METHOD OF KJELDAHL 351 



weight of the substance, which will give the per cent, of nitro- 

 gen therein. 



Then ' = 1.66 = per cent, of nitrogen in sample. 



"T * O 



A more detailed description of the method of making the titra- 

 tion follows: After the distillation is finished the condensing- 

 tube is rinsed with a little water, after which the sulfuric acid 

 nnneutralized in the receiver is determined. It is advisable first 

 to test the reaction of the distillate with litmus paper before going 

 any further ; for if at any time all the acid should be found neu- 

 tralized it will be necessary to add a sufficient quantity of one- 

 twentieth normal sulfuric acid before adding the potassium iodid, 

 etc., otherwise the determination will be irreparably lost. Add 

 to the contents of the flask 10 cubic centimeters of the potassium 

 iodid and two cubic centimeters of the potassium iodate solutions, 

 described further on, and the sodium thiosulfate is then run in 

 from a burette till the fluid, which is constantly kept agitated by 

 shaking the flask, shows only a bare trace of yellow coloration 

 from the iodin still present. Starch solution is then added, and 

 the blue color obtained is at once removed by additional thio- 

 sulfate solution. When some experience has been gained, the 

 eve is able to discern, with great certainty, even the slight color- 

 ation caused by only a small trace of free iodin. 



In regard to the sensitiveness of the end reaction and the ac- 

 curacy of the result, this method of titration leaves nothing to be 

 wished for. The strength of the thiosulfate solution is deter- 

 mined in exactly the same manner, and with starch as an indica- 

 tor. For this purpose, measure 10 cubic centimeters of one- 

 twentieth normal sulfuric acid into an erlenmeyer, add 120 cubic 

 centimeters of ammonia-free water, 10 cubic centimeters of potas- 

 sium iodid solution, and two cubic centimeters of iodate solution ; 

 add thiosulfate solution till the fluid shows only the above men- 

 tioned light yellow tint, then add starch, and finally thiosulfate. 

 In this way the strength of the thiosulfate is ascertained, which 

 of course, must be occasionally redetermined, under exactly the 

 same conditions as obtain in the nitrogen determinations, and 



