Electrochemical oxidation of some catecholamines such as dopamine (1), L-dopa (2), and methyldopa (3) has been studied in various pH values, using cyclic voltammetry. The results indicate participation of catecholamines (1-3) in intramolecular cyclization reaction to form the corresponding o-quinone derivatives (1d-3d). In various pHs, based on ECE mechanism, the observed homogeneous rate constants (k obs ) of cyclization reaction were estimated by comparing the experimental cyclic voltammetric responses with the digital-simulated results. Also, the cyclization rate constants (k cyc ) were calculated using microscopic acidic dissociation constant of ammonium groups. The significant differences in electrochemical behavior, k obs and k cyc , of L-dopa (2) and methyldopa (3) with dopamine (1) are due to the effects of the side chain carboxyl group. C

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Um método espectrofotométrico simples e sensível para a determinação de dopamina, metildopa, levodopa e catecol, puros, em formulações farmacêuticas, e em água, é descrito. O método é baseado na reação de oxidação com periodato, seguida pelo acoplamento oxidativo com ácido 4-aminobenzóico em pH 4, levando à formação de produtos coloridos. O gráfico de calibração foi linear no intervalo de 0,20-18,00 mg L -1 , 0,10-20,00 mg L -1, 0,10-20,00 mg L -1 e 0,10-13,00 mg L -1 para dopamina, metildopa, levodopa e catecol, respectivamente. Dados analíticos para determinação do composto puro são apresentados juntamente com a aplicação do método proposto à análise das formulações farmacêuticas e amostras de água. Os resultados são favoravelmente comparados com métodos relatados.A sensitive and simple spectrophotometric method for the determination of dopamine, methyldopa, levodopa and catechole in either pure form, pharmaceutical formulations and in water is described. The method is based on their oxidation reaction with periodate followed by oxidative coupling with 4-aminobenzoic acid at pH 4 leading to the formation of colored products. The calibration graph was linear in the range of 0.20-18.00 mg L -1 , 0.10-20.00 mg L -1 , 0.10-20.00 mg L -1 and 0.10-13.00 mg L -1 for dopamine, methyldopa, levodopa and catechole, respectively. Analytical data for determination of the pure compound is presented together with the application of the proposed method to the analysis of some pharmaceutical formulations and water samples. The results compared favorably with reported methods.

A rapid, simple, and accurate spectrophotometric method is presented for the determination of fluoxetine by batch and flow injection analysis methods. The method is based on fluoxetine competitive complexation reac

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