Spectrophotometric determination of trimethoprim in pure form and in pharmaceutical preparations using bromothymol blue, bromocresol green and alizarin red S

Simple, sensitive and selective methods for the determination of trimethoprim (TMP) in pure form and in pharmaceutical formulations are described. The methods are based on the reaction of TMP as a pi-electron donor with bromothymol blue (BTB), bromocresol green (BCG) and alizarin red S (ARS) as electron accepters. The coloured products are quantified spectrophotometically at their corresponding lambda(max). Beer's law is obeyed in case of BTB in the range 2.9-23.2 mu g/ml (CHCl3), 2.9-20.0 mu g/ml(CH2Cl2) and 5.0-29.0 mu g/ml (ClC6H5), in the case of BCG 2.927.5)mu g/ml (H2O/alc.)2.9-18.3 mu g/ml (CHCl3) and 2.9-20.3 mu g/ml(CH2Cl2) and for ARS in the range 3.0-12.0 mu g/ml in H2O/alc medium. The specific absorptivities, molar absorptivities, Sandell sensitivities, standard deviations and percent recoveries are evaluated. Application of the suggested methods to dosage forms is presented and compared with the pharmacopoeial method. The interference from additives and sulfa compounds, especially sulfamethoxazole, has been overcome by extraction into chloroform or methylene chloride.