ACIDITY 299 



I find that pure normal chromium salts (strong electrolytes) in solu- 

 tion always have exactly the same absorption of light {i.e., the same 

 color) no matter what the concentration. It is only in the case of the 

 strong electrolytes that the color is not changed with the concentration. 

 The color of the weaker electrolytes depends upon the concentration. 

 I suppose that in the solutions of the weaker electrolytes there is always 

 a greater or smaller quantity of undissociated salt and that this salt not 

 only causes the changes in color but also reduces the electric conduc- 

 tivity. These color relations found in connection with electrolytic 

 dissociation can best be explained by changing Arrhenius' hypothesis 

 in the following manner. We suppose that the strong electrolytes always 

 are completely separated into ions and that this is the reason why they 

 always have the same color in all concentrations. If changes of color 

 take place in solutions of an electrolyte, the ions have more or less 

 entered into combination with each other — the dissociation is not 

 complete. 



If this hypothesis is correct, then the decrease in molecular con- 

 ductivity and in molecular depression of the freezing point that accom- 

 panies the increase in concentration must be due to the action of the 

 electric charges of the ions on each other. The molecular conductivity 

 is diminished not because the number of ions is decreased but because 

 the ions move more slowly. It seems to me there can be no doubt that 

 the electrolytic friction must increase with the ion concentration, both 

 because the positive and the negative ions will more frequently collide 

 than the neutral molecules and also because the electric field around 

 the ions, increasing with the concentration, will create about the ions a 

 water mantle of increasing thickness. 



The change in migration velocity to which Noyes referred is 

 due, then, according to Bjerrum, to the electrolytic friction of the 

 ions. They are thus prevented from exercising full freedom in 

 motion, with the result that their kinetic energy is lowered, and 

 their osmotic pressure and conductivity reduced. It is the 

 electrolytic interference that prevents twice as many ions in a 

 salt solution having twice the (osmotic, etc.) effect of an equi- 

 molecular concentration of sugar. 



Anomalous behavior of electrolytes is greater at high than at 

 low concentration. This is true because the higher the concen- 

 tration the closer are the ions and the greater is the interionic 

 electrical friction. In very dilute solutions, the ions are farther 

 apart, their mutual electrostatic attractions are negligible, and 

 the ions can exercise their full kinetic energy. 



