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.