 6 microg/ml of 2-mercaptoethanol as an anti-oxidant and stabilizer for ascorbic acid, which is difficult to handle at its microg/ml level. Upon matrix absorbance correction, spiked samples that are known to contain UV-absorbing substances produced an average recovery of 101% with a RSD of 1.2%. The methods were used for the rapid and simple determination of ascorbic acid in soft drinks, preserved fruit juices and pharmaceuticals and the results thus produced compared with those obtained by previously checked methods involving titration with iodine, chloranil 2,6-dichlorophenolindophenol, and HPLC. When there was a disagreement between the results, this was traced to the presence of substances which are known to interfere in comparison methods.