ELECTROLYTIC SOLUTIONS 



29 



conductivity equals the molecular conductivity divided bv 

 the volume. 



The conductivity of an electrolyte is proportional to the 

 number of ions in a volume of the solution containing one 

 gramme-molecule. Let M y be the conductivity for complete 

 dissociation and M„ the molecular conductivity at the volume 



V. Then — - = — 1 = — = « the degree of dissociation. This 

 M„ nk n ° 



is OstwakTs law, which says that the degree of dissociation is 



equal to the ratio of conductivity when the gramme-molecule 



occupies a volume V, to its conductivity when the solution is 



so dilute that dissociation is complete. Hence the degree 



of dissociation may also be determined by comparing the 



electrical conductivities of two solutions of different degrees 



of concentration. 



Fig. i.— ISeforc the passage of the current. 



+ 



Fig. 2. — After the passage of the current. 



Velocity of the Ions. — If the electrolytic cell is divided into 

 two segments by means of a porous diaphragm, we shall find 

 after a time an unequal distribution of the solute on the two 

 sides. For instance, with a solution of sulphate of copper, 

 after the current has passed for some time there will be a 

 diminution of concentration in the liquid on both sides of the 

 diaphragm, but the loss will be very unequally divided. Two- 

 thirds of the loss of concentration will be on the side of the 

 negative electrode and only one-third on the positive side. 

 In 1853, Hittorf gave the following ingenious explanation of 

 this phenomenon : — 



