GENERAL METHODS FOR ISOLATING BASES 121 



shows no longer a white precipitate (silver compound of bases) but at 

 once a brown precipitate (of silver oxide). The addition of barium 

 hydroxide in excess would now precipitate both the histidine and the 

 arginine fraction, but a separation of these may be effected by utilising 

 the fact that histidine silver is precipitated by an ammoniacal silver 

 solution but arginine silver is not. Hence, after adding enough silver 

 nitrate, baryta is added in small quantities until a drop of the clear 

 supernatant or filtered solution no longer gives a white precipitate 

 with a reagent which is prepared by adding ammonia to 10 per cent, 

 silver nitrate until the silver oxide has just dissolved. 



The histidine fraction, which is thus precipitated by baryta, is 

 filtered off, and the precipitate, after washing, is suspended in water 

 in as fine a state of division as possible. If a suitable centrifuge is 

 available this means of separation is greatly to be preferred. The 

 silver is then removed with hydrogen sulphide, or with hydrochloric 

 acid, a little sulphuric acid being first added to precipitate adherent 

 baryta. The barium sulphate formed can be readily filtered off with 

 the silver sulphide or chloride. 



Baryta in excess is now added to the filtrate of the " histidine " 

 fraction, and precipitates the silver compounds of the " arginine " frac- 

 tion, which are treated in the same way. 



The former fraction may contain histidine, /3-iminazolyl-ethylamine, 

 carnosine and creatinine, the latter arginine, agmatine and methyl- 

 guanidine. The separation is not always quite sharp, however. Thus 

 Reuter found adenine (a purine base) in the histidine fraction of the 

 bases from Boletus edulis and trimethyl-histidine in the arginine fraction 

 from this same fungus. In Kutscher's examination of mushroom 

 extract trimethyl-histidine altogether escaped precipitation by silver 

 and appeared in the lysine fraction. 



After the silver precipitate of the arginine fraction has been filtered 

 off, the solution may still contain various bases constituting the so- 

 called " lysine " fraction. The excess of baryta is removed by sul- 

 phuric acid and that of silver by hydrochloric acid ; then the bases 

 remaining in solution are precipitated by phosphotungstic acid, and 

 after recovery from the phosphotungstic precipitate, they are separated 

 by mercuric chloride or by other means. 



Potassium bismuth iodide and potassium tri-iodide are more or less 

 general precipitants for bases and have been chiefly used in investi- 

 gations on plant alkaloids, but only to a slight extent for the separation 

 of animal bases. Potassium bismuth iodide (Dragendorff's reagent, 

 modified by Kraut) gives brick red and generally amorphous precipitates 



