162 TANNINS. 



The l)est results would probably be obtained by extracting 

 directly with alcohol, evaporating the tincture in a partial 

 vacuum, treating the residue with water, quickly filtering and 

 estimating, at once with gelatine or permanganate of potassium. 

 (Cf. §§ 61, 52, VIL and XII.) In standardizing the solutions, 

 it may be useful to remember that, according to Giinther's 

 experiments, quercitannic acid, though differing greatly in other 

 respects from gallotannio acid, possesses the same quantitative 

 action on permanganate of potassium. It must be observed 

 that tannic acid is deposited when its solution is completely 

 saturated with chloride of ammonium ; it is advisable, therefore, 

 when precipitating with gelatine, to follow the directions given 

 for titrating catechu-tannic add. Quercitannic acid is sparingly 

 soluble in ether; ferroso-ferric salts produce inky nuxtures with 

 its aqueous solution; other of its properties are mentioned in 

 ^ 49, 51 The lead salt obtaijied by precipitation with a 

 flight excess of the acetate contains 56 to 57 per cent, of oxide, 

 the copper salt 29-5 per cent. The oak-red produced artificially 

 from the tannic acid is identical with the phlobaphene that occurs 

 naturally in the bark. It is likewise coloured black by iron salts, 

 yields protocatechuic acid and phlordglucin when fused with 

 potash, and possesses the properties of a phlobaphene as 

 enumerated in ^ 108, 160. 



The tannins of the pine^ Urch, many species of auum, etc., 

 which have been but little investigated, may possibly resemble 

 quercitannic acid in many of their essential chara'cters; 



FUix tannic aeid^ is resolved, on boiling with acids, into glucose 

 and red flocks of filix-red ; the latter closely resembles cinchona- 

 red. 



CincTumortannia acSi * undergoes a similar decomposition with 

 production of cinchona-red. Its lead salt is somewhat easily 

 soluble in acetic acid. 



' Compare Kawalier, Wiener Akad. Ber. xi, 354 et aeq, ; Buchleder luid 

 ICawalier, ibid. xxlx. 22 ei seq. ; Wittstein Vierteljahresschr. i. pract. Fbanu. 

 iii. 14, 1854. 



^ See Malin, Chem. Centralblatt, xii. 468, 1867. For tannaspidic acid and 

 ptentannic acid, the former o{ whicli Malin'believes to be impure filix-red, see 

 Xiuck, ibid, 657, 676, 1851. Compare further Grabowski, Annal. d. Chem. 

 und Pbarm. oxUi. 279, 1867. 



* Compare Rembold, Annal. d. Chem. und Pharm. cxiiii, 270, 1867, and 

 Schwarz, Chem. Centralblatt, 193,. 1852. 



