Accurate, rapid and sensitive extraction-colorimetric method for the determination of erythromycin and its esters, depending on the formation of an ion-pairs with chromotropic acid (I), chromotrope 2B (II), chromotrope 2R (III), arsenazo I (IV), arsenazo III (V), benzocaprol red (VI) and acid ethyl blue (VII) is described. The calibration curves resulting from the measurements of absorbance-concentration relations (at the optimum reaction conditions) of the extracted ion-pairs are linear over the concentration range 0.4–56 μg ml−1 with a relative standard deviation (RSD) of 1.3% for 25 μg ml−1 erythromycin. The detection limit, quantification limit, the molar absorptivity and Sandell sensitivity for erythromycin ion-pairs were evaluated. The interference from excipients commonly present in dosage forms and common degradation product was studied. The method is highly specific for the determination of stearate and succinate esters in dosage forms. The method has been compared to the official method and found to be simple, accurate (t-test) and reproducible (F-test). The developed method was applied for bulk drug, esters and some of their dosage forms without interferences from additives and excipients.
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