Spectrophotometric investigation of metoprolol-benzyl orange reaction and its application to the assay in pharmaceutical dosage forms.

A sensitive spectrophotometric method for the determination of metoprolol tartrate in tablets and ampoules is presented. Using spectrophotometric measurements, it was found that metoprolol tartrate and benzyl orange form a chloroform soluble ion-pair complex with an absorption maximum at 401 nm. The composition of the ion-pair complex was determined by applying Job's method to equimolar solutions of metoprolol tartrate: benzyl orange (1:2); molar absorptivity 7.39 x 10(3) mol-1 cm-1. Extraction of the ion-pair complex in chloroform was accomplished easily at a Britton-Robinson's buffered optimum pH = 5.2, mu = 0.1 mol/dm3. The relative stability constant, calculated according to the method of Sommer and Job's non-equimolar solutions, was log K = 9.72 (avg. value). Beer's law was obeyed up to 3.42 micrograms/ml of metoprolol tartrate (the detection limit was also 3.42 micrograms/ml). The precision of the method was checked at three different concentrations. The RSD (n = 7) varied from 0.51 to 2.03%. Reproducibility was examined by analysing Lopresor tablets and ampoules. Recoveries varied from 99-101%. The reported method, applied to the assay of metoprolol tartrate in tablets and ampoules, gives precise and reproducible results.