Kinetic Spectrophotometric Analysis and Spectrofluorimetric Analysis of Ciprofloxacin Hydrochloride and Norfloxacin in Pharmaceutical Preparations Using 4-Chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) Mohamed H. Abdel-Hay, Ekram M. Hassan, Azza A. Gazy and Tarek S. Belal* Pharmaceutical Analytical Chemistry Department, Faculty of Pharmacy, University of Alexandria, Elmessalah 21521, Alexandria, Egypt Simple, reliable, sensitive and accurate kinetic spectrophotometric and spectrofluorimetric methods were proposed for the determination of ciprofloxacin hydrochloride (CPX) and norfloxacin (NRX) in pure form and in pharmaceuticals. The methods are based on coupling the studied drugs with 4-chloro-7- nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) in the presence of alkaline borate buffer. Spectrophotometric measurement was achieved by recording the absorbance at 477 nm after a fixed time of 20 and 15 min on a water bath adjusted at 70 ± 1 °C for CPX and NRX, respectively. The same product exhibited emission peaks at 540 nm. The different experimental parameters affecting the development and stability of the color were carefully studied and optimized. The absorbance concentration plots were linear over the ranges 3-18 and 2.5-15.0 mg/mL for CPX and NRX, respectively, while the fluorescence concentration plots were linear over the ranges 0.06-0.36 and 0.05-0.30 mg/mL for CPX and NRX, respectively. The limit of detection of the kinetic method was about 0.2 mg/mL for both drugs while the fluorescence mea- surement enabled their detection at a concentration of about 0.012 mg/mL. The proposed methods were successfully applied for the assay of the two drugs in their commercial products. The results obtained were statistically compared with those obtained by reference HPLC and spectrophotometric methods. The stoichiometry of the reaction was determined and the reaction pathway was postulated. Keywords: Ciprofloxacin hydrochloride; Norfloxacin; NBD-Cl; Kinetic spectrophotometry; Spectrofluorimetry; Pharmaceutical preparations. INTRODUCTION Ciprofloxacin hydrochloride (CPX), chemically known as 1-cyclopropyl-6-fluoro-4-oxo-7-(piperazin-1- yl)-1,4-dihydroquinoline-3-carboxylic acid hydrochloride and norfloxacin (NRX), chemically known as 1-ethyl-6- fluoro-4-oxo-7-(piperazin-1-yl)-1,4-dihydroquinoline-3- carboxylic acid, are fluoroquinolone antibacterial drugs with a wide spectrum of activity. They are bactericidal and act by inhibiting the A subunit of DNA gyrase which is es- sential in the reproduction of bacterial DNA. Both are ac- tive against many Gram-negative and Gram-positive aero- bic and most anaerobic bacteria. CPX has been used in the treatment of a wide range of infections while NRX is used mainly in the treatment of urinary tract infections. NRX is also used for the treatment of gonorrhea. 1 The official methods in the BP 2 and the USP 3 for the assay of CPX either in pure form or in different pharmaceu- tical dosage forms are performed using HPLC. The official assay of pure NRX involves a potentiometric non-aqueous titration procedure, while NRX ophthalmic solution and tablets have been assayed using HPLC. 2,3 Several analytical methods have been reported for the determination of the two fluoroquinolones in pharmaceuti- cal preparations and/or biological fluids, including: flow- injection chemiluminescence, 4 derivative spectrophotome- try, 5 TLC-fluorescence densitometry, 6 spectrofluorime- try, 7,8 HPLC, 9 spectrophotometry, 8,10-14 atomic absorption spectroscopy, 12,14 conductometry, 12 direct and differential- pulse polarography 15 and square-wave adsorptive voltam- metry. 16 Recently, simultaneous determination of three fluoroquinolones including CPX and NRX was carried out using linear-sweep stripping voltammetry with the aid of chemometrics 17 and liquid chromatography-tandem mass spectrometry with solid-phase extraction. 18 818 Journal of the Chinese Chemical Society, 2008, 55, 818-827 * Corresponding author. Tel: +20-3-4871317; Fax: +20-3-4871351; E-mail: tbelaleg@yahoo.com