Novel colorimetric assay of indomethacin using 4-carboxyl-2,6-dinitrobenzene diazonium ion.

A simple, sensitive and direct colorimetric method for the determination of indomethacin either in pure form or in capsules has been developed. The method is based on the diazo coupling reaction between indomethacin and a highly reactive arenediazonium ion, 4-carboxyl-2,6-dinitrobenzene diazonium ion, with the consequent formation of an azo dye. The reaction is fast and gave an orange azo dye in ethyl acetate. The assay was carried out at 470 nm and the azo adduct was stable for three hours. Beer's law is obeyed in the concentration range of 3.3-11 microg mL(-1) of indomethacin. Optimization studies established an optimum reaction time of 20 minutes at 30 degrees C and the drug-to-reagent ratio of 1:2 for optimal detector response. The method developed has a low limit of detection of 0.90 microg mL(-1) and is precise (RSD 2.3%). The new method has been successfully applied to the determination of indomethacin in capsules and the method is of equivalent accuracy as the official (BP) spectrophotometric method. The new method could find application as a simple analytical method for the assay of indomethacin in capsules.