72 THE ISOLATION AND [CH. 



Willstatter (245), 1913. The flower-pigment of Centaurea Cyanus. 



In Centaurea flowers, according to Willstatter, there are three 

 modifications of one anthocyanin pigment: a purple pigment (cyanin), 

 which is itself a free acid ; a blue pigment, which is the potassium salt 

 of the purple, and which constitutes the greater part of the colouring 

 matter of the flower ; a red pigment, which is the oxonium salt of the 

 purple with some organic acid (other oxonium salts can be obtained 

 artificially with inorganic acids, such as hydrochloric acid). Cyanin, 

 moreover, isomerises to a colourless form which is an acid too, and 

 forms colourless alkali salts 1 ; there is also a colourless isomer of the 

 blue pigment. When the anthocyanin of the flower is extracted with 

 water, the deep blue solution rapidly loses its colour; this is due to 

 the above isomerisation, and the colourless solution, on addition of 

 acid, will become as red as a solution of the original blue pigment would 

 on acidification. The red modification also loses colour in absence of 

 acid, i.e. if sufficiently diluted with water or alcohol. On concentration, 

 a colourless solution will return to its original colour, blue, red or violet, 

 as the case may be. 



For preparation of the blue pigment on a large scale, dried flowers 

 ground to a fine powder were employed. The blue pigment can be 

 extracted rapidly with water or very dilute alcohol, but change to the 

 colourless isomer tends to take place. This change can be prevented, 

 however, by addition of much sodium nitrate or chloride to the pigment 

 solution. The cyanin salt can then be precipitated from the water 

 solution with alcohol in which it is insoluble. It is further purified 

 by fractional precipitation with alcohol from water solution. 



Cyanin is a glucoside, but on hydrolysis it gives the free pigment, 

 cyanidin, and sugar. The pigment, cyanidin, like the glucoside, cyanin, 

 forms crystalline oxonium salts with hydrochloric acid. 



The method of isolation of the blue pigment, in greater detail, is as 

 follows. The powder of ground flowers is mixed with sand, extracted 

 with water or 20 % alcohol, and filtered, and finely powdered, sodium 

 nitrate is added. The deep blue solution is then mixed with 96 % 

 alcohol (2-5 vols. alcohol : 1 vol. extract), and the pigment is preci- 

 pitated out in blue flakes, which are separated by a centrifuge. The 

 pigment is further purified by taking up in water, and precipitating 



1 " Das Cyanin isomcrisiert sich zu einer farblosen Modifikation, wclche glcichfalls 

 sauer 1st und farblose Alkalisalze bildet." The alkali salts of the isomer are, however, 

 definitely stated to be yellow later in the paper. 



