Abstract: A highly sensitive spectrophotometric method was developed for the determination of trace amounts of Iodine based on the oxidation of I- to I3- by IO3- and the further reaction of I3- with acridine red to form the ion-association complex [AO][I3]in dilute phosphoric acid solution.In the presence of polyvinyl alcohol the solution is transparent.Theλmaxof the absorption spectrum is 584 nm against reagent blank.It was discovered that the absolute value of theλmaxis proportional to the IO3- concentration.The molar absorptivity isε=5.27×105 L/mol·cm for normal assay at 584nm.Beer’s law is obeyed in the range of 0～7.0μg/25ml.The method has been applied to the determination of iodine in biological samples with satisfactory results.

Key words: iodine, acridine red, ion-association, spectrophotometry