Flow-injection spectrophotometric determination of nicotinic acid in micellar medium of Ncetylpyridinium chloride

A simple flow-injection (FI) method with spectrophotometric detection at 440nm is proposed for the determination of nicotinic acid, which is based on the formation of a polymethine dye at room temperature. In the FI system, nicotinic acid is hydrolyzed with 0.2M cyanogen bromide at pH 5 (acetic-acetate buffer) in a non-micellar medium. The glutaconic aldehyde formed is injected into a stream containing 0.1M aniline at pH 7, in a 0.1M micellar medium of the cationic surfactant N-cetylpyridinium chloride (NCPC), to produce the polymethine. The performance of NCPC is compared with that of other surfactants: cetyltrimethylammonium bromide (CTAB, cationic), sodium dodecyl sulphate (anionic), and Triton X-100 (non-ionic). In the presence of NCPC and CTAB, the coupling reaction increases its sensitivity in ×3.3 and ×2.6 factors, respectively. This effect is explained by the higher concentration of glutaconic aldehyde and aniline on the micelle surface due to electrostatic attraction and hydrophobic association to the cationic surfactant. The dispersion coefficient of the FI system was D=2.4, and the sampling rate, 95 samples per hour. The limit of detection was 1.2×106M and the coefficient of variation 1.0 and 0.8% for 1×10-5 and 1×10-4M nicotic acid, respectively. The optimized manifold was applied to the determination of commercial pharmaceuticals containing nicotinic acid, with recoveries in the 96.7-101% range