Spectrophotometric Determination of Metronidazole and Tinidazole via Charge Transfer Complexation Using Chloranilic Acid

A new spectrophotometric method was developed for the determination of two important nitroimidazoles; metronidazole (MZ) and tinidazole (TZ). The method was based on the charge-transfer (CT) complexation reaction of reduced forms of metronidazole and tinidazole as n-electron donors and chloranilic acid (CAA) as pi-electron acceptor to form a purple-colored complex with a new absorption band at 520 nm which was adopted as the analytical wavelength. Molar absorptivities of 2.741 x 10(2) L M(-1) cm(-1) and 2.681 x 10(2) L M(-1) cm(-1) were obtained for MZ and TZ, respectively. Optimization of reducing agent and time of reduction revealed the superiority of metal hydrides over reducing metals. Reduction of MZ and TZ was completed at 30 degrees C within 10 min. Optimizations of temperature and time for the complexation reaction revealed that the reaction was completed at 30 degrees C within 5 min. A 60: 40 mixture of 1,4-dioxane: acetonitrile was found to be the best diluting solvent for optimal detector response. The complexes were stable at room temperatures for weeks. Beer's law was observed in the concentration of 5-40 mu g ml (MZ) and 4.8-79.2 mu g ml(-1) (TZ) with low limits of detection of 1.88 and 0.74 mu g ml(-1), respectively. Overall recoveries of MZ and TZ from quality control samples were 103.19 +/- 2.05 (%RSD = 1.99, n = 12) and 101.63 +/- 1.41 (%RSD = 1.39) over three days. There was no interference from commonly utilized tablet excipients. No significant difference existed between the results of the new method and the BP titrimetric procedures (p > 0.05). The new CT procedure described in this paper is simple, fast, convenient, accurate and precise and has the novelty of carrying out the reactions at room temperature compared to previously described procedures. The new method could be adopted as an alternative procedure for the quality assessment of MZ and TZ in bulk and dosage forms.