54 THE PROPEKTIES AND [CH. 



is based on the fact that new, or fairly new, wine gives a green colour 

 on neutralisation with ammonia, but after treatment with hydrochloric 

 acid and neutralisation, a dark greenish-blue colour. This behaviour 

 was incorrectly explained by Erdmann as being due to the splitting 

 of the wine pigment into two pigments by the hydrochloric acid. 

 Erdmann separated the above two pigments by shaking with amyl 

 alcohol ; the violet-red amyl alcohol layer gives with dilute ammonia 

 first a bright green, then a brownish-green colour, while the acidified 

 water solution becomes indigo-blue on neutralisation. As a matter 

 of fact no breaking up is brought about by the acid, but during fermen- 

 tation of the grape juice, a portion of the anthocyanin has been hydro- 

 lysed to anthocyanidin. The effect of ammonia on the wine pigment 

 is to give a blue coloration rapidly passing to a green decomposition 

 product. But, as explained by Willstatter, if the anthocyanin is first 

 acidified and then made alkaline, the blue colour is more stable. On 

 shaking up the acidified wine with amyl alcohol, the latter takes up 

 the small portion of anthocyanidin as an oxonium salt. If the wine 

 is heated with hydrochloric acid, or if the hydrochloric layer of the 

 Erdmann reaction is heated, the whole is hydrolysed, and the antho- 

 cyanidin can be extracted quantitatively with amyl alcohol. But the 

 hydrolysis does not happen in the cold. Willstatter further points out 

 a source of error in the reaction. Grape juice from fresh berries gives, 

 after acidification with hydrochloric acid, a little pigment in the amyl 

 alcohol. If the amyl alcohol is then washed with dilute sulphuric 

 acid, it is nearly decolorised. The small amount of pigment in the 

 amyl alcohol is not hydrolysed, but is due to the fact that hydrochloric- 

 amyl alcohol takes up a little of the anthocyanin oxonium salt, whereas 

 sulphuric-amyl alcohol takes practically none. Hence it is advisable 

 to use sulphuric acid for the test. The statement by Weigert, that the 

 'Weinroth' group gives the Erdmann reaction, is regarded by Will- 

 statter as erroneous, for the latter maintains that, apart from the small 

 amount of glucoside salt which can be washed out again from the 

 amyl alcohol, the pigment of flowers, berries and leaves remains com- 

 pletely in the water-acid layer. 



The sensitiveness of anthocyanin to acids and alkalies has suggested 

 its use as an indicator. In fact these were the first reactions to attract 

 the attention of chemists (see p. 8). Its use in this way has been 

 revived from time to time by Pellagri (146). Sacher (215) and others 

 but without any permanent success. 



Reactions with iron salts. Here again the actual colour reaction 



