Flow Systems with MnO₂-coated Open Tubular Reactors for Spectrophotometric Determination of Ascorbic Acid in Pharmaceutical Products

ABSTRACT

A novel strategy for preparing polyethylene open tubular reactors coated with MnO₂(s) was pioneered for flow analysis in order to minimize the inherent drawbacks of packed solid reagents. As an application, the determination of ascorbic acid in pharmaceuticals was selected. The method involved reduction of Mn(IV) by ascorbic acid, release of the formed Mn²⁺ ion, reaction with formaldoxime, and spectrophotometric monitoring. The influence of flow pattern was investigated by using two similar flow systems relying on constant or pulsed flow delivered by peristaltic or solenoid pumps. With pulsed flow, system versatility and ruggedness were improved, and the analytical sensitivity (0.01143 L mg⁻¹ ascorbic acid), detection limit (0.6 mg L⁻¹ ascorbic acid), and sampling rate (60 h⁻¹) evaluated compared to constant flow were improved by 2.1%, 40.0%, and 6.5%, respectively. At the same time, the reagent consumption (2.8 mg per determination) and analytical precision (relative standard deviation of approximately 2.5%) were maintained. One can conclude that the mixing conditions did not limit the analyte/reagent interaction. The results were in agreement with the British Pharmacopoeia method.

KEYWORDS:

• Ascorbic acid
• MnO₂-coated open tubular reactor
• multi-commuted flow system
• multipumping flow system
• pharmaceutical products
• reagent immobilization

Acknowledgements

J. L. M. Santos and F. R. P. Rocha are thanked for supervising the steps of this work carried out at the Porto University (Porto, Portugal) and for critical comments.

Additional information

Notes on contributors

Mario A. Feres

Mario A. Feres is currently associated with Claretian Faculty, Rio Claro, Brazil.