732 ■ VII. VITAMINS A 



In the case of foods enriched with vitamin A, satisfactory results may be 

 obtained by correction of the E (1%, 1 cm., 328 myi) value with a blank 

 obtained from an extract of the unenriched food.^^^ A fairly general proce- 

 dure for the determination of vitamin A in food products is given by Oser 

 et aU^' 



A more complete discussion of the spectrometric properties of vitamin A 

 and of the carotenoids is given in Morton's monograph," as well as in the 

 review of LoofbouroWj^^"* Recent developments in this field are included 

 in the reviews of Morton, ^^^ Hickman,^** and Embree.^^^ 



(4) Reaction with Antimony Trichloride (Carr-Price Test) 



One of the earliest recognized tests for vitamin A was the reaction be- 

 tween vitamin A and inorganic chlorides in anhydrous solvents. Under 

 such conditions a development of color results. Of the inorganic chlorides, 

 antimonj^ trichloride has been used most effectivly in an anhydrous chloro- 

 form solution. This reaction forms the basis for the widely employed 

 Carr-Price method. ^^ A similar reaction is given by zinc chloride, ^^ 

 arsenic chloride, ^^ ferric chloride, ^^ and stannic chloride. ^^ However, the 

 latter salts have proved less satisfactory than the antimony trichloride for 

 quantitative work, partly because of the rapidity with which the color 

 fades. 



Vitamin Ai gives rise to two distinct spectral bands in the reaction with 

 antimony trichloride. The principal one has a maximum absorption at 

 620 mf^, while the second, Avhich is partially masked, shows an inflection in 

 the curve at 583 m/x. Morton^'^' reports the value of the extinction E 

 (1%, 1 cm.) at 617 m/x as 6000 ± 200, while Baxter and Robeson^^ cite a 

 value of 4800 at 622 m/x. Similarly, vitamin A2 reacts with antimony tri- 

 chloride to give two bands ; the more effective one has a maximum extinc- 

 tion at 693 ran, while the weaker band has a maximum absorption at 645 m/x. 



The relative proportion of vitamins A2 and Ai can therefore be deter- 

 mined by the relationship between absorption at 693 and at 620 m/x after 

 the addition of the antimony trichloride reagent. However, the weaker 

 vitamin A2-antimony trichloride band at 645 m/x extends as far as the 

 620 m/x area. Morton" assumes that the absorption at 620 m/i in the ab- 

 sence of vitamin Ai is about one-seventh of that at 693 m/x. The following 

 formula is proposed to calculate the ratio of A2 to Ai from the spectro- 

 photometric data after the addition of antimony trichloride. 



2" P. B. Hawk, B. L. Oser, and W. H. Summerson, Practical Physiological Chemistry, 

 12th ed., Blakiston, Philadelphia, 1947, p. 1047. 



2" B. L. Oser, D. Melnick, and M. Pader, Ind. Eng. Chern., Anal. Ed., 15, 724-729 

 (1943). 



235 R. A. Morton, Ann.- Reo. Biochem., 11, 365-390 (1942). 



"6 N. D. Embree, Ann. Rev. Biochem., 16, 323-358 (1947). 



