Extractive spectrophotometric methods for the determination of doxepin hydrochloride in pharmaceutical preparations using titanium (IV) and iron (III) thiocyanate complexes

Two simple, precise, and accurate extractive spectrophotometric methods have been developed for the determination of doxepin hydrochloride in pharmaceutical preparations. The methods are based on the formation of ion association complexes of doxepin with titanium (IV) and iron (III) thiocyanate complexes in acidic medium. The produced compounds are insoluble in water but well soluble in some organic solvents. They are extracted with mixtures of butyl alcohol-chloroform (2:3, v/v) and (1:4, v/v) and measured spectrophotometrically at 400 and 490 nm for DOX-Ti-SCN and DOX-Fe(III)-SCN methods, respectively. Beer's law was obeyed in the concentration ranges of 5-50 and 3-30 μg/ml with molar absorptivity of 7.12 × 103 and 1.36∙× 104 l mol-1 cm-1 for DOX-Ti-SCN and DOX-Fe-SCN systems, respectively. The proposed methods have been successfully applied for the analysis of the drug in dosage forms. No interference was observed from common pharmaceutical adjuvants. The methods have been also used for the determination of the drug in the presence of its degradation product. Statistical comparison of the obtained results with the reference methods shows excellent agreement and indicates no significant difference in accuracy and precision.