Colorimetric estimation of melatonin in pharmaceutical formulations

Three simple and sensitive colorimetric methods (A-C) for the determination of melatonin in bulk samples and in pharmaceutical formulations are descri bed. They are based on the formation of coloured species by reaction of nin hydrin with the drug (method A, lambda (max) 397 nm) by oxidation of the in dol moiety in melatonin with potassium persulphate (method B, lambda (max) 450 nm) or by reduction of osmium (VIII) (method C, lambda (max) 516 nm). R egression analysis of Beer-Lambert plots showed good correlations in concen tration ranges between 0.8-14.2, 70.0-140.0 and 2.0-10.0 mug/mL for methods A, B and C, respectively. The molar absorptivity, Sandell sensitivity and detection limit were calculated. For more accurate analysis, Ringbom optimu m concentration ranges were calculated. The validity of the proposed method s was tested by analysing pharmaceutical formulations containing melatonin. The relative standard deviations were less than or equal to0.95% with reco veries 99.0-101.33%.