Simple and practicable methods for the determination of astemizole in pharmaceuticals using bromate-bromide and two dyes

One titrimetric and two spectrophotometric methods, which are simple and sensitive, are described for the determination of astemizole (AST) in bulk drug and formulations. The methods use bromate-bromide mixture and two dyes, methyl orange and indigo carmine. In titrimetry (Method A), astemizole is treated with a known excess of bromate-bromide mixture in acid medium and after the bromination reaction is ensured to be complete, the residual bromine is back-titrated iodometrically. In spectrophotometric methods, the excess of bromine is estimated by treating it with a fixed amount of either methyl orange (Method B) or indigo carmine (Method C) and measuring the change in absorbance either at 520 or 610 nm. In all the methods, the amount of bromate reacted corresponds to the drug content. Titrimetric method is applicable over 4-16 mg range and the calculations are based on a 1:0.666 (AST: bromate) reacting ratio. In spectrophotometry, the calibration graph is found to be linear over 0.5-4.0 µg mL-1 (Method B) and 1.25 12.5 µg mL-1 (Method C) with molar absorptivity values of 6.6 x 10(4) L mol-1 cm-1 and 2.1 x 10(4) L mol-1 cm-1, respectively. The limits of detection and quantification are reported for methods B and C. The statistical evaluation of the methods was examined by determining intra-day and inter-day precision. The methods were applied to the determination of AST in tablets and syrups and the results were found to agree well with the label claim. The accuracy and reliability of the methods were further ascertained by parallel determination by a reference method and by calculating the Student's t-value and F-value at the 95% confidence level, and by recovery studies using standard addition technique.