ELECTRICAL CONDUCTIVITY, DISSOCIATION, IONIZATION 19 



obtain figures relating to the same quantity of salt at each dilution. If 

 we multiply the specific conductivity by the volume of solution in c.c. 

 which contains 1 gram-equivalent (see page 22), a value will be secured 

 which represents the conducting power of a gram-equivalent. This is 

 known as the equivalent or molecular conductivity * and is represented by 

 the sign A. When it is determined for progressively diluted solutions, 

 A gradually increases, indicating that the efficiency of the electrolyte itself 

 as a conductor increases with dilution, because it dissociates more. The 

 extent of this increase is found to become less and less as dilution 

 proceeds. By plotting the values of the molecular conductivity of suc- 

 cessive dilutions as a curve, the value at infinite dilution can be ascertained 

 by extrapolation. This value is represented by A c . 



Now, let us see how these facts bear out the theory of electrolytic dissocia- 

 tion. According to this hypothesis the conductivity depends on the num- 

 ber of ions (see page 17), and since it is at a maximum at infinite dilu- 

 tion, the value AOC must represent the total number of ions that can le pro- 

 duced "by the dissociation of 1 gram-equivalent, and A thai at some other 

 dilution. If, therefore, we divide A by AOC we obtain a value (called a) 

 which must represent the degree to which the electrolyte is ionized at the 

 various dilutions at which A is measured. From what has been said re- 

 garding the osmotic pressure of similar solutions, it is evident that the 

 value a could also be calculated by finding the extent to which the de- 

 pression of freezing point A is greater than would be expected from the 

 number of dissolved molecules. As a matter of fact, it has been found 

 that the two methods yield practically identical values for many substances, 

 thus furnishing almost incontrovertible proof in support of the dissociation 

 hypothesis. In the cases of weak acids and bases, it is possible to secure 

 a value, called the dissociation constant (K), which represents the rela- 

 tive values of a at all dilutions. Since the activity of acids and bases 

 is dependent upon the number of H- and OH-ions, respectively, set free 

 by dissociation, it follows that it must be proportional to K. It will be 

 necessary, however, to postpone a further consideration of the application 

 of this constant until we have studied mass action (page 23). 



Biological Applications. The practical value of a knowledge of the 

 laws of electrical conductivity rests, not so much on any direct application 

 that can be made of it in explaining physiological processes, as on the es- 

 sentially important bearing which it has in enabling us to understand the 

 nature and operation of other physicochemical laws. Without a clear com- 

 prehension of the elemental laws of dissociation, it is impossible to con- 

 sider such problems as those which concern the activities of enzymes (mass 



*In other words, the molecular conductivity is the specific conductivity divided by the number of 

 gram-equivalents contained in 1 c.c. 



