52, 53. GALLIC AND CATECHUIC ACIDS. 47 



with gelatine in the following manner : A solution of that sub- 

 stance the value of which, in terms of the tannic acid to be esti- 

 mated, has been previously ascertained is run into the solution 

 in which tannin is to be determined as long as precipitation 

 occurs. The gelatine solution should be mixed with some salt, 

 diminishing the solubility of the tannate of gelatine. For the 

 latter purpose the addition of alum has been recommended 

 (Miiller 1 ). The proposal of Schulze 2 to use chloride of ammonium, 

 or of Lowenthal to add common salt and -^ vol. of hydrochloric 

 acid (specific gravity 1*12), appears better. A solution of gallo- 

 tannic acid may be saturated with these salts ; but in the case 

 of other tannins (from oak, willow, and elm bark) a smaller 

 quantity might be preferable. If Lowenthal's modification be 

 adopted, it is advisable to stir the liquid vigorously for five 

 minutes after each addition of the gelatine solution. It has- 

 already been determined by Giinther that the various tannins 

 differ in the amount of gelatine they are capable of precipitating. 

 He found that 100 parts of gelatine precipitate, in the presence 

 of chloride of ammonium, 77 parts of gallo-tannic acid (according 

 to Johanson 120 parts of dry tannic acid), 132 (Lehmann, 139) of 

 catechu-, 130 kino-, 130 to 132 rhatania-, 130 oak-bark-, and 168 

 of tormentilla-tannic acid. As is well-known, gallic and catechuic 

 acids do not precipitate gelatine. 



53. Gallic and Catechuic Acids. If one of these two substances 

 is to be looked for, the tannic acids should be first precipitated by 

 gelatine, the excess of gelatine by alcohol ; and after the alcohol 

 has been removed by distillation under diminished pressure, gallic 

 or catechuic acid may be isolated by shaking with ether or acetic 

 ether. If care has been taken to avoid using a large excess of 

 gelatine, the treatment with alcohol might be omitted ; and in 

 many cases it would be possible to agitate even the aqueous solu- 



1 Archiv d. Pharm. xxxviii. 147 (1845). Gauhe did not succeed in his en- 

 deavour to find an indicator (iodide of starch) to show the final reaction in 

 titrating. Compare Zeitschr. f. anal. Chem. v. 232 (1866). Neither was 

 Cech quite satisfied with an iron solution used for the same purpose. See also 

 Hallwachs (loc. cit.}. 



Zeitschr. f. anal. Chem. v. 455 (1866). Compare also Salzer, ibid. vii. 70 

 1868), and Johanson, ' Beitr. z. Chemie der Eichen, Weiden, und Ulmenrinde,' 

 Diss. Dorpat, 1875, pp. 72, 76. Also Lehmann (loc. cit.). A more recent 

 critical review of the more important methods for estimating tannic acid by 

 Lowenthal will be found in the Zeitschr. f. anal. Cfremie, xvi. 33, and 201 

 (1877), and xx. 91 (1881). 



