SPECTROPHOTOMETRIC DETERMINATION OF VITAMIN-E (ALPHA-TOCOPHEROL) USING COPPER(II)-NEOCUPROINE REAGENT

The possibility of the utilization of the copper(II)-neocuproine spect rophotometric method, which has previously been shown to permit the de termination of various reducing agents, to the determination of vitami n E was investigated. The molar absorptivity for vitamin E was found t o be (2.1 +/- 0.1) x 10(4) 1 mol(-1) cm(-1) and Beer's law was obeyed between 2.4 x 10(-6) and 9.0 x 10(-5) M concentrations of alpha-tocoph erol. The relative standard deviation of the slope of the absorbance v s. concentration plot was 2.1%. The results obtained by the copper(II) -neocuproine method were compared with those achieved by both the stan dard HPLC and the widely used iron(III)-bathophenanthroline method by means of a t-test which showed that the precision of the developed met hod was not essentially different from those of the others. The develo ped method was successfully applied to three commercial samples, two i n dragee and one in ampoule form. The alpha-tocopheryl acetate contain ed in the samples, which did not respond directly to the Cu(II)-neocup roine reagent, was subjected to alkaline hydrolysis prior to the analy sis of the hydrolysis product, i.e., alpha-tocopherol. The molar absor ptivity due to Cu(I)-neocuproine at 450 nm against a reagent blank ind icated a two-electron oxidation of vitamin E by Cu(II)-neocuproine, wh ich may be slightly enhanced by solvent effects. Copper(II)-neocuproin e is an oxidant of strength comparable to that of Fe(III)-bathophenant hroline. The developed method, although less sensitive, is easy to use in conventional laboratories, unlike the Fe(III)-bathophenanthroline method, which requires specially prepared reagents and solvents. The m ethod is free from interferences from such common reductants as ascorb ic acid and Fe(II) salts, found in pharmaceutical formulations, after washing the formulation with water and collecting vitamin E in the eth er extract for subsequent analysis.