Synthesis of Bis-Schiff Base Reagents and their Application in the Spectrophotometric Determination of Mesalazine via Azo Dye Formation

Mesalazine (5-aminosalicylic acid) is an important first-line anti-inflammatory drug widely used in the management of inflammatory bowel diseases, particularly ulcerative colitis, making its reliable determination in pharmaceutical formulations important for routine quality control. In this study, bis-Schiff base reagents were synthesized by condensation of 4,4′-diaminoazobenzene with substituted aromatic aldehydes. Among the synthesized derivatives, 4,4′-bis[(4-hydroxybenzylidene)amino]azobenzene (SH1) was selected as the chromogenic reagent because it provided a superior analytical response compared with SH2. The novelty of the proposed method lies in the use of laboratory-synthesized SH1 as a coupling reagent for diazotized mesalazine, producing a color-stable azo product in alkaline medium with a maximum absorbance at 530 nm. The analytical procedure is simple and low-volume, employs a predominantly aqueous final reaction medium, and does not require chromatographic separation or solvent-extraction steps, thereby reducing solvent and instrumental requirements. Under optimized conditions, Beer’s law was obeyed over the concentration range of 2.0–20.0 μg/mL, with a molar absorptivity of 7.580 × 10³ L/mol/cm and Sandell’s sensitivity of 0.0202 μg cm⁻². The method showed recoveries of 99.20–100.83%, with RSD values of 0.70–0.80%, and a limit of detection of 0.1540 μg/mL. Good selectivity and robustness were observed, and the method was successfully applied to the determination of mesalazine in Pentasa® tablets. These findings support the applicability of the proposed SH1-based spectrophotometric method for routine pharmaceutical quality-control analysis.