Spectrophotometric Determination of Methyldopa and Levodopa by Oxidative Coupling Reactions using the synthesis reagent (2-amino-5-(para-aminophenyl)-1,3,4-thiadiazole) and Investigation of Biological Activity

Based on oxidative coupling reactions utilizing the synthesized organic reagent 2-amino-5-(para-aminophenyl)-1,3,4-thiadiazole in acidic media, and with potassium dichromate as an oxidizing agent, a spectrophotometric method has been established for determining methyldopa and levodopa. For these drugs, the resultant complex exhibits peak absorption at 401.5 nm and 415 nm, respectively. The method adheres to Beer s law in the ranges of 1-55 µg ml^-1 and 2.5-170 µg ml^-1, with molar absorption coefficients of 0.24 x 10⁴ and 0.116 x 10⁴ L mol^-1 cm^-1, respectively. The quantification limit (LOQ) is set at 1.5769 µg ml^-1 for methyldopa and 3.0616 µg ml^-1 for levodopa, yielding recovery rates of 100.14% and 100.34%, and relative standard deviation rates of 2.748% and 0.779%. The nature of the resulting complex was examined using the continuous variation method (Job s method) and molar ratios, revealing a 1:2 ratio (reagent to drug compound) for both drugs. This method has been successfully applied to pharmaceutical formulations.