42 SUBSTANCES SOLUBLE IN ALCOHOL 



mined from the organic matter in the dried precipitate. In this 

 way the tannin in rhatany, tormentilla, sumach, divi-divi, myro- 

 balans, knopper-galls, oak-bark, and willow-bark may generally 

 be satisfactorily estimated. Gallo-tannic acid at times also yields 

 good results, 



n. Acetate of Copper has been suggested by Sackur^ as a 

 precipitant for tannin. The composition of the precipitate is, 

 however, seldom constant even when working with the same 

 tannic acid ; and here,, too, it has proved advisable to precipitate 

 in tolerably concentrated solirtions, not to wash too long, and to 

 estimate the tannin gravimetrically, as above described. 



III. Stanrvous Chloride and Ammormcal Stannous Chloride, which 

 have been recommended by Eisler-Beunat^ and Persoz, * for the 

 estimation of tannin, precipitate most tannic acids less completely 

 than the two foregoing reagents. The precipitates moreover form 

 slowly, but are in the majority of cases tolerably constant in 

 composition. On account of the solubility of the precipitate in 

 water, the estimation will here, too, be most accurate when the 

 washing is not continued too long, the precipitate dried, impreg- 

 nated with nitrate of ammonia, ignited, and the resulting oxide 

 of tin weighed. The loss by ignition gives the weight of the 

 tannin. But since the advantage in obtaining precipitates of 

 constant composition cannot coiiipensate for the deficfencies of the 

 method already mentioned, I have not further thought of employ- 

 ing the precipitation with stannous chloride for the purposes we 

 have now in view. 



IV. Tartar Emetic, which ha.s been recommended by Gerland* 

 and KoUer* for the volumetric estimation of tannin, will yield 



^ GerbeTzeitnng, xxxi. 32.' See also V^lff, Erit. Blatter f. Porst und 

 Jagdwissensch. xliv. 167 ; Fleck, Wagner's Jahresber. f. techn. Chem. Jg. 

 1860, p. 531 ; Hallwachs, Zeitsohr. f. anal. Chem. v. 234 (1866). ' 



" Zeitschr. f, anal. Chem. ii. 287 (1863). 



* Traits de I'lmpression des Tissus, i. 282. The results obtained by the 

 method recommended by Persoz, in which the amount of tannin is calculated 

 from the volume of the precipitate, are, according to Gauhe (Zeitschr. f. anal. 

 Chem. iii. 130, 1864) and Cech (Stud, iiber quant. Best, der Grerbsauren, In- 

 augural Dissertation, Heidelberg, 1867), too high. I avail myself of this 

 opportunity to draw attention to the works of the two Isst-named authors, 

 which are intended as a critical review of the more important methods of 

 estimating tannic acid. (See Procter, Pharm. Joum. Trans. [8], vii. 1020 ; 

 Allen, Commercial Organic Analysis, London, 1879.) 



* N. Jahrb. f. Pharm. xxvi. 20 (1866) ; (Amer. Joum. Pharm. xxxv. 519). 



" Koller employed this method in estimating the tannic acid in orange-peel 

 (N. Jahrb. f. Pharm. xxv. 206, 1866). 



