476 AGRICULTURAL ANALYSIS 



One of the side tubules carries a burette, containing five per 

 cent, sulfuric acid, the other one is filled with a hyposulfite solution 

 of known strength. The middle tubule serves to introduce a 

 glass tube through which carbon dioxid or illuminating gas 

 passes for the purpose of driving out the air from the solution 

 and the flask. If carbon dioxid be used it should be generated 

 by the action of sulfuric acid on marble. The cork holding 

 this is furnished with a slot or valve to permit the exit of the 

 air and the excess of the gas. 



Before inserting the middle stopper, a few cubic centimeters 

 of potassium iodid solution and a few drops of thin starch paste 

 are added, the potassium salt being always used in excess of 

 the nitrite supposed to be present. 



After the air has all been expelled from the flask, the ana- 

 lytical process is commenced, the carbon dioxid current being 

 slowly continued. At first, a few drops of the dilute sulfuric 

 acid are allowed to flow into the flask. As soon as the liquid is 

 colored blue, a sufficient quantity of the thiosulfate solution is 

 added to discharge the color. The successive addition of acid 

 and thiosulfate is continued until another portion of the acid 

 fails to develop the blue color, thus indicating that all the nitrite 

 has been decomposed. From the volume of thiosulfate used, the 

 quantity of nitrite is calculated. 



The Thiosulfate Solution. The thiosulfate solution is con- 

 veniently prepared, when a large number of analyses is to be 

 made, by dissolving 25 grams of pure crystallized sodium thio- 

 sulfate in loo cubic centimeters of water and diluting any con- 

 venient part thereof to 100 or 1000 cubic centimeters, accord- 

 ing to the supposed strength of nitrite solution under examination. 



For fixing the strength of the solution dissolve 3.348 grams of 

 pure iodin in a solution of potassium iodid and make the volume 

 up to one liter. Each cubic centimeter of this solution corre- 

 sponds to one milligram of nitrous acid. A given volume of the 

 iodin solution is titrated against the thiosulfate, but it is best 

 not to add the starch paste until the greater part of the iodin has 

 been removed. The starch paste is then added and the titra- 

 tion continued until the blue color has been discharged. Ten 



