Spectrophotometric determination of vitamin E (alpha-tocopherol) using copper(II)-neocuproine reagent

Abstract

The possibility of the utilization of the copper(II)-neocuproine spectrophotometric method, which has previously been shown to permit the determination of various reducing agents, to the determination of vitamin E was investigated. The molar absorptivity for vitamin E was found to 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-tocopherol. The relative standard deviation of the slope of the absorbance vs. concentration plot was 2.1%. The results obtained by the copper(II)-neocuproine method were compared with those achieved by both the standard HPLC and the widely used iron(III)-bathophenanthroline method by means of a t-test which showed that the precision of the developed method was not essentially different from those of the others. The developed method was successfully applied to three commercial samples, two in dragee and one in ampoule form. The alpha-tocopheryl acetate contained in the samples, which did not respond directly to the Cu(II)-neocuproine reagent, was subjected to alkaline hydrolysis prior to the analysis of the hydrolysis product, i.e., alpha-tocopherol. The molar absorptivity due to Cu(I)-neocuproine at 450 nm against a reagent blank indicated a two-electron oxidation of vitamin E by Cu(II)-neocuproine, which may be slightly enhanced by solvent effects. Copper(II)-neocuproine is an oxidant of strength comparable to that of Fe(III)-bathophenanthroline. 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 method is free from interferences from such common reductants as ascorbic acid and Fe(II) salts, found in pharmaceutical formulations, after washing the formulation with water and collecting vitamin E in the ether extract for subsequent analysis.