§1 51, 5a ESTIMATION OF TANNIN. 41 



and dried as directed in § 47. One advantage in employing 

 alcohol to extract the tannin, as already recommended by Loewe, 

 is the exclusion of the vegetable mucilage (so-called pectin) and 

 similai; substances which may under certain conditions introduce 

 a veiy great error into the estimation. Another reason in favour 

 of tho use of alcohol is to be found in the fact that, if the material 

 contains a large quantity of alb.uminous matter, water mil fre-. 

 quently only partially remove the tannin, and that many tannins 

 are much more easily decomposed by evaporation in an aqueous 

 than ill an alcoholic-solution. It may happen, it is true, that cold 

 absolute alcohol will- not in some cases extract the whole of the 

 tannin from vegetable substances that are very rich in albumen, 

 but even in such cases I would prefer treating the residue, after 

 extraction with ether, with boiling alcohol to exhausting it with 

 water. (See also §§"95, 162.) 



Special cinphasis must, however, be laid on the importance of 

 getting rid of the whole of the alcohol by distillation, if that men- 

 struum has been employed, as almost all the following determina- 

 tions of tannin are made in aqueous solution, and, the admixture 

 of even small quantities of alcohol might cause great error. 



§ 62. Let us now review the more important methods that 

 have been recommended for the estimation of tannin. 



I. Acetale of Z^arf. -^Pribram ^ has proposed precipitation with 

 neutral acetate of lead. If care be taken not to introduce too 

 great an excess of the precipitant, the precipitation of most 

 tannins is toleraljly complete, and it is only in the case of gallo- 

 tannic acid, catechu-, kino-, and caffeo-tannic acid that part remains 

 in solution on account of the slight solubility of the lead salt. 

 But aa the precipitates are not invariably of constant composition 

 it is difficult to estimate the tannin by titration with lead solution. 

 Some of the precipitates (tannic acids from oak- and willow-bark) 

 are decomposed by prolonged washing, with water, the tannic acid 

 partly passing into solution and undergoing change. It was for 

 these reasons that I have recommended the precipitation to be made 

 in not over-dilute solutions, and directed that the washing should 

 not be continued too long, and that the tannin shoidd be deter- 



' Zeitsehr f. anaL Chemie, t. 455 (1866). Compare also Jacobson, Chem. 

 teohn. Eepert. 1866, ii. 85 ; Stdn, Schweiz. polyt. Zeitsehr. u. 169 : Gietl. 

 Zeitsehr. f. anal. CTiemie, xi. 144 (1872) ; and Schmidt, Zeitsehr. d. osterr. 

 Apothekervereins, xii-p. 3?4 (1874) ; (Am. Joum. Phaim. 1874, 427). 



