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 (_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 O₂. 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.