116 HYDROGEN ION CONCENTRATION 



ions with their free charges this should happen at much greater 

 dilution. Furthermore, it cannot possibly be true that the con- 

 ductivity of a concentrated electrolyte solution be simply an 

 additive function composed of the sum of the conductivities of its 

 ions, as it is in the case of very dilute solutions, for it is impossible to 

 conceive that ions in a concentrated state could pass in the proximity 

 of each other without producing a variety of force effects. Hence 

 it follows that conductivity measurements made in concentrated 

 solutions of ions cannot give us an accurate measure of the degree of 

 dissociation; in the case of strong electrolytes at least they must 

 simulate too low a degree of dissociation. Since this apparent degree 

 of dissociation value is always large, i.e., very little less than 1, the 

 above mentioned authors proposed the hypothesis to the effect that 

 the so-called strong electrolytes are always completely dissociated. 



This hypothesis cannot as yet be proven as being absolutely 

 exact. It is possibly analogous to such an acknowledged theory as, 

 for example, the one asserting that in the solid crystallized state 

 KCl does not exist in undissociated molecules but only as K+ and Cl~ 

 ions definitely spatially arranged. But if instead of "totally" we 

 should sa3^ "almost totally" or "practically totally" dissociated, then 

 we would at all events approach the true state of the matter much 

 closer than did the older assumptions. We shall assume then with 

 Bjerrum that in a 1 A?" KCl solution, for example, the molecular 

 species KCl is practically non-existent, and that each of the K+ and 

 Cl~ ions are present in a 1 M concentration. But the active mass of 

 the K+ or Cl~ ions in a 1 iY KCl solution is to be taken, however 

 less than 1000 times that in a 0.001 N solution. 



Bjerrum was led to the establishment of his hypothesis of the total 

 dissociation of strong electrolytes by his observation that the absorp- 

 tion of light by solutions of colored strong electrolytes was inde- 

 pendent of concentration and of other factors, provided there is no 

 formation of complex ions. The chromium salts afforded convenient 

 experimental material, because in solutions of these the formation 

 of complex salts occurs very slowly, and it is possible for a time to 

 work with complex-free solutions, which is not so easy with such other 

 metal salts as those of iron. It was found that the light absorption 

 of a gram-molecule of any chromium salt was quite independent of the 

 concentration of its solution and of the acid anion with which it was 

 combined. Since in all weak colored electrolytes (indicators) changes 



