60 



AROMATIC COMPOUNDS 



CHjO-HEXAOXYDIPHENIC ACID 

 P-&ALLOYL 



COOH 



The hydrolyzable tannins are usually amorphous hygroscopic, yellow-brown sub- 

 stances which dissolve in water (especially hot) to form colloidal rather than true solu- 

 tions. The purer they are, the less soluble they are in water and the more readily they 

 may be obtained in a crystalline form. They are also soluble at least to some extent in 

 polar organic solvents, but not in non-polar organic solvents like benzene or chloroform. 

 From aqueous solution the tannins may be precipitated by mineral acids or salts. 



Gallotannins and ellagitannins are reviewed by Mayer (27). 



HUMIC ACID 



Humic acid is a very poorly defined entity originally prepared by extracting basic 

 substances from humus with dilute acid and then extracting the residue with dilute am- 

 monium hydroxide. Acidification of the ammoniacal extract precipitates a crude mixture 

 known as humic acid. It is generally believed that the humic acid in soil is derived from 

 the lignin or carbohydrates of decaying plants; but it may also contain nitrogen, phosphorus 

 and sulfur (28). The following has been suggested as a possible structural unit (29): 



COOH 



The molecular weight has been estimated as 1200-1500. 



