208 TANNINS 



PHLOBAPHENES. 



Among the products of the decomposition of tannins by 

 boiling with acids must be mentioned the substances known 

 as Phlobaphenes. The name derived from the Greek (0Xoto? 

 bark, and jBafyr) dyeing) was first given by Stahelen and Hoff- 

 stetter,* in 1 844, to a red-brown substance obtained by them 

 by adding water to an alcoholic extract of bark which had 

 previously been extracted with ether to remove fats or waxes. 

 It has since been shown that aqueous extracts of bark contain- 

 ing tannin, deposit from solution a substance known as oak- 

 red or phlobaphene, and that this substance is more rapidly 

 produced by warming concentrated solutions of tannin with 

 sulphuric acid. 



Inasmuch as phlobaphenes are produced by any process 

 which tends to remove water, such as heating tannins to a high 

 temperature or prolonged boiling or heating under pressure, 

 they are regarded as anhydrides of the tannins ; besides being 

 thus produced artificially, they occur also in nature side by 

 side with the tannins from which they can be produced. 



They are red-coloured substances and are practically insolu- 

 ble in water though they dissolve in solutions containing tannic 

 acid ; also they dissolve in alcohol and in alkaline solutions. 



The formation of phlobaphenes by treatment of a tannin 

 with acid is characteristic of pyrocatechol tannins (p. 211) in 

 just the same way as ellagic acid is produced from pyrogallol 

 tannins. 



A number of different phlobaphenes are known, such as 

 kino-red, catechu-red, oak-bark red, etc. 



TANNINS AS GLUCOSIDES. 



At one time it was thought that the tannins were sub- 

 stances of a glucosidic nature and occurred in the plant in 

 combination with a carbohydrate complex such as glucose; 

 but while this is undoubtedly so in some cases it is by no 

 means universal. 



To determine whether a tannin is a glucoside or not the 

 following procedure is recommended by Procter. f 



The Tannin must first be carefully purified from glucose, 



* Stahelen and Hoffstetter : " Annalen," 1844, 51, 63. 



f- Procter: " Leather Industries Laboratory Book, London," and ed., 1908. 



