Ethambutol-Cobalt (II) ions complexation spectral characteristics and applications for quantitative analysis

Ethambutol (EMB) has no significant absorption characteristics beyond 200 nm due to its aliphatic saturated nature. Thus, direct EMB determination in the UV range is not feasible due to its very low molar absorptivity and must requiring a derivatization reaction. Since EMB can act as a chelating agent that disrupts one of several metal-containing enzyme systems in the nucleic acid structure of mycobacteria. This chelating property can be used for quantitative analysis of EMB by formation of UV-Vis absorbing metal complexes. The study in this work describes simple, accurate and sensitive spectrophotometric procedure for the determination of EMB in its pure form and in pharmaceutical formulations. The method depends on the interaction of EMB with cobalt (II) ions in presence of ammonia solution (4 N). The absorbance was measured either at 250, 348 or 521 nm respectively. All parameters affecting the reaction were carefully studied and optimized. Beers-Lamberts law was obeyed within the concentration ranges 5-25, 40-240 and 100-700 μg ml-1 for the three studied wavelengths respectively. The developed method was validated according to the ICH guidelines and applied to pharmaceutical formulation analysis with good recovery ranges. The results were compared to those obtained by an official method and found satisfactorily matched and no significant differences were found within the 95% confidence level. The proposed procedures were suitable for simple routine work and quality control analysis of EMB.