CARBON IN ORGANIC SUBSTANCES BY THE KJELDAHL METHOD. 



749 



25 c.c. of baryta solution was now added, and being directly discharged into the acid and 

 mostly neutralised, the danger of change of titre by exposure to air of the flask was 

 reduced to a minimum. More oxalic acid was now added till the pink colour just 

 disappeared. The baryta solution which had adhered to the wall of the pipette mean- 

 while had run together, and there was always at least one drop could be got out of it 

 by closing the upper end with the finger and warming the bulb with the hand. This 

 restored the colour again, which would probably disappear with one-half drop of the acid. 

 Using a burette graduated to tenths of a c.c. one can easily work to one-twentieth c.c. 



Second. — Reading the burette. 



By the time the titration was finished the oxalic acid solution had time to drain 

 down, and thus the reading gave the exact number of c.c.'s used. 



Example of the analysis of uric acid ; quantity used, 7 milligrams. 



Titre of baryta solution — 25 c.c. require 45*4 c.c. acid solution. 



After the experiment. 



No. 1 Tube. 



1st Titration, 

 2nd 



25 c.c. require 41 "8 c.c. acid solution. 

 41-85 c.c. „ 



Mean = 41 "825 c.c. 





No. 2 Tube. 





1st Titration, 



.... 



25 c.c. require 44'3 c.c. acid solution 



2nd 



. 



„ 44-3 c.c. 



Mean = 44 - 3 c.c. 



Subtracting from original baryta solution 



45-4 

 45-4 



41-825 

 44-3 



3575 

 11 



Total carbonic acid, 4'675 c.c. 

 = 2 "51 milligrams carbon. 



2'51 milligrams of carbon in 7 milligrams uric acid = 35 - 86 per cent, of carbon. 

 The amount of carbon theoretically is 35 - 71 per cent. 



„ „ calculated from combustion, 3 6 "5 3 per cent. 



Table I. contains the results obtained by this method from five different substances 

 compared with the results got by combustion. The combustions were made for me in 

 the Public Health Laboratory by Mr Maitland Gibson, B.Sc 



Note. — If the air be allowed to sweep through the flask during the heating of the sulphuric acid, some 

 of this latter will almost certainly be found in the baryta solution. The solution of barium nitrate and 

 silver nitrate is to fix any trace that might come over by accident. Conducting the method as described, no 

 sulphuric acid ever gets to (D 4 ). 



