Determination of Doxycycline Hyclate by Batch and Reverse Flow Injection Analysis Based on the Oxidative Coupling Reaction with 3-Methyl-2-benzothiazolinone Hydrazone hydrochloride (MBTH)

New, simple, and sensitive batch and reverse FIA spectrophotometric methods for the determination of doxycycline hyclate (DCH) in pure form and in pharmaceutical preparations were proposed. These methods are based on an oxidative coupling reaction between doxycycline hyclate and 3-methylbenzothiazolinone-2-hydrazone hydrochloride (MBTH) in the presence of ammonium ceric sulfate in acidic medium, forming a green water-soluble dye with a maximum absorbance at 626 nm. A calibration graph shows that Beer's law is obeyed over the concentration range of 1-80 and 0.5-110 μg/mL for the batch and reverse FIA (r-FIA) methods, respectively, with a detection limit of 0.325 μg/mL of DCH for r-FIA methods. All different chemicals and physical experimental parameters affecting the development and stability of the colored product were carefully studied. The proposed methods were successfully applied for the determination of DCH in pharmaceutical preparations.