A simple and reliable spectrophotometric method for the determination of ascorbic acid in pharmaceutical preparations

In this study, a simple spectrophotometric method based on the reaction between ascorbic acid and the copper(II)-ammonia complex is presented for the determination of the vitamin C content of pharmaceutical preparations. During this reaction, ascorbic acid is oxidized and the copper(II)-ammonia complex is reduced to the copper(I)-ammonia complex, and the absorbance decrease at 600 nm (lambda(max) for the copper(II)-NH3 Complex) is measured. Stirring the final solution in the presence of air leads to the primary absorbance again being obtained, which indicates that the copper(I)-NH3 complex is quantitatively oxidized to the copper(II)-NH3 complex by O-2. The linear dynamic range of the calibration curve is 0.8-6 mmol with a detection limit of 0.26 mmol. The relative standard deviation for eight repeated experiments is 2.4%, which shows that the proposed method has a good repeatability. Finally, this method was used in the analysis of the vitamin C content of different pharmaceutical preparations, such as multivitamin tablets and syrups, vitamin-C tablets and powders, and effervescent tablets. The obtained results are in good agreement with iodimetric data.