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 described. They are based on the formation of coloured species by reaction of ninhydrin with the drug (method A, lambda (max) 397 nm) by oxidation of the indol moiety in melatonin with potassium persulphate (method B, lambda (max) 450 nm) or by reduction of osmium (VIII) (method C, lambda (max) 516 nm). Regression analysis of Beer-Lambert plots showed good correlations in concentration 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 optimum concentration ranges were calculated. The validity of the proposed methods was tested by analysing pharmaceutical formulations containing melatonin. The relative standard deviations were less than or equal to0.95\% with recoveries 99.0-101.33\%.