New colorimetric methods for microdetermination of melatonin in pure and in dosage forms

Two simple, rapid, sensitive and accurate methods (A and B) for the microdetermination of melatonin in pure form and in pharmaceutical formulations are developed. Method A is based on the formation of tris(o-phenanthroline)iron(II) complex (ferroin) upon reaction of melatonin with an iron(III)-o-phenanthroline mixture in sodium acetate-acetic acid buffer media. The ferroin complex is colorimetrically measured at lambda(max) 510 nn against reagent blank Method B is based on the reduction of Fe (III) by the drug which forms a colored complex (lambda(max) 522 nm) with 2,2(-)-bipyridyl. Optimizations of the experimental conditions are described. Beer's law is obeyed in the concentration range 0.4 - 6.4 mu g ml(-1) for A and 0.4 - 7.4 mu g ml(-1) for B, respectively. The developed methods are applied successfully for the determination of melatonin in pure-form and in melatonin tablets without any interference from common excipients.