iv] REACTIONS OF ANTHOCYANINS 55 



is difficult to estimate unless the pure pigment is tested. The flavones 

 give green or brownish-green colorations with iron salts, the tannins 

 green or blue. Though anthocyanins on the whole appear to give 

 green-blue, green, or sometimes brown reactions, if tannin or flavone 

 be present it is obvious that no reliance can be placed on the results. 

 In a few cases, however, where anthocyanin pigment has been obtained 

 approximately pure, one of the above colour reactions was given. 

 Such a result indicates that anthocyanin has probably hydroxyl groups 

 of the phenol type. 



Reactions with sodium bisulphite. An interesting reaction of antho- 

 cyanins is that given with sulphur dioxide and bisulphites It was 

 well known at a very early date that flowers containing anthocyanin, 

 or extracts of the pigment, are bleached by sulphur dioxide gas, and 

 that the colour is again restored by stronger acids. Boyle (107) writes : 

 'That Roses held over the Fume of Sulphur, may quickly by it be 

 depriv'd of their Colour, and have as much of their Leaves, as the Fume 

 works upon, burn'd pale, is an Experiment, that divers others have 

 tried, as well as I. But (Pyrophilus) it may seem somewhat strange 

 ...That, whereas the Fume of Sulphur will,... Whiten the Leaves of 

 Roses; That Liquor, which is commonly call'd Oyl of Sulphur... does 

 powerfully heighten the Tincture of Red Roses." Further observations 

 on this bleaching action were made by Kuhlniann (118), Hiinefeld (120) 

 and Schonbein (123). Solutions of anthocyanin decolorised by sulphur 

 dioxide have been employed by Kastle to test the relative 'strengths' 

 of acids. Kastle (196) is of the opinion that the decolorisation is not 

 caused by reduction, and the same view is held by Grafe (197, 222), 

 Avho prepared bisulphite derivatives from the anthocyanins of both 

 Althaea and Pelargonium by addition of sodium bisulphite. Both 

 products were colourless, but the red colour returned on addition of 

 a trace of a stronger acid. Grafe concludes that the anthocyanins 

 contain aldehyde colour-producing groups, which form additive com- 

 pounds with bisulphites, whereby the linkiugs in the molecule and 

 the resultant colour are changed. 



Action of nascent hydrogen. A reaction which would appear to be 

 one of reduction is that produced by treating acid solutions of antho- 

 cyanin with zinc dust. The colour rapidly disappears and the solution 

 remains colourless if air be excluded. On exposure to air, if the reducing 

 action is not very violent, the colour returns, the surface of the liquid 

 becoming coloured before the deeper layers. Kastle (196) does not 

 consider the reaction to be of the nature of reduction, since the colour 



