162. PURIFICATION. 155 



when warmed with a dilute acid, gallic and ellagic acids together 

 with a phlobaphene. 1 



$ 162. Purification. The ease with which tannins decompose 

 renders their preparation in the state of purity desirable for 

 accurate investigation a matter of considerable difficulty, and we 

 may confidently assert that this has not been attained with the 

 majority of the substances belonging to this class that have 

 hitherto been described. 



In preparing pure tannins the following hints may be useful, 

 in addition to those given in 49 to 51 and 60 : 



1. If a tannic acid is to be separated from the alcoholic extract 

 it is very advisable, after evaporating, to mix at once with a con- 

 vsiderable quantity of water. Alcohol dissolves phlobaphenes and 

 resinous substances, together with tannic acid, and strong aqueous 

 solutions of the latter are known to be capable of taking up the 

 former, even if otherwise insoluble in water ( 160). 



2. In precipitating the aqueous filtrate with acetate of lead, it 

 is advisable to add the reagent in successive portions, rejecting 

 the first (generally more deeply coloured) and last precipitates, as 

 they are usually contaminated with foreign substances to a con- 

 siderable extent. 



3. The lead precipitates should be washed and treated with 

 sulphuretted hydrogen as rapidly as possible, to avoid decomposi- 

 tion of the tannate of lead. 



4. The filtrate from the sulphide of lead should be evaporated, 

 if possible, in a partial vacuum, and only to the consistence of a 

 thin syrup. The remainder of the water may be evaporated over 

 sulphuric acid and lime at the ordinary temperature, the opera- 

 tion being completed in vacuo. 



It will frequently be found advantageous to shake the filtrate, 

 before evaporating, with ether or acetic ether, which would 

 remove any gallic acid that might be present. 



Many tannins may be purified by dissolving in water, adding 

 chloride of sodium, and removing the tannic acid by shaking 

 with acetic ether or a similar solvent. This method has been 

 successfully used by Loewe for sumach-tannic acid and some 

 others, 2 and by Eaabe for rhatania-tannic acid. 3 It should be 



1 Beitriige zur Chemie der Nymphaceen, Diss. Dorpat, 1881. 

 : Zeitschr. f. anal. Chem. xii. 128 ; xiv. 35, 44 (Journ. Chain. Soc. xxvii. 171 ; 

 xxviii. 75). 

 3 Pharin. Zeitschr. f. llussland, 577, 1880. 



