294 TANNINS 



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 100° 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 

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

 solution when a red precipitate proves the presence of glucose. 



CATECHOL TANNINS. 



As stated above the catechol tannins are distinguished from 

 the pyrogallol tannins by their colour reaction with iron salts 

 and more especially by their property of giving rise to red 

 coloured substances or phlobaphenes on boiling with dilute 

 mineral acids. 



The catechol tannins comprise the various products known 

 commercially as cutch or catechu as well as the tannins 

 obtained from quebracho, mangrove, oak, birch, pine, larch, 

 and fir barks and also canaigre, but only a few of these can be 

 described. 



CUTCH OR CATECHU. 



The term cutch is applied to a number of somewhat 

 similar products obtained by evaporating down aqueous ex- 

 tracts of various woods such as acacia, mangrove, mahogany 

 etc. ; the commercial products differ slightly in their composi- 

 tion according to their source of origin. 



I. Gambler catechu obtained from the leaves and young 



