A new, simple and low cost spectrophotometric method for the determination of methyldopa in pharmaceutical preparations was developed. The method was based on the coupling of methyldopa with 2,6-dichloroquinone-4-chlorimide (DCQ). The absorbance maximum (λ max) of the resulted colored product was at 400 nm. Different buffers were used to determine the optimal pH for the reaction. About 1% w/v acetate buffer with pH 8.0 gave the optimal pH required for the reaction. Of the different solvents tried, water and ethanol were found to be the most suitable solvents. Beer's law was obeyed in concentration range of 4-20 μg/ml methyldopa. The correlation coefficient was found to be (r = 0.9975). The limit of detection and limit of quantification were 1.1 μg/ml and 3.21 μg/ml, respectively. The reaction ratio between methyldopa and DCQ was studied and found to be 1:3. The work included the study of the possible interference of hydrochlorothiazide found in combination with methyldopa tablets. The method was validated and results obtained for the assay of two different brands of methyldopa tablets were compared with the BP method (colorimetric). The repeatability and reproducibility of the developed method were evaluated and the obtained results quoted. The derivative formed as a result of the reaction of methyldopa with DCQ was isolated and its possible mechanistic pathway was suggested.
An accurate stability-indicating method has been developed for the analysis of Torsemide (TOR) in bulk and pharmaceutical dosage forms. The methods used the zero-order spectrum (0D) of TOR aqueous solution (measured at 285 nm) and the instrumentally differentiated first (1D) and second (2D) derivative spectra (measured at 311 nm and 282 nm, respectively). The effect of light, acid (HCL) and alkali (NaOH) on the stability of TOR were studied using the new methods. ICH guidelines were used to validate the new methods. Regression analysis of Beer's plots showed a good correlation coefficient not less than 0.998. These methods reported great inter-day and intra-day precision. Excipients interference was not detected due to the achievement of good recovery percentages (97.60 - 101.45 ± 2.7 %, n=3). Good assay results ranged from 99.0 ± 1.7% to 100.0 ± 2.5%; the developed methods obtained n=3. The 2D model proved its ability to be used as a stability indication method of TOR analysis. TOR was unstable in acids and bases with or without heating. Its degradation follows the first-order kinetics. However, its aqueous solution was proved to be stable under sunlight. The established methods demonstrated good precision, sensitivity and accuracy at 95% confidence level.
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