»8 VITAMINS A AND CAROTENES 



concentration unit, c is the concentration in grams per liter, and d is the 

 thickness of the layer in centimeters. 



The extinction coefficient k is often given as the molar extinction coeffi- 

 cient e (gram-moles per liter). 



EM 



€ = r- = k-M (M = molecular weight) 



c-d 



In vitamin chemistry the term E^^cm. is commonly used, according to the 

 formula 



_ ^ 



^^^'^ -^l^VdCcm.) 



which immediately gives the concentration c in per cent. In practice, it 

 generally refers to the material being examined; e.g., fish liver oils, which 

 show at 328 m/i an i5'|^°m. oi 17.5, contain 1 % vitamin A. Furthermore, the 

 color reaction with glycerol dichlorohydrin is important for it gives a stable 

 red color-^** With regard to the quantitative determination, this color re- 

 action offers some advantages in comparison to the Carr-Price reaction. The 

 color is stable for 2 to 10 minutes, and the reagent is considerably less 

 affected by moisture than antimony trichloride in chloroform. The maxi- 

 mum of absorption for the determination of vitamin A alcohol with this 

 reagent lies near 550 m/x, E\'^^^ = 1150 to 1250. However, this color reaction 

 is not yet sufficiently tested to warrant general acceptance for quantitative 

 purposes, but its importance may increase.^ In principal, the same methods 

 serve for the determination of carotene and vitamin A after appropriate 

 preparation. Thus, measurement of the quantitative extinction of the light 

 absorption maxima and comparison with a standard curve represents in 

 both cases the most precise method. Beside this method the old colorimetric 

 or photometric procedure is still frequently used for the determination of 

 /3-carotene. Standard j8-carotene, azobenzene,^ or potassium bichromate 

 solutions^ are used as comparison solutions for this procedure (see below). 

 Besides, as mentioned above, the blue color with antimony trichloride in 

 chloroform or the red color with glycerol dichlorohydrin is employed for 

 the quantitative determination, especially for vitamin A. Here also the 

 measurement of the intensity is done with a colorimeter, with a photometer, 

 or better with a spectrophotometer. 

 The determination with the Carr-Price reaction gives rise to several 



sb A. E. Sobel and H. Werbin, /. Biol. Chem. 159, 681 (1945); Anal. Chem. 19, 107 



(1947). 

 s P. B. Miiller, Helv. Chim. Acta 30, 1172, 1188 (1947). 



■» R. Kuhn and H. Brockmann, Hoppe-Seyler's Z. physiol. Chem. 206, 41 (1932). 

 6 A. Krogis, Biochem. Z. 287, 266 (1936). 



