Colorimetric determination of catecholamines by 2,3,5-triphenyltetrazolium chloride

Development and validation of a simple spectrophotometric method for the determination of methyldopa in both bulk and marketed dosage formulations

A simple, precise, sensitive, rapid, specific and economical spectrophotometric method was developed to determine methyldopa (MTD) content in bulk and pharmaceutical dosage formulations. The proposed method was based on the formation of a colored product from the nitrosation reaction of MTD with sodium nitrite in an acid medium. The resultant nitroso derivative species reacts further with sodium hydroxide and is converted it into a more stable compound. This yellow nitrosation product exhibited an absorption maximum at 430 nm. Beer's Law was obeyed in a concentration range of 6.37 to 82.81 μg mL-1 MTD with an excellent coefficient of determination (R2 = 0.9998). No interference was observed from common excipients in formulations. The results showed the method to be simple, accurate and readily applied for the determination of MTD in pure form and in pharmaceutical preparations. The analytical results obtained for these products using the proposed method are in agreement with those of the Brazilian Pharmacopoeia procedure at a 95% confidence level.

A new spectrophotometric method for the determination of methyldopa

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.

Spectrophotometric determination of some catecholamine drugs using sodium bismuthate

A novel spectrophotometric method is described for the determination of epinephrine (EP) and norepinephrine (NE). The method is based on the development of a red colour with sodium bismuthate, as a sensitive chromogenic reagent, in aqueous medium at pH 3. Oxidation of these catecholamines produces aminochrome derivatives which can be measured spectrophotometrically at 486.0 nm. Calibration graphs are linear in the range 4.8-800 (micromol l(-1)) for epinephrine bitartarate and 4.8-600 (micromol l(-1)) for norepinephrine bitartarate with detection limits of 0.26 (micromol l(-1)) and 2.46 (micromol l(-1)) for epinephrine and norepinephrine bitartarate salts, respectively. The method has successfully been applied to determination of these catecholamines in pharmaceutical preparations.

Stability-indicating high-performance liquid chromatographic assay for alpha-methyldopa in sustained-release capsules

A stability-indicating high-performance liquid chromatographic assay has been developed for the analysis of alpha-methyldopa (MD) in sustained-release capsules and in the presence of MD decomposition products and an MD industrial impurity, 3-O-methyl-methyldopa (MMD). The method utilizes reversed-phase chromatography (cyano-bonded column), an acidic mobile phase containing sodium heptanesulphonate as ion-pairing reagent and UV detection. Detector responses were linear in the ranges 0.5-200 microgram/ml for MD and 0.2-100 microgram/ml for MMD. The mean recoveries of MD from authentic sample and sustained-release capsules were 100.09 +/- 0.38 and 100.38 +/- 0.46%, respectively. The recovery of MD added to degraded MD, were 99.69% by the proposed method and 153.13% by the US Pharmacopeial (USP) spectrophotometric method. The method is sensitive, accurate and rapid and can be used in routine analysis for MD.

Sensitive colorimetric assay of carbidopa and methyldopa using tetrazolium blue chloride in pharmaceutical products

A colorimetric method for the assay of carbidopa and methyldopa either in pure form or in pharmaceutical preparations is described. The method is based on the reduction of tetrazolium blue chloride in a non-aqueous alkaline medium by the substances analysed, and the measurement of the absorbance of the pink-coloured diformazan solution, which is caused by this reduction. The spectra of the aforesaid solutions exhibit an absorption maximum at lambda = 525 nm. The apparent molar absorptivities and Sandell's sensitivities (in l.mol-1.cm-1 and ng.cm-2, respectively) are 1.2.10(5) and 1.9 for carbidopa and 7.07.10(4) and 3.0 for methyldopa. The solution of diformazan formed obeys Beer's law in the concentration range of 0.05-4.0 ppm for carbidopa and 0.1-6.0 ppm for methyldopa. The slope and intercept of the corresponding regression line equation were obtained with a correlation coefficient of 0.9999 for carbidopa and 0.9994 for methyldopa. The variables affecting the development of the colour are investigated and the conditions are optimized. Compared with other procedures this method showed to be more simple and rapid, highly sensitive, precise and accurate. Results obtained by application of the proposed method and the official one, were in good agreement, while statistical comparison by the Student's t-test shows no significant difference between the two sets of the results.

Difference spectrophotometric assay of 1,2-diphenolic drugs in pharmaceutical formulations II: Germanium dioxide reagent

A rapid, difference UV spectrophotometric assay of formulated drugs containing a 1,2-diphenolic group is described. In this assay are utilized the bathochromic shift of the absorption band of 1,2-diphenolic substances from approximately 280 to approximately 287 nm and the concomitant hyperchromic effect induced by the addition of germanium dioxide to an aqueous solution of the drug buffered at pH 6. The absorbance of the solution of the complexed drug relative to that of an equimolar solution of the free drug, which is maximum at approximately 292 nm, is proportional to the concentration of the drug and is unaffected by the presence of other UV-absorbing substances in the formulations that lack the 1,2-diphenolic moiety. Applications of the assay are described for formulations containing epinephrine, isoetharine, isoproterenol, levodopa, and methyldopa.