Two simple, rapid, sensitive and accurate methods (A and B) for the microde
termination of melatonin in pure form and in pharmaceutical formulations ar
e developed. Method A is based on the formation of tris(o-phenanthroline)ir
on(II) complex (ferroin) upon reaction of melatonin with an iron(III)-o-phe
nanthroline 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 for
ms a colored complex (lambda(max) 522 nm) with 2,2(-)-bipyridyl. Optimizati
ons of the experimental conditions are described. Beer's law is obeyed in t
he 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 t
he determination of melatonin in pure-form and in melatonin tablets without
any interference from common excipients.