ELECTRICAL CONDUCTIVITY, DISSOCIATION, IONIZATION 17 



in the solution before any electric current passes through it, the ions 

 being however uniformly distributed throughout that is, arranged so 

 that the negative charges of the anions precisely neutralize the positive 

 charges of the cations. The electric current causes the electrodes to be- 

 come charged, the one positively, the other negatively, so that an attrac- 

 tive force is exerted on the ions of opposite sign. This causes the nega- 

 tively charged ions to migrate towards the positive electrode, and the 

 positively charged, towards the negative electrode. It is this migration 

 of the ions that endows the solution with conducting qualities. 



In water, or in a solution of a nonelectrolyte, molecules of H 2 or non- 

 electrolyte exist thus: 



H 2 H 2 H 2 



H 2 H 2 H 2 

 H 2 H 2 H 2 



In a solution of an electrolyte, many of the molecules split into ions thus : 



Na+ Cl- Na* Cl- Na+ Cl- 



Na* Cl- Na+ Cl- Na+ Cl- 

 Na+ Cl- Na+ Cl- Na+ Cl- 



When an electric current passes through a solution of an electrolyte, 

 the ions tend to arrange themselves thus: 



Cathode- Anode* 



Na+ Na+ Na* Cl- Cl- Cl- 



Na* Na+ Na+ Cl- Cl- Cl- 



Na+ Na+ Na+ Cl- Cl- Cl~ 



It follows from the above considerations that the conductivity of a sub- 

 stance in solution will depend on the degree to which it undergoes dissocia- 

 tion. Furthermore, if we assume that in so far as osmotic pressure 

 phenomena are concerned, each ion behaves in the same way as a mole- 

 cule, then it follows th.at the electrical conductivity must be proportional 

 to the extent to which the osmotic pressure is greater than we should ex- 

 pect it to be from the amount of substance actually dissolved. 



In the Determination of the Conductivity it is obviously necessary to 

 use standard conditions of depth and width of the fluid through which the 

 current is passed, and to have some standard of comparison. The value 

 is then known as the specific conductivity, the standard for comparison 

 being the conductivity of a hypothetical liquid which, if enclosed in a 

 centimeter cube, would offer a resistance of 1 ohm between two opposite 

 sides of the cube acting as electrodes. The actual determination is usu- 



