48 THE PROPERTIES AND [CH. 



Qualitative Reactions. 



With respect to qualitative reactions, the necessity, already men- 

 tioned in connection with the properties of anthocyanin, for a guarantee 

 of the purity of the pigment used is of paramount importance. Crude 

 extracts invariably contain other substances which may modify, or 

 completely alter, the reactions of the pigment itself. In the next 

 chapter, accounts are given of special methods for purification and 

 analyses of authocyauin, and on studying these, the futility of applying 

 tests to any but pure material will be recognised at once. It "is only 

 after careful extraction, and purification by means of analyses, that 

 the reactions of any pigment can be determined with certainty, and 

 qualitative tests on impure extracts are to a large extent worthless. 

 Nevertheless, numbers of observations have been made on more or 

 less impure material, and the following account deals with the more 

 important results. 



With alkalies. When an aqueous or alcoholic extract of anthocyanin 

 is treated with alkali, the pigment turns green 1 and often finally 

 yellow. Sometimes a blue colour precedes the green ; with very dilute 

 alkali, or with solutions of salts having a weakly alkaline reaction, a 

 blue colour only may appear. Similarly red, purple and blue flowers 

 placed in ammonia vapour as a rule turn green. 



With acids. Anthocyanins almost invariably turn bright red with 

 acids, though the shade may vary in different cases. 



With lead acetate. Anthocyanin extracts are generally precipitated 

 by lead acetate, and the colour of the precipitates is usually some shade 

 of green or blue; occasionally it is red. 



The reactions of anthocyanins with acids and alkalies have formed 

 a subject for discussion from time to time. The whole matter is so 

 bound up with the views of those who have worked on the pigments 

 that something of the nature of a historical summary must be included. 



The question first to be considered is whether the green coloration 

 given when tissues and crude extracts containing anthocyanin are 

 treated with alkalies is a reaction of anthocyanin alone, or is the com- 

 bined result of reactions with anthocyanin plus reactions with other 

 substances present in the cell or solution, and this can only be determined 

 satisfactorily by testing pure pigments. In most cases, when white 

 flowers are treated with alkalies, or exposed to ammonia vapour, a bright 

 yellow colour is developed, indicating a reaction of alkalies with a class 



1 In some cases such solutions are slightly dichrolc, green and red. 



