Acta Pharm. 56 (2006) 189-202

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Original research paper 

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


Department of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Ibadan, Nigeria
Accepted February 13, 2006

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 3.3-11.55 g mL-1 of indomethacin. Optimization studies established an optimum reaction time of 20 minutes at 30 oC 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 g mL-1 and is precise (RSD of 2.3%). The new method has been successfully applied to the determination of indomethacin in capsules and the method is of equivalent accuracy (p > 0.05) with the official (BP) spectrophotometric method. The new method could find application as a simple analytical method for the assay of indomethacin in capsules.

Keywords: indomethacin assay, colorimetry, 4-carboxyl-2,6-dinitrobenzene diazonium, diazo coupling reaction