§ 165. CATECHVia ACID, ETQ. 157 



tannic acid good results can bo obtajiied by using a 1 per cent, 

 solution of gelatine in Jsaturalod solution <jf ciilori'de of ammonium 

 as indicated ih § 52, xii. ; chloride of animoniuni should be added 

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

 may be diluted within, eertain limits without affecting the result 

 to any notable extent, and thaJt it is advisable to promote the . 

 subsidence o;f 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 oti a watch-glass with dark background. The tannin 

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

 ride of animonium solution. Each cc of the reagent indicates 

 0'0139 gram catechu-tannic acid. No other constituetits of 

 catechu are precipitated by gelatine. 



OaUchuic acid (§ 161), which is easily converted into catechu- 

 tannic acid, should not be neglected in d*; ermining the value of 

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

 enee in the amount of permanganate of potassium required (of. 

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

 which catechu-tannic acid. Mone is remevod). The results he 

 obtained were, however, somewhat too highj 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 penasjiganate of potassium, reckojiing i'Si parts of catechiiic 

 acid for aymTy 16. parts of oxygen consumed. 



Rhatania^tmnie acid, like -the two preceding substances, yields 

 phlproglucin iind protocatechuic acid when fused with potash.* 

 Fof this tannin also Giinther recommends the estimation with 

 solution of gelatiue, calculating 0-01302 to 0'01323 gram rhatar 

 nia-taimic acid for every cc. of gelatine solution. The load pre- 

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

 contains, according to Gttnther, 31-26 per Ciqnt. of oxide of lead. 



^ Lehmann, ' Vergl. Untars. einiger Catechu uftd Gainbicr-Proben,' Diss. 

 Dorpat, 41, 1380 ; Phann. Zeitaohr. f. Busslaud, No. 18, 1881. 



^ Compare Eaabe, lof;. eU. Kaabe contests the gliioosldai chariicter of 

 rhatania-tannip acid, and is of <)pinion that it simply loses^ water when converted 

 into rhatania-i-ed. Sei also Cheni. Centralblatt, xii. 467, 1867 ; Annal. d. 

 Chem. und Phsirln. crfiji..274, 1867, in which Grabowski, like Wittstein, still 

 maintains the production of glucose in the Jeoomposition. 



