Spectrophotometric determination of alendronate in pharmaceutical formulations via complex formation with Fe(III) ions

The formation of the complex between alendronate, non-chromophoric bisphosphonate drug important for the treatment of a variety of bone diseases, and iron(III) chloride in perchloric acid solution was studied. The stoichiometric ratio of alendronate to Fe(III) ions in the chromophoric complex was determined to be 1:1. The conditional stability constant was log K′_ave=4.50 (SD=0.15), indicating that the Fe(III)–alendronate complex is a complex of medium stability. The optimum conditions for this reaction were ascertained and a spectrophotometric method was developed for the determination of alendronate in the concentration range 8.1–162.5 μg ml⁻¹, the detection limit being 2 μg ml⁻¹. The method was validated for the direct determination of alendronate in tablet dosage formulations.