CHEMICAL COMPOSITION OF PROTEIN MOLECULE 77 



to Ae/4. 



granular precipitate with neutral sodium sulphate solution. If the red 

 colour disappears in the process two or three more drops of alkali are 

 added. A large excess of barium chloride should be avoided. 



The barium phosphotungstate is filtered off using the same funnel, 

 paper and precautions as before, except that larger portions of wash water 

 may be used. The final washing must give no reaction for chlorides. 



The filtrate and washings are concentrated 

 in vacua, until they are reduced in volume to 50 

 c.c. The barium phosphotungstate, which sepa- 

 rates out, is filtered off, and the filtrate and 

 washings received in a 200 c.c. double-necked 

 distilling flask ; the volume is then again reduced 

 and made up to 50 c.c. in a measuring flask. 



7. Estimation of Arginine. Since arginine 

 is quantitatively decomposed by boiling with 

 alkali with the loss of half of its nitrogen, its 

 estimation is performed before that of the total 

 nitrogen contained in the solution. 



25 c.c. of the solution are placed in the 200 

 c.c. flask of the apparatus shown in fig. 5. 



The Folin bulbs are connected to the flask 

 either by a ground glass joint, or by a heavy 

 piece of rubber tubing. These bulbs contain 

 15 c.c. of decinormal acid, coloured with alizarin 

 sulphonate. 12-5 grams of solid potash and a 

 piece of porous porcelain are added to the 

 solution in the flask, and the solution is boiled 

 gently* for exactly six hours. Nearly all the 

 evolved ammonia diffuses into the bulbs. 



The bulbs are disconnected and 100 c.c. of 

 water are poured through the condenser into 

 the flask. The flask is connected with the 

 condenser of a Kjeldahl distilling apparatus, 



and the remainder of the ammonia driven off and collected in the 

 acid from the bulbs, which has been transferred to a suitable receiver. 

 Not more than 100 c.c. of the solution must be distilled over as the 

 strong potash may destroy the other substances in the solution. 



The excess of acid in the receiver is titrated. Each c.c. of acid 

 neutralised by the ammonia corresponds to 0*0028 gram arginine- 

 nitrogen, or 0*0056 gram in the total solution. 







FIG. 5. 



Reproduced by kind permis- 

 sion from the Journal of 

 Biological Chemistry. 



