The Yellow Pigments. 17 



C. Thb Yellow Pigments. 



Although the yellow pigments may have physiological import- 

 ance in carbon-assimilation there is not much to be said in regard 

 to their chemistry. They both give non-fluorescent yellow solutions 

 stable in alkaline but very easily dissociated in acid media. 



Carotin is identical with the yellow pigment of carrots. It is 

 an unsaturated hydrocarbon of the formula C^gHj^ crystallising 

 in rhombohedra with a lustrous blue surface, but appearing red in 

 transmitted light. It is easily soluble in chloroform, carbon- 

 disulphide and benzene, soluble with difficulty in petrol ether and 

 ether, and in even boiling methyl and ethyl alcohol ; in the cold it 

 is almost insoluble. 



Characteristic of it is its distribution between petrol ether and 

 methyl alcohol. If to a solution in petrol ether is added methyl 

 alcohol containing a little water, the alcohol layer remains colour- 

 less. 



It undergoes auto-oxidation. If it stands in air it becomes 

 bleached and increases in weight by 35% in dry air and by 41% in 

 moist air. With the halogens it forms addition compounds. 



It gives a red solution in carbon-disulphide and a deep blue 

 solution in concentrated sulphuric acid. 



Xanthophyll has the formula C^fiH^^O^. The crystals are 

 pleochromatic often with a steel blue lustre. In transmitted 

 light they are yellow and only red where two or more cross one 

 another and in this way are easily distinguishable from those of 

 carotin although the colour of the two pigments in solution is very 

 similar. The behaviour of xanthophyll with sulphuric acid and 

 halogens is the same as that of carotin. It is insoluble in petrol 

 ether, the solvent is not even coloured ; in methyl alcohol it is 

 soluble with difficulty but more easily than carotin. It is also 

 soluble with difficulty in carbon-disulphide. In ether it is more 

 soluble, and is easily soluble in chloroform. Like carotin it under- 

 goes auto-oxidation and a solution of xanthophyll bleaches in 

 presence of air very quickly, much quicker than carotin. 



If a xanthophyll solution in methyl alcohol is mixed with 

 petrol ether and a little water added, the greatest part of the 

 pigment remains in the methyl alcohol layer. 



Although carotin and xanthophyll give very similar solutions 

 it is difficult to compare the colour intensities of the two because 

 the colour varies with the solvent and the concentration. The 

 carotin is always stronger and in dilute solutions they are not 

 comparable because the shade varies. 



