134 EXPERIMENT STATION. [Jaii. 



EuU Method {Wijs' Solution). 



Reagents. — Carbon tetrachloride, dry and free from oxidi- 

 zable products. 



Iodine solution according to Wijs^: 13 grams of resubli- 

 mated iodine to 1,000 cubic centimeters of glacial acetic acid 

 (99.5 per cent.), free from oxidizable j)roducts. After the 

 iodine is completely dissolved the solution is treated with pure 

 dry chlorine gas- until the iodine has been converted into mono- 

 chloride. The completion of the reaction is indicated by a dis- 

 tinct change, the solution becoming transparent, cherry red, and 

 the titer with thiosulfate doubled. As it is desirable to have a 

 slight excess of iodine it is advisable to retain a small quantity 

 of untreated solution to add in case of necessity. 



N/10 sodium thiosulfate (hyposulfite) solution: 24.830 

 grams of sodium thiosulfate are dissolved in water and made up 

 to a liter at 20° C. 



Potassium bichromate solution: 3,8G57 grams of dry potas- 

 sium bichromate, free from sodium bichromate, are dissolved in 

 water and made up to a liter at 20° C. This solution will keep 

 indefinitely, without changing, and is used for standardizing the 

 thiosulfate solution; 100 cubic centimeters of potassium bi- 

 chromate solution will liberate 1 gram of iodine from a potas- 

 sium iodide solution. 



Potassium iodide solution: 1G5 grams of neutral potassium 

 iodide, free from iodine and iodate, to 1,000 cubic centimeters 

 of water. Iodate is said to be frequently present in commercial 

 potassium iodide and yields free iodine with hydrochloric acid. 



Starch paste : 1 gram to 200 cubic centim'eters of water. The 

 solution is prepared by boiling ten minutes. 



Process of Standardizing the Thiosidfate Solution. — Twenty 

 cubic centimeters of potassium bichromate solution are ac- 

 curately measured into a 300 cubic centimeter Erlenmeyer flask 

 and 10 cubic centimeters of potassium iodide solution and 5 

 cubic centimeters of concentrated hydrochloric acid added. Thio- 

 sulfate solution is run in gradually until the brownish yellow 



' Wijs' solution, with the same active reajrent (iodine monochloride) , has largelj' re- 

 placed that of Ilubl because of its far greater staljility and more rapid aljsorption. 

 » Well washed, and then dried by being passed through concentrated sulfuric acid. 



