vm] AKOMATIC COMPOUNDS 115 



(as may be the case in a crude extract) they give a green colour, due to 

 mixture of bhie and yellow. In solution in neutral alcohol and water 

 many anthocyan pigments lose colour, and this is said to be due to the 

 conversion of the pigment into a colourless isomer which also gives a 

 yellow colour with alkalies (Willstatter and Everest, 30); hence even a 

 solution of a pure anthocyan pigment may give a green coloration with 

 alkali due to mixture of blue and yellow. The isomerization can be 

 prevented or ^lessened by addition of acids, or of neutral salts which form 

 protective addition compounds with the pigment. With lead acetate 

 anthocyan pigments give insoluble lead salts, blue if the pigment is 

 pure, or green, as in the case of alkalies, if it is mixed with flavone or 

 flavonol pigments, or the colourless isomer. 



When anthocyan pigments are treated with nascent hydrogen, the 

 colour disappears but returns again on exposure to air. It is not known 

 what reaction takes place. 



Expt. 111. The reactions of anthocyanins and anthocyanidins. Extract petals of 

 the plants mentioned below with boiling alcohol in a flask. Note that the anthocyan 

 colour may disappear in the alcoholic extract. Filter off some of the alcoholic extract 

 and make the following tests {a) and (6) with it : 



{a) Add a little acid and note the bright red colour. 



(6) Add a little alkali and note the green colour. 



The decolorized petals, after filtering off the extract, should be warmed with a 

 little water in an evaporating dish. The colour is brought back if pigment is still 

 retained by them. 



Evaporate the remainder of the alcoholic extract to dryness and note that the 

 anthocyan colour returns. Dissolve the residue in water and continue the following 

 tests, taking a little of the solution in each case : 



(c) Add ether and shake. The anthocyan pigment is not soluble in ether. 



(o?) Add acid. A bright red colour is produced. 



(e) Add alkali. A bluish-green or green colour is produced which may pass to 

 yellow. 



(/) Add basic or normal lead acetate solution. A bluish-green or green precipitate 

 is produced. 



{g) Add a little sulphuric acid and then amyl alcohol and shake ; the latter does 

 not take up any of the red colour, indicating that the pigment is in the anthocyanin 

 (glucosidal) state. 



(A) Heat a little of the solution on a water-bath with dilute sulphuric acid and 

 then cool and add amyl alcohol. The colour will pass into the amyl alcohol, indi- 

 cating that the pigment is now in the anthocyanidin (non-glucosidal) state. 



{i) Acidify a little of the solution with hydrochloric acid and add small quantities 

 of zinc dust. The colour disappears. Filter off the solution and note that the colour 

 rapidly returns again. 



For the above reactions it is suggested that the following flowers be used as 



8—2 



