35 AGRICULTURAL ANALYSIS 



inserted to prevent any loss of ammonia. To prevent bumping, 

 some zinc dust is added, securing an evolution of hydrogen dur- 

 ing the progress of the distillation. In this case the bumping is 

 prevented until near the end of the operation, when it begins 

 anew, probably by reason of the separation of solid sodium 

 sulfate. After the end of the distillation, the excess of acid re- 

 maining in the receiver is determined by a set alkali solution, 

 and thus the quantity of ammonia obtained easily calculated. 

 Kjeldahl, however, preferred to titrate the solution after adding 

 potassium iodate and iodid, a mixture which in the presence of 

 a strong acid sets free a quantity of iodin equivalent to the free 

 acid present. The iodin thus set free is titrated by a set solu- 

 tion of sodium thiosulfate, using starch as an indicator. The 

 merits of this method are sharpness of the end reaction and the 

 possibility of using only a small- quantity of the nitrogenous body 

 for the combustion. The sulfuric acid used in the receiver is 

 made of the same strength as the thiosulfate solution ; viz., about 

 one-twentieth normal. Thirty cubic centimeters of this were 

 found to be the proper amount for use with substances oxidized 

 in such quantities as to produce ammonia enough to neutralize 

 about half of it. The titration is carried on as follows : A few 

 crystals of potassium iodid are dissolved in the acid mixture ob- 

 tained after the distillation is completed, then a few drops of the 

 starch-paste, and finally a few drops of a four per cent, solution 

 of potassium iodate. The iodin set free is then oxidized by the 

 addition of the one-twentieth normal sodium thiosulfate solution 

 until the blue color disappears. 



Example: Sulfuric acid used, 30 cc. 

 Equivalent to sodium thiosulfate, 30 cc. 

 Blank combustion required, 29.8 cc. thiosulfate solution. 



Combustion of 0.645 gram of bar- 

 ley required, 14.5 cc. 

 Thiosulfate corresponding to bar- 

 ley, 15.3 cc. 



In the computation it is more simple to multiply the corre- 

 sponding number of cubic centimeters of thiosulfate by seven, 

 half the atomic weight of nitrogen, and divide the product by the 



