§ 165. CäTECHÜIC acid, ETC. 157 



tannic acid good results can be obtained by using a 1 per cent. 

 solution of gelatine in saturated solution of chloride of ammonium 

 as indicated in § 52, xii. ; chloride of ammonium should be added 

 to the tannin solution also.^ Lehmann has shown that the liquid 

 may be diluted within certain limits without affecting the result 

 to any notable extent, and that it is advisable to promote the 

 subsidence of the precipitate by adding powdered glass and 

 vigorously stirring. He determines the end of the experiment by 

 removing a drop with a filtering tube and testing it with solution 

 of gelatine on a watch-glass with dark background. The tannin 

 solution should be mixed with an equal volume of saturated chlo- 

 ride of ammonium solution. Each cc. of the reagent indicates 

 0'0139 gram catechu-taunic acid. No other constituents of 

 catechu are precipitated by gelatine. 



Catechuic acid (§ 151), which is easily converted into catechu- 

 tannic acid, should not be neglected in determining the value of 

 a catechu. Lehmann endeavoured to estimate it from the differ- 

 ence in the amount of permanganate of potassium required (cf. 

 § 52, vii.) before and after precipitation with gelatine (by 

 which catechu-tannic acid alone is removed). The results he 

 obtained were, however, somewhat too high, since an infusion of 

 catechu contains other substances besides catechuic and catechu- 

 tannic acids that act upon permanganate of potassium. A more 

 successful process consisted in removing the catechuic acid by 

 shaking with ether, as directed in § 151, and then titrating it 

 with permanganate of potassium, reckoning 4 "84 parts of catechuic 

 acid for every 16 parts of oxygen consumed. 



Ehatcmia-tannic acid, like the two preceding substances, yields 

 phloroglucin and proto catechuic acid when fused with potash. ^ 

 For this tannin also Günther recommends the estimation "svith 

 solution of gelatine, calculating 0-01302 to 0-01323 gram rhata- 

 nia-tannic acid for every cc. of gelatine solution. The lead pre- 

 cipitate, which is tolerably stable but not quite insoluble in water, 

 contains, according to Günther, 31 '26 per cent, of oxide of lead. 



■* Lehmann, 'Vergl. Unters, einiger Catechu und Gambier- Proben,' Diss. 

 Dorpat, 41, 1880; Pharm. Zeitschr. f. Russland, No. IS, 1881. 



^ Compare Raabe, loc. cit. Raabe contests the glucosidal character of 

 rhatania-tannic acid, and is of opinion that it simply loses water when converted 

 into rhatania-red. See also Chem. Centralblatt, xii. 467, 1867 ; Annal. d. 

 Chem. und Pharm, cxliii. 274, 1867, in which Grabowski, like Wittstein, still 

 maintains the production of glucose in the decomposition. 



