Extraction-Free Spectrophotometric Assay of Ethionamide in Pharmaceutical Using Two Sulphonphthalein Dyes as Ion-Pair Agents

Abstract

Two simple, sensitive, selective, rapid and reproducible visible spectrophotometric methods are presented for the determination of ethionamide (ETM) in bulk drug and tablets by employing two sulphonphthalein dyes: bromocresol green (BCG method) and bromocresol purple (BCP) as chromogenic agents. The methods are based on the formation of intensely yellow colored ion-pair complexes upon the reaction of the drug with the cited dyes in chloroform medium. The 1:1 (drug:dye) complexes formed were spectrophotometrically measured at 460 nm without involving the extraction step. Optimization of the experimental conditions is described. Beer’s law is obeyed in the concentration range 1–10 µg ml⁻¹ in both methods, with molar absorptivity values of 1.45 × 10⁴ and 1.68 × 10⁴ l mol⁻¹ cm⁻¹ with BCG method and BCP method, respectively. The calculated limits detection and quantification were 0.02 and 0.07 µg ml⁻¹ with both methods. The suitability of the methods for the quantitative determination of ETM was proved by validation in accordance with the requirements of the ICH guidelines. The coefficients of variation were < 1.65% for intra-day and < 1.90% inter-day analysis, and the accuracy expressed as relative error was ≤ 2%. The methods were applied to the determination of ETM in tablets and the results agreed well with the label claim and those obtained by the reference method. The accuracy of the methods was further ascertained by recovery study via standard-addition procedure. The possible reaction pathway is also discussed.