§ 52. ESTIMATION OF TANNIN. 45 



If a solution contain both gallic and tannic acid, or catechin 

 and catechu-tannic acid, both may be approximately estimated 

 by Löwenthal's method. (See also § 164, et seq.) The addition 

 of gelatine as directed by him introduces only a slight source of 

 error, which may be generally neglected. 



VIII. Chlorinated Lime. — Löwenthal ^ has titrated with chlori- 

 nated lime in the presence of indigo carmine in the same way as 

 with permanganate of potassium, but the estimations generally 

 yield too high results in consequence of the impurities present. 



Cech^ has already expressed an unfavourable opinion of the 

 propositions of Commaille^ and Millon'* to make the separation 

 of iodiiie from iodic acid by tannin the basis of a method for its 

 quantitative estimation. The decolourization of a solution of 

 iodine by tannic acid in the presence of carbonate of soda has 

 been recommended by Jean^ for the quantitative estimation of 

 tannin. He states that 1 part of gallo-tannic acid decolourizes 

 4 parts of iodine, and reserves to himself the determination of 

 the value of other tannins in terms of iodine. He admits that 

 gallic acid also acts upon iodine, and advises, when both are pre- 

 sent, first to make a total estimation, and then determine the 

 gallic acid alone in a second portion of the liquid, after the tannic 

 acid has been removed by gelatine or hide. The solution of 

 tannic acid for standardizing should contain 1 part in 1,000 of 

 water. Before titrating, 2 cc. of a 25 per cent, solution of cryst. 

 carbonate of soda should be added for every 10 cc. of tannic acid 

 solution. It must be observed that here, too, many organic com- 

 pounds would act in a similar manner to tannin. 



IX. Oxidation. — For the estimation of tannin Mittenzwey*" has 

 availed himself of the fact that an alkaline solution of tannic acid 

 rapidly absorbs oxygen from the air. In the analysis of plants this 

 method will seldom be of any value. Cech has already shown that 

 it yields unsatisfactory results Avith the tannins usually employed. 



X. Titration ivith Cinchonine. — Wagner "^ has proposed titration 

 Avith sulphate of cinchonine, using acetate of rosaniline as an 



1 Loc. cit. " Loc. cit. 3 Compt. rend. lix. 599 (1864). 



■* Annales de Chimie at de Phys. [3], xii. 26. 



5 Zeitschr. f. anal. Chem. xvi. 123 (1877). 



^ Journ. f. pr. Chemie, xci. 81, and Zeitschr. f. anal. Chemie, iii. 484 (1864). 

 See also Terreil, Zeitschrift des österr. Apothekervereins, Jg. xii. 377 (1874). 



^ Zeitschr. f. anal. Chem. v. 1 (1866). See also Salzer, ibid. vii. 70 (1868) ; 

 Buchner, ibid. 139 ; Clark, Amer. Journ. Pharm, xlviii. 558 (1876). 



