274 TANNINS AND [CH. XII 



H 2 S : after removing H 2 S by warming, the solution is 

 neutralized exactly and tested for glucose by ordinary 

 tests (see p. 282). If the presence of much glucose is 

 indicated, the original tannin was certainly a glucoside, 

 but if only a small quantity of glucose is present, another 

 experiment should be tried with a more carefully purified 

 residue or lead precipitate. 



Glucosides. 



After the removal of tannins some glucosides may still 

 remain in solution. No general method can be given for 

 detecting these substances, but if their presence is sus- 

 pected, special tests may be tried for those glucosides 

 likely to be present in any particular case. 



A glucoside can generally be separated from aqueous 

 solution by shaking with some appropriate solvent im- 

 miscible with water, such as amyl alcohol, benzene, 

 chloroform, etc. (not ether), and it can then be identified 

 by evaporating off the solvent and applying tests to the 

 residue. 



E.g. To detect salicin. 



The aqueous solution is shaken with an equal volume 

 of amyl alcohol, which extracts this glucoside completely 

 from solution in water. 



The amyl alcohol is then separated from the water 

 and a part of the solution is cautiously heated till all 

 the solvent (amyl alcohol) is evaporated off and a dry 

 residue obtained. Salicin may be identified in this resi- 

 due by its reaction (intense red colour) with strong 

 sulphuric acid, and also by dissolving another part of 



