80 SUBSTANCES SOLUBLE IN WATER. 



95. Estimation of Total Albumen, (a) Precipitation with 

 Tannin. Another portion (25 cc.) of the aqueous extract is 

 mixed with half its volume of a concentrated solution of salt and 

 a solution of tannin and acetic acid in dilute alcohol (20 grams 

 tannin, 3 7 '5 cc. glacial acetic acid, 400 cc. spirit made up to a 

 litre with water) added as long as a precipitate is produced. 

 This is then rapidly filtered off, washed with water and dried. 

 The albumen contained in it may be determined by estimating 

 the nitrogen and multiplying by 6'25 (see 224), or by extracting 

 the tannin from the powdered precipitate by boiling with alcohol, 

 collecting and weighing the residue. (Cf. 229.) 



This estimation of albumen should be compared with the 

 previous estimations of legumin ( 93) and albumen ( 94). If the 

 determination by tannin yields a higher result, the difference may 

 be taken to represent albuminous substances not precipitated by 

 hydrochloric acid or by boiling with acetic acid. (Compare also 

 the remarks on peptone in 232.) 



As already observed in 51, in working with substances con- 

 taining a large quantity of tannin, the results obtained by pro- 

 ceeding as directed in 92, et seq., cannot be quite accurate, as part 

 of the albuminous matter is retained by the tannin in the residue 

 insoluble in water. This undissolved albumen may be determined 

 as directed in 96, 224. 



Amongst the substances which facilitate the solution of albumen 

 we may include arabin. Giinsberg 1 has proved that albumen, of 

 animal origin at least, is precipitated by gum from slightly acid 

 solutions, but redissolved by an excess. Dextrin is said to differ 

 from gum in not redissolving the precipitated albumen when 

 added in excess. 



96. Total Nitrogen. It is advisable to determine the total 

 nitrogen in the substance under examination before and after 

 exhaustion with water ; the difference represents the nitrogen in 

 the substances removed by that menstruum. If from this differ- 

 ence the nitrogen contained in the albumen estimated according 

 to 93 to 95 is deducted, the remainder will be nitrogen that 

 has been dissolved by water in the form of ammoniacal salts, 

 amides, alkaloids, nitrates, etc. The following estimations should 

 be made with the object of determining as far as possible in what 

 state this remaining nitrogen exists. 



1 Journ. f. pract. Chem. Ixxxviii. 239. 



