4 COLOUR REACTIONS 7 



however, that for the complete identification of a carotenoid it is necessary 

 to carry out a chromatographic purification and to isolate the pigment in the 

 crystalline state. 



4. COLOUR REACTIONS 



The polyene pigments are well known to give blue or violet solutions with 

 a variety of strong acids such as concentrated sulphuric acid, hydrochloric 

 acid, perchloric acid, trichloracetic acid, and with acid chlorides, such as 

 antimony trichloride or arsenic trichloride. These colourations, although not 

 specific, can be used as qualitative tests^*. 



(a). Reaction with concentrated sulphuric acid: This reaction is carried out 

 by carefully forming a layer of concentrated sulphuric acid under an ethereal 

 solution of the pigment. The sulphuric acid layer acquires an intense dark 

 blue to blue-violet or, occasionally, greenish-blue colour which disappears on 

 the addition of water^^. 



(b). Other strong acids: Fuming nitric acid produces a transient blue 

 colouration. A number of observations have also been published recently 

 regarding the blue colouration produced by concentrated aqueous hydro- 

 chloric acid. It appears that the following carotenoids colour concentrated 

 aqueous hydrochloric acid blue: 



(i). Aldehydes, e.g. j3-citraurin, ^-apo-2-carotenal. 



(ii). Some carotenoids containing several hydroxyl groups, e.g. fucoxanthin, 

 azafrin. , 



(iii). Carotenoid epoxides and their furanoid transformation products, e.g. 

 violaxanthin, auroxanthin, xanthophyll epoxide, flavoxanthin, j8-carotene di- 

 epoxide, aurochrome. 



In practice, the hydrochloric acid reaction is carried out in the following way : 

 The pigment is dissolved in a little ether and concentrated aqueous hydro- 

 chloric acid is added to the solution. After shaking, the acid layer is coloured 

 blue. With a number of hydrocarbon epoxides, such as a-carotene mono- 

 epoxide, the blue colouration is very weak and only persists for a short time. 

 For further details, reference should be made to the description of this reaction 

 in the sections on individual carotenoids. 



(c). Antimony trichloride in chloroform solution (Carr-Price reagent) : 



Similarly to vitamin A, carotenoids give dark blue colourations with the 

 Carr-Price reagent^^. These blue colourations often have characteristic 

 absorption maxima, and can be used for the quantitative estimation of 

 carotenoids^'. 



References p. 8-g. 



