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Rasha M. Youssefa*, Essam F. Khamisa, Sameh E. Younisb, Fawzi A. El-Yazbia

a Faculty of Pharmacy, Department of Pharmaceutical Analytical Chemistry, University of Alexandria, El-Messalah, Alexandria 21521, Egypt.
bFaculty of Pharmacy and Drug Manufacturing, Department of Pharmaceutical Chemistry, Pharos University of Alexandria, Somouha, Canal El Mahmoudia Street, Alexandria, Egypt.


Two graphical spectrophotometric techniques have been developed for the determination of ionization constants of entacapone (ENP). The first one depends on plotting the relationships between absorbance values at three λmax against different pH values. Consequently, the pKa values are corresponding to the pH of drug solution at the inflection points in these plots. The second one is based on plotting the derivative spectrophotometric titration curves and interpolating pKa at D1/2. Both techniques have been successfully applied to evaluate two ionization constants of ENP. On the other hand, three selective, sensitive and validated spectrophotometric methods have been developed for the determination of drug in bulk powder and tablets. Method A depends on measuring the first derivative spectrophotometric peak-to-peak amplitudes of ENP in methanolic solution at 368405 nm. This method is highly sensitive, so it allows the determination of ENP in human plasma, where linear correlation was achieved in the range 0.9-2.4 g mL-1. Method B is pH-induced difference spectrophotometry (∆A) and its first derivative (∆D1). This method involves measurement of analytical signal values of drug alkaline solution against its acidic solution from peak to peak at (299-365) nm and (266-340) nm for ∆A and ∆D1, respectively. Method C is based on the oxidative coupling reaction with 3-methylbenzothiazolin-2-one hydrazone (MBTH) in the presence of Ceric ammonium sulphate, Ce (IV), as an oxidant to produce deep-green colored species measurable at 535 nm. All the proposed methods were validated in compliance with ICH guidelines.

Keywords: Ionization constants - derivative spectrophotometric - pH-induced difference spectrophotometry – MBTH – spiked human plasma.

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