174 



Gooeh and Norton — lodometric 



and acid, and to raise the liquid to actual boiling only when 

 the space above the liquid in the retort and in the connecting 

 tube is filled as completely as possible with iodine vapor, while 

 the liquid in the receiver begins to rise in the tube. 



The action of atmospheric oxygen upon the solution of 

 hydriodic acid must, however, be also taken into acconnt. It 

 is a familiar fact that when a considerable excess of strong 

 hydrochloric acid is allowed to act in contact with air upon 

 potassium iodide (free from iodate) dissolved in a little water, 

 the mixture is colored by free iodine. The amount of iodine 

 liberated by atmospheric action is insignificant when the acid 

 is very dilute, but is considerable when the acid is strong, and 

 increases with time and rise in temperature, as shown in the 

 experiments recorded in the accompanying table. 













MoO a 









Percentage 







equivalent 





KT 





of HOI 



Time 



Temperature, 



to iodine 





taken. 



Yolume. 



in aqueous 



in 



Centigrade. 



found. 



Remarks. 



grm. 



cm 3 . 



acid. 



minutes 





grm. 





1 



66 



2 



1 



23° 



none 





1 



66 



2 



10 



23° 



O'OOOl 





1 



66 



24* 



10 



23° 



0-0017 



^ Diluted 



1 



66 



24* 



4 



^ From 23° 

 ! to the 

 [ boiling 



J point 



0-0067 



1 to 500 cm " 

 ! before ti- 

 j trating 

 with 

 J Na o S,0, 



1 



66 



24* 



10 



0-0121 



Even the precaution to conduct the operation in an atmos- 

 phere of carbon dioxide does not eliminate all chance of error 

 of this sort unless the liquid of the mixture — the hydrochloric 

 acid — is free from air. The experiments of the following 

 statement, which were conducted in the apparatus and man- 

 ner previously described, show this point clearly. Thus, 40 cm3 

 of unboiled acid, sp. gr. 1*12, introduced enough air into the 

 apparatus to cause an error of 0*0013 grm. reckoned in terms 

 of molybdenum trioxide, while the iodine set free by the 

 action of the residual acid of this experiment upon another 

 gram of potassium iodide introduced without admission of air 

 corresponded to only 0'0002 grm. in terms of molybdenum 

 trioxide. The use of acid of sp. gr. 1*1, freshly boiled in the 

 air, obviously reduces the error due to the unboiled acid, but 

 even in this case the effect of included oxygen was not wholly 

 obviated. 



*This corresponds nearly to sp. gr. 1*12. 



