Novel colorimetric assay of mefenamic acid using 4-amino-3,5-dinitrobenzoic acid (ADBA)

Purpose: To develop a novel colorimetric assay method for mefenamic acid capsules.
Method: The new method (aromatic ring derivatization technique) is based on a diazo coupling reactionusing diazotized 4-amino-3,5-dinitrobenzoic acid (ADBA) as a chromogenic derivatizing reagent.
Result: Optimization studies showed that the coupling reaction is very fast and completed in less than 1 minute. A 1:1 drug to reagent stoichiometric ratio was obtained for the azo dye formed. The azo adduct formed exhibits bathochromic shift with absorption maximum (λ_max) at 490 nm, which was selected as the analytical wavelength. Lower limit of quantitation of mefenamic acid was 1 μ g/ml. The assays were linear over the concentration range of 1 - 6 μ g/ml and reproducible. This new method has been successfully applied in the assay of mefenamic acid capsules with accuracy similar to the official (B.P) titrimetric method of assay (p>0.05) and has the advantages of speed, high sensitivity, lower limit of detection and can be automated.
Conclusion: The method developed could find application in in-process quality control of mefenamic acid capsules.

Keywords
Mefenamic acid assay, 4-amino-3,5-dinitrobenzoic acid, colorimetry, diazotization