PHLOBAPHENES 297 



the other hand, under similar circumstances give rise to ellagic 

 acid. 



OAK-BARK TANNIN OR QUERCITANNIC ACID. 



Besides the undoubted pyrogallol tannin of oak galls 

 which is practically never used for tanning, the oak is the 

 source of two other tannins which have in the past been used 

 very extensively for tanning — these are oak-wood and oak- 

 bark tannins. The former of these, which is sometimes 

 described as quercic or quercinic acid, is probably also a pyro- 

 gallol tannin, since it gives a blue colour with ferric chloride 

 and is not precipitated by bromine water ; the tannin of 

 oak bark, known as quercitannic acid, is, however, a catechol 

 tannin, and is quite distinct from the tannin of oak galls; 

 it differs from the latter in giving with iron salts a green 

 colour instead of a bluish-black, and moreover on hydrolysis 

 it yields no glucose. 



Although much work has been done on the oak-bark 

 tannins by various workers,* notably Etti, Bottinger, and 

 Lowe, nothing definite is known as yet regarding their 

 constitution. 



Procter summarizes the present state of our knowledge by 

 saying that, on the whole, it seems probable that the principal 

 tannin of oak bark is a purely catechol tannin, and that the 

 galhc and ellagic acids which have been detected in it are due 

 to an admixture of the gallotannic and ellagitannic acids 

 present in oak wood. 



A great many more catechol tannins are known, but too 

 little is known about their composition to justify their in- 

 clusion here. 



PHLOBAPHENES. 



Among the products of the decomposition of catechol 

 tannins by boiling with acids must be mentioned the substances 

 known as Phlobaphenes. The name derived from the Greek 

 {(pXoio'i — bark, and /3a(f>'t] — dyeing) was first given by 



* Etti : " Monatshefte," 1880, l, 265. Bottinger : " Ber. deut. chem. 

 Gesells.," 1883, 16, 2712. Lowe : " Monatshefte," 1883, 4, 515. Freuden- 

 berg and Vollbrecht : " Annalen." 1922, 429, 284. 



