iv] REACTIONS OF ANTHOCYANINS 53 



glucosides (anthocyanins), which can be hydrolysed artificially with 

 the formation of sugar and non-glucosidal pigments (anthocyanidins). 

 From a dilute acid solution anthocyanidins are completely removed 

 by amyl alcohol, whereas anthocyanins are not taken up at all by this 

 solvent. This may be demonstrated experimentally by extracting 

 fresh material containing anthocyanin with dilute sulphuric acid. 

 After nitration and addition of amyl alcohol, no pigment is taken up 

 by the alcohol. But if the solutions are heated for one half to three 

 quarters of an hour on a water-bath, the anthocyanins are hydrolysed, 

 and on addition of amyl alcohol, the anthocyanidins are quantitatively 

 removed. 



2. Anthocyanin is itself an acid and in the free state is purple. 



3. There is a blue modification which is the potassium salt of the 

 purple. 



4. There is a red modification, which is the oxonium salt of antho- 

 cyanin; the pigment may be combined with either organic or inorganic 

 acids. 



5. In Centaurea, as well as in some other plants, all three forms 

 of anthocyanin readily change to a colourless isomer; with the red 

 form this only occurs in absence of excess of acid. The change to a 

 colourless isomer can be prevented by adding neutral salts to the 

 anthocyanin solution ; the anthocyanin forms additive compounds 

 with these substances thereby preventing the isomeric change. 



As regards the colour reactions with alkalies, Willstatter, as we 

 have already seen, gives the following explanation. With alkalies 

 a blue salt is formed, which may become green owing to mixture with 

 flavones or the colourless isomer (see pp. 50, 72), if these are present 

 in the extract. The blue salt is also unstable, and with excess of 

 alkali passes to a greenish or even yellow decomposition product. But 

 if a neutral mineral salt is present, the blue salt is rendered stable. 

 This may be brought about by either (1) acidifying the anthocyanin 

 and then neutralising or (2) by adding the neutral salt (NaCl, NaN0 3 ). 

 Thus, for example, if pigment of Bilberries or Grapes (which presumably 

 contain little flavone) are treated with alkali, it gives a greenish colour. 

 But if treated first with some salt, it gives a blue colour, and the same 

 result may be brought about by acidification and subsequent neutralisa- 

 tion. 



It is now possible to explain the Erdmann reaction, which, according 

 to Willstatter, has hitherto been misunderstood. The following account 

 is given more or less in Willstatter's words. The Erdmann reaction 



