484 UNIVEESITY OF VIRGINIA PUBLICATIONS 



PART II. 

 DIFFUSION OF IODINE IN POTASSIUM IODIDE. 



The work previously quoted showed plainly that, "An increase in the 

 concentration of the potassium iodide produces a marked acceleration of 

 the reaction." As no data bearing upon the rate of diffusion of iodine in 

 KI solutions of varying concentrations were available, a few rough determi- 

 nations were made by Van Name and Edgar. These, however, owing to 

 the experimental difficulties encountered and to the lack of time to push 

 further a side investigation, only gave a rough confirmation of the hypothe- 

 sis. During the winter of 1911-1912 Dr. Graham Edgar made a few 

 determinations of the same kind at the University of Virginia. The 

 results of these preliminary investigations showed that it was very prob- 

 able that the original hypothesis was correct as to iodine diffusing more 

 rapidly in strong than in weak KI solutions, also that the value of the diffu- 

 sion constant was somewhere about 1.1. It also developed that experi- 

 mental difficulties were much greater than has been anticipated. 



As previously stated Nernst has deduced an expression for the rate of 

 diffusion of electrolytes in terms of the velocity of migration of the ions 

 formed. This expression was deduced on purely theoretical grounds, 

 making the assumption that the electrolyte was completely dissociated. 

 The complete expression is: 



D = 0.4485-^^. (l + 0.0034 (t - 18)) 

 u + V 



where D is the diffusion constant, u and w the transport numbers, and t 

 is the temperature on the Centigrade scale. 



Making the assumption that the KI is completely dissociated, and that 

 the ions would migrate at the same velocities in the KI solution as in water, 

 and using the values given by Nernst for the K ion, and that given by 

 Bredig for the I3 ion we obtain for the diffusion constant 1.21, (16) (17) (18). 

 This value is in all probability too great for the reason that the triiodide 

 can not be completely dissociated. In fact the theoretical values thus 

 obtained for a very large number of salts are in nearly all cases much larger 

 than the results of actual experiments performed by Scheffer and by 

 Oholm (16) (19) (20). In this connection it is of interest to note that the 

 theoretical constant similarly calculated for KI is 1.47 and that actually 

 found by Oholm using a solution of 0.01 normal was 1.46. This solution 

 was, about five times as dilute as the solution of KI3 used by Van Name 

 and Edgar, to say nothing of the immense excess of KI always present, 



