286 ANNUAL OF SCIENTIFIC DISCOVERY. 



ON THE PERCENTAGE OF TANNIN IN DIFFERENT SUBSTANCES. 



The following article is contributed to the Polytcchnisches Centralblatt, 

 by Prof. Fehling : 



Among the various substances which precipitate tannin from solution, 

 such as gelatine, quinine, animal skin, &c., the latter has hitherto been 

 recommended as the most appropriate for determining the percentage of 

 tannin. This method of valuation has been preferred, because it represents 

 in miniature the operation to which the results refer. There are, however, 

 no detailed directions for its application ; and in repeated trials made by 

 the author, under a variety of conditions, he has found that the tannin is 

 never perfectly precipitated, and that the solutions soon become mouldy. 

 Experiments with solution of quinine, freshly precipitated oxide of iron 

 or alumina, did not give more satisfactory results. He then tried gelatine 

 in solution ; and instead of weighing the precipitate obtained, by adding 

 an excess of gelatine, preferred adopting the volumetrical method, estimat- 

 ing the quantity of solution of gelatine of known centigrade value re- 

 quired to precipitate the tannin. For this purpose, it is indispensable that 

 the precipitate should separate readily ; but with most kinds of tannin 

 this is not the case. The author has found it advantageous to use a dilute 

 solution of gelatine, and to have the liquids quite cold. His mode of 

 operating is as follows : 



The solution gelatine is prepared by digesting 10 grms. of dry gelatine 

 (containing about 18 or 19 per cent, of water) in water for twelve hours, 

 and then applying heat until the solution is complete. The volume is 

 then made up to 1 litre. 



For the purpose of determining the centigrade value of the gelatine so- 

 lution, 0.2 grm. of pure tannic acid, dried at 212 F., is dissolved in 100 

 or 120 grms. of water, and the gelatine solution added from a graduated 

 burette until the precipitation is complete. Filtration is generally neces- 

 sary towards the end of the operation, or, as a substitute, the following 

 plan may be adopted : A narrow open glass tube is covered at one end 

 with some tolerably thick linen bound tight by a cord ; on immersing this 

 covered end in the liquid, and sucking out the air by the mouth at the 

 other end, a portion is rendered clear by passing through the linen, and 

 may be poured into a tube and tested with gelatine. 



The author found that the 0.2 grm. of pure dry tannic acid required 

 from 32.5 to 33 cub. centims. of the gelatine solution for perfect precipita- 

 tion ; when the gelatine solution is some days old, a larger quantity is 

 necessary, 35, 38, or even 40 cub. centims. It is therefore necessary in all 

 cases, when the gelatine solution has been kept any time, to determine 

 its centigrade value by means of tannic acid immediately before making 

 any experiments with it. 



If it is required to estimate the value of oak or other barks for tanning, 

 they are first dried in a warm room, powdered finely, digested in quan- 



