172 



THE ESTIMATION OP PROTEINS. 



caustic soda, followed by 10 c.c. of a 10 per cent, solution of 

 potassium sulphide, or 1 gramme of sodium hypo phosphite, are 

 added through the dropping funnel, and the contents of the 

 flask mixed by rotatory shaking ; a flame is placed beneath 

 the distillation flask, cold water run through the condenser, 

 and the contents distilled till about 200 c.c. have passed over. 

 The tap of the dropping funnel is opened, the flame removed, 

 and the flask disconnected from the condenser ; after washing 

 the latter into the flask containing the distillate, the distillate 



N 

 is titrated with alkali solution, using methyl red or cochineal 



Fig. 24. Kjeldahl Apparatus. 



as indicator. An amount of acid equivalent to the alkali used is 

 subtracted from the amount of acid originally added ; the 

 difference represents the acid neutralised by the ammonia distilled. 

 From this figure should be subtracted the figure obtained in a blank 

 experiment i.e., an experiment performed without the addition 



N 

 of milk, but in other respects exactly the same. Each c.c. of 



acid neutralised by the ammonia produced is equal to 0-0014 

 gramme of nitrogen. The percentage of nitrogen multiplied 

 by 6' 39 will give the percentage of total proteins. 



Another form of apparatus gives equally good results if care be 

 used ; the caustic soda and sulphide solution are poured carefully 

 down the side of the distilling flask, so that the alkaline solutions 

 do not mix with the acid ; the bulb tube rapidly inserted, and 

 the contents of the flask mixed. No condenser is used, but 

 the flask containing the standard acid may be placed in cold water 

 (Fig. 24). 



