A new kinetic and thermodynamic studies of spectrophotometric method for the quantitative analysis of micrograms of methyldopa (MD) in pure and pharmaceutical dosage forms has been adopted.The method is based on reaction of MD with ferric chloride and nitroso-R-salt (NRS). MD reduces ferric ion, resulting in the formation of ferrous ion which finally forms a green color complex with NRS which is measured spectrophotometrically at 708 nm. The initial rate and fixed time methods were adopted for constructing the calibration curves. The linearity ranges were found to be 0.5-10 and 0.3-10 µg mL-1 for initial rate and fixed time methods, respectively. Thermodynamics' parameters like free energy change, the enthalpy of formation as well as the entropy were determined for the reaction product .The method was applied successfully for the estimation of MD in commercial dosage forms. Statistical comparison between the proposed and reference spectrophotometric methods showed excellent agreement between the accuracy and precision of the two methods.
