36 
THE TANNINS. 
Risler-Beunat proposed to modify the method by col¬ 
lecting the precipitate of tin tannate, washing, drying, 
and igniting, with the addition of a little ammonium 
nitrate in order to completely convert the residue into 
stannic oxide. The weight of this residue was con¬ 
verted into tannin by a calculation based on the amount 
of residue obtained from a pure dry tannin. Dragen- 
dorff found that the precipitation was incomplete, that, 
it took place slowly, and that the precipitate was partly 
decomposed by washing. 
Gerland suggested the use of tartar emetic in the 
presence of ammonium chloride for the volumetric es¬ 
timation of tannin; the method, however, met with 
little favor until improved by Richards and Palmer, 
who used ammonium acetate instead of the chloride, 
both of which prevent the precipitation of gallic acid 
and assist the precipitation of the tannin. The tartar 
emetic was dried at 100°, and then used in the propor¬ 
tion of 6.730 grammes in one litre; in such a solution 
1 cc. corresponded to 0.010 gramme of digallic acid. 
The ammonium acetate was made by saturating glacial 
acetic acid with strong solution of ammonia, and 1 cc. 
of this solution added to every 25 cc. of the tannin 
solution, which latter should contain from 0.1 to 0.3 
gramme of tannin in every 100 cc. Three or four tests 
should be made in as many beakers placed side by side, 
and the end of the reaction determined by placing a 
drop of the clear supernatant liquid on a white porce¬ 
lain surface, adding a drop of sodium thiosulphate, and 
warming; a red color of antimony sulphide will form 
when the least excess of the reagent has been added. 
Ammoniacal zinc acetate was recommended by 
