Assay of Vitamin C

Significance of the Topic

The quantification of vitamin C (ascorbic acid) is critical in pharmaceutical quality control and nutritional analysis. As an essential antioxidant and enzyme co-factor, accurate determination of its content in dosage forms ensures product efficacy and compliance with pharmacopeial standards.

Objectives and Overview

This study demonstrates a rapid and precise redox titration method for assaying vitamin C in water-soluble tablets according to USP General Chapter <580>. The approach can be extended to oil- and water-soluble vitamins and mineral combinations in various dosage forms.

Methodology and Instrumentation Used

Sample Preparation:

• Accurately weigh and pulverize multiple tablets into a fine powder.
• Transfer a representative portion into a volumetric flask.
• Add metaphosphoric acid and acetic acid to stabilize ascorbic acid and prevent oxidation.
• Fill to volume with carbon dioxide-free water and mix thoroughly.

Redox Titration Procedure:

• Titrate the prepared solution with dichlorophenol-indophenol (DCPIP) until the first equivalence point.
• Monitor the potential change via a polarized platinum electrode to identify the endpoint.
• Perform a blank determination under identical conditions for correction.

Instrumentation Used:

• OMNIS Advanced Titrator with magnetic stirrer and 3S Liquid Adapter for secure reagent handling.
• Double platinum sheet redox electrode for endpoint detection.
• 905 Titrando system with magnetic stirrer and platinum electrode as an alternative setup.

Main Results and Discussion

The method produced well-defined, reproducible titration curves. In six replicate determinations of a water-soluble vitamin C tablet, the mean assay was 97.7% of label claim with a relative standard deviation of 0.23%. These values fall within the USP acceptance range of 90–150%. The blank correction had negligible impact, confirming method specificity.

Benefits and Practical Applications

This titrimetric approach offers:

• High speed and throughput suitable for routine QC laboratories.
• Excellent precision and accuracy in compliance with USP <580>.
• Adaptability to diverse formulations, including capsules, tablets, and suspensions.
• Minimal sample preparation with readily available reagents.

Future Trends and Potential Applications

Emerging developments may include:

• Integration of parallel titration capabilities for higher sample throughput.
• Automation and networked titration systems with real-time data exchange.
• Miniaturized and compact titrators for on-site analysis in production lines.
• Coupling of electrochemical endpoints with spectrophotometric detection for multi-analyte assays.

Conclusion

The described redox titration method provides a robust, fast, and precise determination of vitamin C in pharmaceutical forms. Compliance with USP <580> ensures regulatory acceptance and supports efficient quality control workflows.

References

• USP General Chapter <580> “Vitamin C Assay,” United States Pharmacopeia.