Quantitative determination of lansoprozole in capsules and spiked human urine by spectrophotometry through ion-pair complex formation reaction

Two simple, rapid and sensitive extraction-free spectrophotometric methods are proposed for the determination of lansoprazole (LAN) in bulk drug and in its capsule formulation. The methods are based on the interaction of LAN in dichloromethane (DCM) with acidic sulfonphthalein dyes, namely, bromocresol purple (BCP) in method A and bromothylmol blue (BTB) in method B to form stable, yellow-colored, ion-pair complexes peaking at 400 and 430 nm, respectively. The parameters that affect the reaction were carefully optimized, and under the optimized conditions, linear relationships were obtained in the ranges of 0.5–15.0 and 1.25–20.0 μg mL^?1 LAN for method A and method B, respectively. The molar absorptivity values are calculated to be 2.10 × 10⁴ and 1.50 × 10⁴ L mol^?1 cm^?1 for method A and method B, respectively. The composition of the ion-pairs was found to be 1:1 by Job’s method of continuous variations and the conditional stability constant (log K_f) of the complexes has been calculated. The proposed methods were applied successfully to the determination of LAN in capsules as well as in spiked urine sample with good accuracy and precision. The results obtained by the proposed methods were compared favorably with those of the reference method.