42 SUBSTANCES SOLUBLE IN ALCOHOL. 



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

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

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

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

 good results. 



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

 precipitant for tannin. The composition of the precipitate is, 

 howeA'er, seldom constant even when working with the same 

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

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

 estimate the tannin gravimetrically, as above described. 



III. Stannous Chloride and Ammoniacal Stannous Chloride, which 

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

 estimation of tannin, jirecipitate 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 compensate for the deficiencies 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 has been recommended by Gerland* 

 and Koller^ for the volumetric estimation of tannin, will yield 



^ Gerberzeitung, xxxi. 32. See also Wolff, Krit. Blätter f. Forst und 

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

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



2 Zeitschr. f. anal. Chem. ii. 287 (1863). 



^ Traite de l'Impression 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, über quant. Best, der Gerbsäuren, In- 

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

 <ipportunity to draw attention to the works of the two last-named authors, 

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

 estimating tannic acid. (See Procter, Pharm. Journ. Trans. [3], vii. 1020 ; 

 Allen, Commercial Organic Analysis, London, 1879.) 



* N. .Tahrb. f. Pharm, xxvi. 20 (1866) ; (Amer. Journ. Pharm, xxxv. 519). 



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

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



