4 BULLETIN OF THE UNIVERSITY OF WISCONSIN 



of 10 cc., the tubes were used over again. As the test tubes 

 were new, special precautions in cleaning had to be exer- 

 cised. To remove the alkalies that are given off by new 

 glassware, I followed the advice of Professor Ostwald and 

 steamed the tubes for about five minutes by means of an 

 apparatus figured on p. 295 of his "Hand- und Hilfsbuch 

 zur Ausftihrung physico-chemischer Messungen. " 



In mixtures where the components enter in equivalent pro- 

 portions it was found that the free iodine began to crystal- 

 lize out when about 40 per cent, had been liberated. To get 

 this iodine into solution a few cubic centimeters of a strong 

 potassium iodide solution were added. But as this excess 

 of potassium iodide might possibly enter into the reaction 

 and so slightly increase the amount of sodium thiosulphate 

 used, the iodine of several samples was shaken out with 

 carbon bisulphide and determined. A comparison of re- 

 sults obtained by these two methods, 1 shows that the ad- 

 dition of a few cubic centimeters of cold potassium iodide 

 during titration has little or no effect on the result. 



The equation expressing the reaction shows that for 

 every molecule of potassium iodide present one atom of 

 iodine is liberated. In titrating the iodine with sodium 

 thiosulphate the following reaction takes place, 



2 Na 2 S 2 O 3 + I 2 = Na 2 S 4 O 6 + 2 Na I. 



Hence the per cent, of iodine liberated at any time from 

 the given sample, originally containing 20 cc. of potassium 

 iodide in deci-normal solution, is obtained by dividing by 

 forty the number of cubic centimeters of sodium thiosul- 

 phate used, the thiosulphate being twentieth-normal. 



Presentation of Eesults. The results obtained are divided 

 into five sections. Section A includes the results obtained 

 from a mixture containing equivalents of the salts as deci- 

 normal solutions. The effect of a definite excess of one 

 of the several components upon the speed of the reaction 



1 Sec. B, III, series (1) and (3), and Sec. C, series (4). 



