Spectrophotometric determination of B6 vitamins by coupling with diazotized P-sulfanilic acid in micellar medium of N-cetylpyridinium chloride

The water-soluble B-6 vitamins pyridoxine, pyridoxal and pyridoxamine were determined by coupling with the diazonium ion of p-sulfanilic acid, in a micellar medium of N-cetylpyridinium chloride (NCPC), with spectrophotometric measurement in the visible region (450 nm). A stable azo dye, with increased sensitivity with respect to the derivative obtained in non-micellar medium, was rapidly formed. The addition of toxic mercury(II) acetate, recommended in a previous procedure to develop the colour for pyridoxine and pyridoxamine, was not required in the presence of NCPC. The procedure was applied to the determination of pyridoxine hydrochloride in commercial pharmaceutical preparations, with recoveries between 94 and 107% and repeatabilities below 1%. The differences in the coupling reaction rates of the B-6 vitamins, in the presence of boric acid and NCPC micellar medium, permitted the differential-kinetic analysis of binary mixtures of the three B-6 vitamins.