26] Colorimetric analysis of vitamin A and carotene

Abstract

This chapter discusses an overview of colorimetric analysis of vitamin A and carotene. For analysis of vitamin A and carotene in serum, they are first removed from their protein complexes by stripping with ethanolic KOH, then are extracted with petroleum ether, and quantitated by measuring the color developed upon treatment with 1,3-dichloro-2-propanol containing acetyl chloride. In this study, it was found that the colored species decayed most rapidly when preparations of dichloropropanol were used that contained significant quantities of peroxides. Because vitamin A is highly unsaturated and peroxides are well known to be scavengers of double bonds, it was concluded that one of the sources of the instability of the color was the presence of the peroxides in the activating solution. The purification scheme given in the chapter removes the peroxides effectively. Using this purified DCP, many different activating reagents were evaluated. When DCP was activated with antimony trichloride by codistillation as in the original Sobel-Snow work, the reagent was superior (greater color stability) to that obtained from the use of unpurified DCP. However, this required a second distillation, because the addition of antimony trichloride to the eluate from the alumina column was inactive upon subsequent distillation.