Sensitive and selective spectrophotometric assay of rizatriptan benzoate in pharmaceuticals using three sulphonphthalein dyes

Three simple, rapid, selective and sensitive spectrophotometric methods are described for the determination of rizatriptan benzoate (RTB) in bulk drug and in tablets. The methods are based on the formation of intense yellow colored ion-pair complexes between RTB and sulphonphthalein acid dyes, namely, bromophenol blue (BPB), bromocresol purple (BCP), bromothymol blue (BTB) in chloroform medium. The colored products are measured at 425 nm (RTB-BPB complex, RTB-BCP complex) and 420 nm (RTB-BTB complex). The reactions were extremely rapid at room temperature and the absorbance values remained constant for 90 min (methods A and B) and over 12 h (method C). Beer's law was obeyed in the concentration ranges of 0.8-16.0, 1.0-20.0 and 1.2-24 μg ml−1 with molar absorptivity values of 1.76 × 104, 1.96 × 104 and 1.63 × 104 l mol−1 cm−1 for BPB, BCP and BTB methods, respectively. The limits of quantification (LOQ) were 0.39, 0.34 and 0.27 μg ml−1 for BPB, BCP and BTB methods, respectively. Other method validation parameters, such as precision, accuracy, robustness, ruggedness and selectivity, were satisfactory. The composition of the ion-pair was found to be 1:1 by Job's method. The proposed methods were successfully applied to the determination of RTB in commercial tablets. No interference was observed from common tablet adjuvants. Statistical comparison of the results with the reference method showed excellent agreement and indicated no significant difference in accuracy and precision.