77. TANNIC ACID. 107 



tate with ferric salts. It also gives a white precipitate 

 when poured into a solution of gelatine ; as long as the 

 gelatine is in excess, this precipitate is soluble in the sti- 

 pernatant liquor when heated, while if the acid is in ex- 

 cess, it is much less soluble. 



If a piece of fresh skin, deprived of its hair by caustic 

 lime, is left for several hours in contact with a solution 

 of tannic acid, the latter is completely absorbed, so that 

 the liquid will give no color with ferric salts. 



Quantitatiye estimation. — The tannic acid in a solution 

 may be determined with considerable accuracy by com- 

 paring the specific gravity of the solution before and 

 after it has been in contact with powdered skin. 



The solution must be as clear as possible and not too 

 dilute ; such a one will answer, for example, as may be 

 obtained by exhausting 20 to 40 grms. of tanner's bark 

 with water, and diluting to 400 or 500 c.c. 



Detennine the weight of the extract obtained from a 

 weighed quantity of the bark, and then determine the 

 specific gravity of the solution accurately with the pik- 

 nometer, or specific-gravity bottle (§ 34, a). Then weigh 

 out 100 c.c. of the solution in a flask, and weigh out also 

 a quantity of finely divided skin, equal to about four 

 times the amount of tannic acid that is supposed to be in 

 this quantity of the solution ; this amount can be ascer- 

 tained approximately from Table lY, taking as the per 

 cent of tannic acid that which is found against the 

 number rej)resenting the specific gravity of the original 

 solution, just determined. 



Soften the skin by soaking it in water, enclose it in a 

 linen bag and press out the water, add it to the weighed 

 solution in the flask, close the flask and shake the mix- 

 ture vigorously, filter through a linen cloth and determine 

 the specific gravity of the filtrate. To the difference be- 

 tween the specific gravity before and after treatment with 

 the skin, add one, seek for the number so obtained in the 



