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

摘要

The possibility of the utilization of the copper(II)-neocuproine spectrophotometric method, which has previously been shown to permit the estimation of various reducing agents, to the estimation of vitamin E was investigated. The molar absorptivityfor vitamin E was found to be (2.1±0.1) X 104 litre 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 Cu(II)-neocuproine method were compared with those achieved by 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 notessentially different from those of the others. The developed method was successfully applied to 3 commercial samples. The alpha-tocopheryl acetate contained in the samples, which did not respond directly to the Cu(II)-neocuproine reagent, was subjectedto 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, whichmay be slightly enhanced by solvent effects. Cu(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.