v] CONSTITUTION OF ANTHOCYANINS 65 



Further investigations were first made upon the water-soluble 

 portion. The potassium salt was prepared by acidifying the water 

 solution of the pigment with sulphuric acid, and bringing to the neutrali- 

 sation point with dilute potash. On standing, the potassium compound 

 of the pigment was obtained as a blue-green precipitate. It was found 

 to contain 13-28 % of potassium which, according to Grafe, agrees 

 fairly well with 14-08 % of potassium calculated on the hypothesis that 

 the pigment is a dibasic acid of the formula C 20 H 30 13 . 



The pigment, as in the case of other anthocyanins, was found to 

 be decolorised on the addition of either a solution of sulphur dioxide 

 in water, or a solution of sodium bisulphite. Grafe does not consider 

 this to be a reduction process, since the red colour does not return on 

 standing in air, but on addition of a stronger acid. He considers it 

 rather to be due to the formation of an additive compound with the 

 aldehyde groups present in the anthocyanin molecule. In the formation 

 of these compounds, certain double linkings are destroyed, thereby 

 depriving the substance of colour. 



The sodium bisulphite compound of the water-soluble pigment was 

 prepared by adding the salt to an alcoholic solution of the pigment. 

 The alcohol was then driven off, and the residue, after purification 

 from sodium bisulphite, was distilled in vacuo. At about 150-160 C. 

 there came over a colourless oily substance distilling with difficulty, 

 which showed a violet-red coloration with a trace of acid. The number 

 of aldehyde groups in the anthocyanin was determined by estimating 

 the sodium bisulphite as sulphate, after oxidation with bromine. The 

 results showed that one molecule of sodium bisulphite combines with 

 one molecule of anthocyanin indicating that one aldehyde group is 

 present in the molecule of the water-soluble pigment. 



In order to find out whether the sodium bisulphite causes any change 

 in the anthocyanin molecule, some of the bisulphite compound was 

 decomposed with acid, the solution neutralised, evaporated to dryness, 

 and taken up with acidified alcohol; on evaporation, a red granular 

 mass was deposited which gave the same combustion results as the 

 original substance. 



The acetyl derivative of the portion soluble in alcohol was made 

 by treating the pigment with acetic anhydride and anhydrous sodium 

 acetate, and pouring into water in which the derivative is insoluble. 

 The acetyl compound came down in red crystals from methyl alcohol. 

 The results of hydrolysing, and estimating the acetic acid, indicated the 

 presence of two hydroxyl groups in the pigment molecule. 



w. P. 5 



