Cxxviii. Spectrophotometric Investi- Gations of the New Vitamin a Colour Test. by Eugene Rosenthal And

COLORIMETRIC determination of vitamin A has frequently been based upon the Carr-Price reaction, namely the production of a blue colour when vitamin A is added to a saturated solution of antimony trichloride in chloroform, which shows two absorption maxima at 606 m,u and at 572 m,u, respectively. This reaction has the following drawbacks: (1) the blue colour is not specific for vitamin A, being given also by carotene, (2) the colour shows a considerable change after the lapse of 30 seconds, rendering colorimetric estimation very difficult. In order to eliminate this difficulty Rosenthal and Erdelyi [1934, 1] devised a new colour test. In this the vitamin-containing material is warmed with a solution of antimony trichloride and catechol in chloroform. The resulting blue colour soon turns into a purplish red. This colour, quite similar to that of a dilute solution of potassium permanganate, is much more stable than the blue colour obtained with the Carr-Price reaction. No other carotenoids except vitamin A produce this colour; the blue colour obtained with other carotenoids does not change on warming. Rosenthal and Erdelyi emphasise the fact that the purplish red hue produced by catechol remains unchanged for some 5 to 10 minutes if fairly pure preparations of vitamin A are used. This fact made possible a more thorough spectrophotometric examination of this purplish red colour. In our investigations we employed the Konig-Martens spectrophotometer at a layer depth of 0*2 cm., a Nitra bulb being used as a light source. The extinction coefficient has been calculated according to the following equation: