CHEMICAL COMPOSITION OF PROTEIN MOLECULE 75 



shown by the turbidity and alkaline reaction of the solution. Air may be 

 introduced through the stop-cock, if distillation starts too rapidly ; the 

 stop-cock serves to release the vacuum when the distillation is finished. 

 The decinormal acid in both flasks is mixed and titrated with deci- 

 normal soda using alizarin sulphonate as indicator, which may be added 

 previous to the distillation. 



Reproduced by kind permission from the Journal of Biological Chemistry. 



4. Humin Nitrogen. The black colouring matter is adsorbed by the 

 lime, which is filtered off on a folded filter paper and washed with water 

 until the washings are free from chlorides. The precipitate and paper 

 are then submitted to Kjeldahl analysis, using 35 c.c. of sulphuric acid. 



5 . Precipitation and Washing of the Phosphotungstate Precipitate. 

 The filtrate from the humin nitrogen is neutralised with hydrochloric 

 acid, returned to the vacuum distillation apparatus and concentrated 

 in to about 100 c.c. 



It is then washed into a 250 c.c. conical flask, and 18 c.c. of con- 

 centrated hydrochloric acid and 1 5 grammes of phosphotungstic acid 1 

 in aqueous solution are added. 



The entire solution is diluted to 200 c.c. with water and heated 

 in a water-bath, until the precipitate has nearly, or quite, redissolved. 

 On cooling, the phosphotungstates separate in a crystalline form. 



After standing for at least forty-eight hours they are filtered off and 

 washed in the following manner : 



1 This should be purified by Winterstein's method by shaking its acid solution with 

 ether ; from the ethereal layer containing the substance the acid is recovered by recrystal- 

 lisation. 



