78 CHEMISTRY OF THE PROTEIDS CHAP. 



following Hausmann's directions, namely, to continue the boiling till 

 no appreciable alteration in the amid-nitrogen- value" occurs ; or by 

 showing that the biuret-reaction is no longer obtainable, a plan adopted 

 by Osborne and Harris. 



Giimbel adds one more precautionary measure : Embden has found 

 that cystin gives off considerable amounts of ammonia if it is boiled 

 for a long time with magnesia, while no ammonia is given off during 

 the distillation with magnesia if the distillation is done in vacuo at a 

 temperature not exceeding 40-42. Distillation at 40 is the safest, 

 because Schwarzschild l has shown that at this temperature even 

 readily decomposable acid-amides, such as asparagin, are not affected. 

 The distillation at low temperatures is especially indicated when sub- 

 stances rich in cystin (hair, horn, etc.) have to be investigated. A 

 possible source of error is further the formation of melanin as 

 shown below. 



2. Dl-AMINO-NITROGEN-DETERMINATIONS 



Hausmann's method is not so perfect for the determination of 

 di-amino-acids as for mono-amino-acids, because 



1. The phosphotungstates of the di-amino-acids are not absolutely 



insoluble, and therefore the values obtained by Hausmann's 

 method must be too low. 



2. Mono-amino-acids in concentrated solutions are also apt to be 



precipitated by phosphotungstic acid. 



3. The nitrogen of the melanins (humin-substances) is estimated 



along with the di-amino-nitrogen, and therefore increases its 

 value. 



Sub. 1. The solubility of arginin-phosphotungstate, according to 

 Gulewitsch, 2 is as follows : On precipitating a solution of arginin- 

 sulphate with phosphotungstic acid, there remains in solution, if 

 sufficient phosphotungstic acid be taken, about 0'07 grm. in 1 litre of 

 fluid (1 : 14,000); while if the phosphotungstic acid is in insufficient 

 amount, the loss amounts to 0'2 grm. per litre (1 : 5000). 



Giimbel determined experimentally the solubility of arginin, as it 

 is set free during Hausmann's process. He made a 1 : 700 solution of 

 arginin chloride, corresponding to 0*118 per cent arginin, and added 

 varying amounts of phosphotungstic acid. He found the precipitate 

 to be most readily soluble in water, less in a mixture of dilute HC1 

 and phosphotungstic acid, and still less in the latter mixture, if the 



1 Scliwarzscliild, ffofmeister' s Beitrdge, 4. 155 (1904). 



2 Gulewitsch, Zeitschr. /. physiol. Chem, 27. 195 (1899). 



