Extractional spectrophotometric analysis of metronidazole, tinidazole, ornidazole and secnidazole bases through acid-dye complexation using bromothymol blue dye

An easy, precise and valid extractional-spectrophotometric technique is described for the assessment of metronidazole (MNZ), tinidazole (TNZ), ornidazole (ONZ) and secnidazole (SNZ) in pure state and in their pharmaceutical formulations. The technique includes first the reduction of above cited drugs using HCl and zinc powder, then the formation of intense yellow colored ion-association complex species (1:3 drug/dye) using bromothymol blue (BTB) in a buffered aqueous acidic medium at pH 3–3.50. The colored products are extracted into dichloromethane and quantitatively determined at 416–420 nm. The experimental operating factors influencing the ion-pairs development were studied and optimized to obtain the maximum color intensity. The Beer plots are obeyed in the concentration ranges 2.50–22.50, 2.50–30, 7.50–35 and 5–30 μgml−1 for MNZ, TNZ, ONZ and SNZ, respectively, with correlation coefficients not less than 0.9995. The proposed technique is recommended for the routine quality control analysis of the investigated drugs in commercial tablets with no observed interference from common pharmaceutical adjuvants. Results of such analysis were statistically validated and through recovery studies, showing excellent agreement with those achieved by the reported techniques.