342 



PROTEINS 



connected without delay to the bulb tube B, to which is 



attached the pipette P, 

 whose lower end is then 

 dipped into the acid in R. 



The flask is then 

 boiled over a sand bath 

 until about 100 c.c. of 

 liquid havedistilled over.* 

 Great care must be taken 

 not to allow the flask F 

 to become cool by ex- 

 posure to draughts or by 

 temporarily lowering or 

 removing the flame, as 

 otherwise the acid in R 

 will rush back and spoil 

 the experiment. 



When the distillation 

 is complete the cork in 

 the large flask F is taken 

 out and the flame is then 



FIG. 4. 



removed ; the pipette P 

 next is disconnected from 



the bulb tube B and carefully washed with water before 

 withdrawal from the flask R. The excess of acid is then 

 titrated back with standard alkali until the pink colour of the 

 solution changes to orange. 



The method of calculating the result may be seen from the 

 following example : 



Weight of substance taken = 0-55 gram. 



Volume of '1045 N acid taken = 50 c.c. 



Volume of -0946 N alkali required for back titration = 15-4 c.c. 



'50 c.c. '1045 N acid = 50 x -1045 c.c. N acid = 5*225 c.c. 

 15-4 c.c. '0946 N alkali = 15-4 x "0946 c.c. N acid = 1*456 c.c. 



Volume of acid neutralized 3*769 c.c. N acid. 



But 3769 c.c. N acid are equivalent to 3*769 c.c. N nitrogen. 



= 3 7 9 x H _ -0^27 gram nitrogen 



Per cent nitrogen 



IPO x -0427 _ 6 

 55 



* It is important to observe that the distillate should remain acid throughout 

 the distillation as indicated by its pink colour ; if at any time it should change 

 to yellow, another 50 c.c. of standard acid should be added at once, and this must 

 of course be taken into consideration in working out the result. 



