CONDUCTION PROCESS IN ELECTROLYTES 23 



tration changes observed at the electrodes. In order to determine the 

 relative amounts of solvent transferred by the two ions, it is necessary 

 that there should be present in the solution some reference substance 

 which remains at rest when the current passes through the solution. The 

 concentration changes may then be referred to this reference substance 

 and the true transference numbers of the electrolyte determined, together 

 with the relative amounts of water associated with the transfer of the 

 charge through the solution. Since the results of such measurements will 

 be discussed in detail in another chapter, it will be unnecessary to proceed 

 further with their discussion here. They have been alluded to at this 

 point merely for the purpose of calling attention to the fundamental 

 assumption underlying the Hittorf method of determining transference 

 numbers. 



That the passage of the current through an electrolyte is accompa- 

 nied by a transfer of matter may also be shown by other means, as, for 

 example, by introducing a surface of discontinuity 13 in the path of a 

 conducting electrolyte. Such surfaces of discontinuity may be observed 

 visually and thus yield a very direct method for demonstrating the trans- 

 fer of matter by means of the current within the body of the electrolyte. 

 If, for example, a solution containing hydrochloric acid is superimposed 

 on a solution containing potassium chloride and a current is passed 

 through the boundary of these solutions in such direction that the more 

 rapidly moving ion, namely, in this case, the hydrogen ion, precedes 

 the more slowly moving ion, the potassium ion, then the boundary be- 

 tween the two solutions will advance in the direction of the positive 

 current. The rate of motion of the boundary under a given potential 

 gradient will depend upon the speed of the carriers. If a solution of an 

 electrolyte is placed between solutions of two other electrolytes, each of 

 which has one ion in common with the first, then, under the action of a 

 potential, the two boundaries will move in opposite directions, the boun- 

 dary between the cations moving toward the cathode and that Between 

 the anions toward the anode. It is of course necessary that the condi- 

 tions for stability of the boundaries should be fulfilled. This requires 

 that at each boundary the more rapidly moving ion shall move in ad- 

 vance of the more slowly moving ion. Allowing for certain corrections 

 which must be made, the ratio of the speeds of the two boundaries is 

 proportional to the current carrying capacities of the two ions. 14 



While the method of moving boundaries may thus be employed for 

 measuring the transference numbers of electrolytes, its chief value, per- 



u Lodge, Brit. Ass. Reports, p. 389 (1886). 

 "Lewis, J. Am. Chem. Soc. 32, 863 (1910). 



