Chapter X 



Quantitative Estimation of Carotinoids 



The small amount of carotinoids in plant and animal tissues, to- 

 gether with the difficulty of securing the pigments free from color- 

 less impurities as well as the great ease with which the pigments oxi- 

 dize, forbid their quantitative estimation by a gravimetric method. 

 The great intensity of the carotinoid pigments and their ready solu- 

 bility in certain organic solvents naturally suggests the possibility of 

 their quantitative estimation by colorimetric methods. The methods 

 which have been proposed have, in fact, been devised on this basis. 



Estimation of Carotin and Xanthophyll 



Arnaud (1887) was the first to propose a colorimetric method for 

 the quantitative estimation of carotin in plant tissues. The method 

 was based on his observation that air dried, or especially vacuum 

 dried leaves (leaves dried in an oven even at low temperature cannot 

 be used, according to Arnaud) do not give up any of their chlorophyll 

 when allowed to remain in contact with low boiling petroleum ether, 

 but permit all the carotin to be extracted. A further essential fea- 

 ture of his method was based on the observation (a single experi- 

 ment only is reported) that the color of carbon disulfide solutions of 

 carotin is directly proportional to the amount of carotin present. 

 With these observations as a basis Arnaud proceeded as follows. 



Twenty gram quantities of air-dried or vacuum-dried, powdered 

 leaves were shaken up with 1 liter of cold petroleum ether in a stop- 

 pered flask for a period of 10 days. The extract was filtered off and 

 exactly 100 cc. evaporated to dryness. The residue was taken up in 

 exactly 100 cc. of carbon disulfide and compared in a Dubosque col- 

 orimeter with a standard 0.001 per cent solution of carotin in carbon 

 disulfide. It is stated that the colorimeter was modified slightly to 

 prevent the volatilization of the solvent and that blue glasses were 

 inserted to improve the sensitiveness of the instrument, but the de- 

 tails in regard to these modifications are not given. The data which 



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