MEASUREMENTS OF ELECTROLYTIC MIGRATION 9 



results of the careful study that has been devoted to solutions of 

 iodine in potassium iodide solutions. Jakowkin and others have 

 shown by means of several physico-chemical methods that KI3 

 and KI are electrolytes of equal degree of dissociation ; the 

 dissociation constant for the reaction F^ ;=i I^ -(- 1 is also known, 

 from these data and a transport determination in solutions con- 

 taining iodine and potassium iodide the mobility of the tri-iodion 

 was calculated, and found to agree with that calculated from 

 conductivity measurements assuming that the dissociation of 

 potassium tri-iodide is the same as that of potassium iodide. 



(ii) In solutions of the haloid salts of cadmium and zinc the 

 transport of the halogen increases rapidly with the concentra- 

 tion, and finally approaches a maximum. This may be in- 

 terpreted by assuming that, when the maximum is reached, the 

 concentration of the simple ions is negligibly small ; by making 

 suitable assumptions as to the formula of the complex its trans- 

 port number may be calculated, and from the experimental 

 numbers for intermediate concentrations the relative amounts of 

 simple and complex ion may be obtained. 



In the case of cadmium iodide ^ the numbers so arrived at are 

 in accord with freezing point, and conductivity, and with the 

 requirements of the mass law. The applicability of the method 

 is restricted by the condition that, if the results are to be reliable, 

 the maximum must be reached before the solution becomes too 

 concentrated. 



(iii) The third method is based on the assumption that in 

 solutions containing a number of ions the migration of each is 

 given by the product of its concentration into its migration 

 velocity. This view was advanced by Hittorf in 1857 ; it has 

 been tested and confirmed by numerous experimenters with 

 known mixtures of two simple salts having a common ion ; and 

 it has been used in the mathematical developments of 

 Kohlrausch." 



Let ?/, V, 2Vy ►v, represent the mobilities of the ions K*, Cd", 



'R. Schrader, Zeit. f. Elektrochem., 3, 498 (1897). E. v. Stackelberg, Zeit. 

 phys. Chem., 23, 493 (1897). K. Hopfgartner., Zeit. phjs. Chem., 2j, 115 

 (1898). H. Jahn, Zeit. phys. Chem., 27, 354 (1898). H. Hotfmeister, Zeit. 

 phys. Chem., 27, 345 (1898). E. Berliner, Dissertation, Berlin, 1902 — to appear 

 this Winter with other work from Jahn's laboratory in the Zeit. phys. Chem. 



*Wied. Ann., 6^, 209 (1897). 



