This study evaluated a zero-order derivative UV spectrophotometric method for the simultaneous determination of binary components, codeine phosphate and diphenyhydramine HCl in cough mixtures. The method was based on the measurement of zero-order derivative amplitudes at isosbestic points, 258 and 264 nm, for the assay of codeine phosphate and diphenylhydramine HCl (0.05 to 0.15 mg/ml). The method does not require any pretreatment or the use of dyes or complexing agent. The scanned spectrum for diphenhydramine HCl standard in 0.1 M HCl showed absorptions at 252 and 258 nm (λmax) while codeine phosphate standard in 0.1 M HCl solution showed maximum absorptions at 284 nm. However, both spectra showed an overlap at 258 nm and 264 nm. The accuracy and precision of the method was evaluated by performing five replicate analyses. The relative standard deviation (rsd) (%) for both codeine phosphate and diphenyhydramine HCl ranged between 2.64 and 8.85% while the standard errors of mean concentrations were below 10 and 0.5%, respectively. The 95% confidence interval presented for students t-test is suggestive of a satisfactory precision and accuracy of the method used. The mean percentage recovery of both drugs from the spiked formulated cough mixtures ranged between 96.99-102.40% and 98.07-101.56% for codeine phosphate and diphenhydramine HCl, respectively. The technique in this study when compared with the BP (2008) method and specification for single dosage forms for codeine phosphate (95.0 - 105.0%) and diphenhydramine HCl (90.0–110.0%) could be of good application in simple analytical assays of such binary components in mixtures.
Key words: Codeine phosphate, diphenhydramine HCl, derivative UV-spectrophotometry, binary components, mixtures.
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