38 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



V. Estimation and Isolation of Arginine. 



The filtrate containing the arginine is saturated with baryta ; the 

 precipitate of the silver salt of arginine, so obtained, is filtered off, and 

 precipitate and filter paper are stirred up in a mortar with baryta, 

 filtered off, and the process repeated till the precipitate is free from 

 nitric acid. It is then suspended in water containing a slight excess of 

 sulphuric acid and decomposed with hydrogen sulphide. The filtrate 

 and washings from the precipitate of silver sulphide and barium 

 sulphate are evaporated down and made up to 500 c.c., or I litre. The 

 amount of arginine is estimated from the amount of nitrogen deter- 

 mined in 25 to 50 c.c. of this solution by Kjeldahl's method. 



The arginine is isolated from the remainder of the solution as 

 nitrate, copper nitrate double salt, or as picrolonate. 



1. As Nitrate. The solution is freed from sulphuric acid by baryta, 

 the excess of which is removed by carbon dioxide, and evaporated 

 down. The last traces of baryta are then removed with a drop of 

 sulphuric acid, the solution is neutralised with nitric acid and evapor- 

 ated to dryness. Arginine nitrate, C 6 H 14 N 4 O 2 . HNO 3 + ^H 2 O is ob- 

 tained as a dry white crystalline mass. The double salt with copper 

 nitrate i then prepared from the nitrate ; a yield of 85-90 per cent, 

 is obtained. 



2. As Picrolonate. The solution is freed from sulphuric acid as 

 described above, and evaporated down to about 10 c.c. The necessary 

 quantity of picrolonic acid (calculated from the nitrogen determination 

 and dissolved in a small volume of hot alcohol) is then added ; the yellow 

 crystals of picrolonate are filtered off after a few days, washed with a 

 small quantity of water and dried at 1 10. The yield of arginine cal- 

 culated from the picrolonate, C 6 H 14 N 4 O 2 . C 10 H 8 N 4 O 5 , which loses its 

 one molecule of water of crystallisation at 1 1 o, is almost quantitative 

 since the picrolonate has the very slight solubility of I part in 1 1 24 

 parts of water. 



