24 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



haps, lies in that it enables us to observe the motion of the electrolyte 

 within the solution visually. 



The results of transference measurements cannot be interpreted with- 

 out a knowledge of the nature of the ions within the solution. The 

 transference numbers are calculated from the observed concentration 

 changes on an assumption as to the nature of the ions themselves. For 

 example, in determining the transference numbers of potassium chloride 

 by the Hittorf method, it is assumed that only potassium is transferred 

 to the cathode and chlorine to the anode. If, however, ions different from 

 those assumed exist in the solution, these will take part in the transfer 

 of electricity and will have an influence upon the observed concentration 

 changes at the electrodes. The question as to whether or not the ions 

 have the simple structure commonly assumed is one which ultimately 

 must be answered on the basis of considerations derived from other prop- 

 erties of these solutions. That complex ions are formed in the case of 

 certain solutions was conclusively shown by Hittorf. 15 He found that 

 in solutions of cadmium iodide the transference number of the cation, 

 as measured, is greater than unity. Since this ion cannot transport 

 more current than the total passing through the solution, it is obvious, as 

 Hittorf pointed out, that the result may be accounted for by assuming 

 that complex cations are formed by means of which iodine is transferred 

 from the anode to the cathode. The effect of this is to lessen the con- 

 centration increase of iodine in the neighborhood of the anode due to 

 the transfer of the iodide ion. 



If either positive or negative ions of more than one kind occur in 

 solution, an equilibrium must exist among them by virtue of which the 

 relative concentration of these ions will be a function of the total con- 

 centration of the salt. In general, with decrease in concentration, the 

 more complex ions break up into simpler ones. It follows, therefore, 

 that if complex ions exist in solution, the transference numbers should 

 vary as a function of the concentration. 



We may now examine the numerical values of the transference num- 

 bers which have been determined for various electrolytes and which are 

 given in Table I. 16 At a concentration of 5 millimols per liter, the cation 

 transference number for sodium chloride, for example, is 0.396. Corre- 

 spondingly, the anion transference number is 0.604. This means that in 

 a sodium chloride solution of this concentration the fraction 0.396 of 

 the current is carried by positively charged carriers, and the remainder 

 by negatively charged carriers. It will be observed that, in general, the 



"Hittorf, loc. cit. 



"Noyes and Falk, J. Am. Chem. Soc. S3, 1436 (1911). 



