PROPERTIES AND DESCRIPTION OF TANNINS 209 



The Tannin must first be carefully purified from glucose, 

 gums, or other bodies likely to interfere. This may be done 

 by extracting according to Pelouze's method (p. 210), or, if 

 the tannin is to be extracted from an aqueous solution, by 

 agitating with ether to remove gallic acid and then saturating 

 the aqueous solution with common salt and shaking with ethyl 

 acetate, which extracts the tannin. The ethyl acetate is then 

 evaporated off, the last traces being expelled by the repeated 

 addition of small quantities of ether. 



Another method is to extract with alcohol and to evapo- 

 rate off the alcohol at as low a temperature as possible, and 

 then to take up the residue with a large volume of water 

 whereby the phlobaphenes (see p. 215) are precipitated and 

 may be filtered off. The infusion is then precipitated with 

 successive small quantities of lead acetate ; the first and last 

 portions are rejected and the middle fraction after washing is 

 suspended in water and saturated with sulphuretted hydrogen. 

 The precipitated lead sulphide is filtered off, and the solution 

 is warmed to drive off excess of gas and then extracted with 

 ethyl acetate. 



Thus purified the tannin, or its washed lead salt, is heated 

 to 1 00 for an hour or more in a sealed tube or boiled in a 

 flask under a reflux condenser with hydrochloric acid (2 per 

 cent). After cooling the mixture is allowed to stand for some 

 time and is then filtered from any deposit which may have 

 formed. The filtrate is shaken with ether to remove gallic 

 acid and the aqueous solution boiled, neutralized with caustic 

 soda and precipitated with basic lead acetate to remove any 

 -unchanged tannin or colouring matter ; the solution is again 

 | filtered and any lead remaining in solution is removed by the 

 ! addition of dilute sulphuric acid, excess of acid being carefully 

 | avoided The solution is then neutralized and once more fil- 

 tered and the clear filtrate heated to boiling with Fehling's 

 solution when a red precipitate proves the presence of glucose. 



PROPERTIES AND DESCRIPTION OF INDIVIDUAL TANNINS. 



As already stated the term Tannin is merely a generic 

 I name for the whole group of substances, though it has been, 

 i and still is, frequently used to mean a particular tannin, namely 



14 



