302 PROTOPLASM 



In accepting these newer theories, we do not thereby alto- 

 gether discard the older concept which ascribes acidity to the 

 hydrogen ion. 



The Dielectric Constant. — Any discussion of dissociation 

 would not be complete without reference to the dielectric constant, 

 which gives us a clue as to why electrolytes dissociate in water. 



Pure water is a very weak electrolyte, i.e., dissociates but little; 

 pure liquid hydrochloric acid is an equally weak electrolyte; yet 

 if these two substances are mixed, the solution is an excellent 

 conductor of electricity, which means that water, though a poor 

 conductor of electricity in itself, has the capacity to confer this 

 property (dissociation) on another substance when the two are 

 intimately associated. That property of water which is primarily 

 responsible for the dissociation of hydrochloric acid in solution is 

 termed the dielectric constant. The dielectric constant is that 

 property of solvents which determines, to a great extent, their 

 dissolving power and those other properties which they confer on 

 substances dissolved in them, such as ionization. 



The dielectric constant depends upon the nature of the medium 

 and is determined by the electrical force or repulsion between two 

 point charges separated by a given distance in the medium; in 

 other words, the repulsive or attractive effect that two charged 

 bodies have upon each other is dependent upon the electric 

 character of the medium that separates them. The dielectric 

 constant is a measure of that character, the unit being the 

 dielectric constant of a vacuum. The value for water is 81; for 

 ethyl alcohol, 26.8; and for acetic acid, 9.7. 



Substances ionize in water because the mutual attraction of the 

 ions of the dissolved substance is not sufficient to hold the ions 

 together in opposition to the electrical barrier set up by the 

 surrounding water. Water molecules are polar, i.e., electrically 

 unsymmetrical. The dielectric constant is the most direct 

 evidence that we have of polarity in molecules. 



Hydrogen Ions. — The formula for dissociation can be expressed 

 again, thus: 



where K is the constant; Ci, the concentration of the cations 

 (H+ in acids); C2, the concentration of the anions; and C, the 



