Investigating the Spectrophotometric Determination of Doxycycline by Ion-pair Complex Formation With Bromothymol Blue

Background: Doxycycline is a widely used antibiotic, commonly formulated as doxycycline hydrochloride (100 mg). The accurate estimation of doxycycline in pharmaceutical formulations is crucial for quality control and therapeutic efficacy. Spectrophotometry is a preferred analytical technique due to its simplicity, rapidity, accuracy, and cost-effectiveness. Objectives: To develop a simple, accurate, and cost-effective spectrophotometric method for estimating doxycycline in pharmaceutical formulations by forming an ion-pair complex with bromothymol blue. The study aims to optimize conditions for complex formation, validate the method's linearity, sensitivity, and accuracy, and apply the procedure to pharmaceutical samples. Methods: The absorbance of doxycycline was measured at 528 nm. The ideal conditions for product formation were systematically studied, including the type and volume of dye, the effects of acid and base, temperature, time, and solvent choice. The method’s linearity was evaluated within the range of 5-45 µg/mL, adhering to Beer-Lambert’s law. Sensitivity was determined using Sandell’s sensitivity; meanwhile, the molar absorptivity (ε) coefficient was calculated. The limits of detection and quantification were also determined. The accuracy and precision of the method were assessed using the percentage recovery and the relative standard deviation. The procedure was applied to the estimation of doxycycline in its pharmaceutical form using direct and standard additive methods. Results: The developed spectrophotometric method demonstrated an absorbance peak for doxycycline at 528 nm. The linearity of the method was established between 5-45 µg/mL, with Sandell’s sensitivity calculated at 0.0625 µg/cm². The molar absorptivity (ε) coefficient was determined at 7.110×10³ L/mol·cm. The limits of detection and quantification were 0.07950 µg/mL and 0.29573 µg/mL, respectively. The percentage recovery ranged from 95.250% to 99.250%, and the relative standard deviation did not exceed 0.4194. The procedure was successfully applied to estimate doxycycline in its pharmaceutical form by the direct method and the standard additive method, confirming the method’s reliability and accuracy. Conclusion: A reliable and straightforward spectrophotometric method was successfully developed to estimate doxycycline using bromothymol blue. The method showed good accuracy, precision, and linearity within a 5–45 µg/mL concentration range. It was effectively applied to pharmaceutical formulations, demonstrating high recovery rates and compatibility, confirming its utility for quality control purposes in pharmaceutical analysis.